EVERYONE Needs a Multi-Meter
A multi-meter can save the average person a lot of money over a few years, even though ladies who choose to buy one may not carry it in their purse when going out for the evening. You will often be able to solve many problems yourself in less time than it would take to get a technician to come to your home. This Instructable will show how to do that simply and easily.
I often meet handy people, both men and women, who do not go near to anything electrical because they are frightened of electricity. It is good to have a strong respect for electricity. It is also good to know how to make basic electrical measurements around the home. A multi-meter is the tool every home ought have. They eliminate the guess work from so many things and can save a lot of money.
A suitable multi-meter for occasional use around the home and automobile can be had for relatively little. I had been a fan of the little meters Harbor Freight often gives out for practically nothing, but something changed. Some I have begin to give erratic readings. I have a similar meter from Home Depot that is fine, but did cost a little more.
Will I Electrocute Myself?
No, that will not happen if you follow a few simple precautions. Most home uses of a multi-meter will be done with the power disconnected, or with very harmless low voltages. Regardless, you will always handle the probes by their well-insulated, completely safe plastic "handles." Usually, you will hold one in each hand.
The black probe is normally associated with the ground or negative terminal. The red probe is normally associated with the "hot" or positive terminal. For household uses illustrated in this Instructable you can ignore all of that and use either probe on any terminal.
The black probe is normally associated with the ground or negative terminal. The red probe is normally associated with the "hot" or positive terminal. For household uses illustrated in this Instructable you can ignore all of that and use either probe on any terminal.
So, What Can a Multi-meter Do?
See the first photo. You can test common batteries, but that is only the beginning of what you can do with a multi-meter. Set the selector to DC volts. Notice the symbols for AC (alternating current) voltage and for DC (direct current voltage) as highlighted by the yellow text boxes. Batteries use only DC voltage. Set the selector to the proper range on the DC scale. Household batteries like AA and AAA are rated at 1.5 volts. On this meter, select the 4 volt range on the DC scale for common batteries. If I were testing a 9 volt battery with this meter, I would need to move the selector to the 40 volt range.
(This meter has an ON/OFF slider switch. Many meters turn on when the round selector switch is moved to any setting range. OFF is at the top of the selector dial on those units. Meters have internal batteries. Turn the meter OFF when finished to save the batteries. Many meters switch themselves off automtically after a few minutes.)
See the second photo. Touch each end of the battery with a probe. Ideally, the red probe touches the "+" end of the battery and the black probe touches the "-" end of the battery, but in practice it does not matter for this purpose. A minus (-) sign will appear in front of the numbers if the red and black wires are reversed when testing the battery. You are interested in only the numbers. A 1.5 volt battery is "dead" when it registers 1 volt or less. If a device does not work, but the 1.5 volt battery registers (for example) 1.38 volts, a new battery will not make it work. There is another problem to be solved. A 9 volt battery is "dead" when it falls to read about 7.5 to 8 volts. But, when a battery is "dead" also depends on the application. I use a 9 volt alkaline battery in a wireless microphone. I have learned by experience that a battery with a voltage below 8.5 volts will not last through a one hour church service.
Shown in the photo are several types of common batteries: a 3 volt camera battery, a 9 volt "radio" battery used in smoke detectors and other things, a button battery used in watches and other instruments, and a common AA battery. In this test the AA battery registers 1.59 volts, which is new in condition.
There are dedicated battery testers you can buy, but a multi-meter costs about the same and does so much more. Some multi-meters come with a battery test function built in. Such a function adds a resistance to test the battery under a load similar to what might be encountered in a device. Here is a way you can add a battery test function to a meter you already own.
(This meter has an ON/OFF slider switch. Many meters turn on when the round selector switch is moved to any setting range. OFF is at the top of the selector dial on those units. Meters have internal batteries. Turn the meter OFF when finished to save the batteries. Many meters switch themselves off automtically after a few minutes.)
See the second photo. Touch each end of the battery with a probe. Ideally, the red probe touches the "+" end of the battery and the black probe touches the "-" end of the battery, but in practice it does not matter for this purpose. A minus (-) sign will appear in front of the numbers if the red and black wires are reversed when testing the battery. You are interested in only the numbers. A 1.5 volt battery is "dead" when it registers 1 volt or less. If a device does not work, but the 1.5 volt battery registers (for example) 1.38 volts, a new battery will not make it work. There is another problem to be solved. A 9 volt battery is "dead" when it falls to read about 7.5 to 8 volts. But, when a battery is "dead" also depends on the application. I use a 9 volt alkaline battery in a wireless microphone. I have learned by experience that a battery with a voltage below 8.5 volts will not last through a one hour church service.
Shown in the photo are several types of common batteries: a 3 volt camera battery, a 9 volt "radio" battery used in smoke detectors and other things, a button battery used in watches and other instruments, and a common AA battery. In this test the AA battery registers 1.59 volts, which is new in condition.
There are dedicated battery testers you can buy, but a multi-meter costs about the same and does so much more. Some multi-meters come with a battery test function built in. Such a function adds a resistance to test the battery under a load similar to what might be encountered in a device. Here is a way you can add a battery test function to a meter you already own.
Your Car Battery
One morning you turn the key in your car, but the motor turns over too slowly to start. You suspect the battery may be dead. Set the meter selector to 15 volts DC or more. Touch the meter probes to the terminal posts on your car battery. The meter will read 12 to 13 volts, but you want to know what the voltage reading is when the battery is under a load. Have someone turn the key while you watch the meter. If the meter reading drops to around 9 volts or less, you need to charge the battery and see if that solves the problem. If you still get a reading of about 9 volts under load after a sufficient period for charging, you probably need a new battery for your car.
Corrosion?
See the first photo. Your car battery may be in good condition, but the starter still barely cranks the engine. The problem could be corrosion at one or both of the battery terminals. Sometimes you will see white or blue powder around the terminals, but often the corrosion is not visible. The photo shows an old-style battery terminal. If your car (or motorcycle or riding lawnmower) has this type of terminal, see the text boxes for where to place the voltmeter probes. Set the voltmeter to a DC setting slightly in excess of 12 volts. Have a helper turn the key to crank the engine. If the battery connections are good, the voltmeter reading should remain at zero (0). If the battery connections are corroded, they will provide a high resistance, perhaps even an open circuit, and current will try to go through the voltmeter as an alternate route. The voltmeter reads the difference in voltage between one probe and the other. That means the voltmeter will show a reading equal to the voltage of the battery. Clean or replace the battery terminal by taking it apart, dissolving the corrosion with baking soda in water, and scraping the parts of the connection. If you use your multi-meter just once for this, you will have saved enough money to pay for it.
See the text boxes in the second photo. The newer side post battery terminals are now more common than the old-style shown in the first photo. Much of the terminal is covered with insulation. One meter probe touches the terminal's bare metal bolt. The other probe can touch a straight pin pushed through the cable's insulation.
(The first photo is from Bing Images.)
See the text boxes in the second photo. The newer side post battery terminals are now more common than the old-style shown in the first photo. Much of the terminal is covered with insulation. One meter probe touches the terminal's bare metal bolt. The other probe can touch a straight pin pushed through the cable's insulation.
(The first photo is from Bing Images.)
Light Bulbs
Is your light bulb burned out, or is there a problem with your lamp? The first test is to check for a circuit through the light bulb. Use a low resistance or a CONT setting. This test will not work with the compact fluorescent bulbs, nor with fluorescent tubes. Those do not have a continuous conductor running through them, like an incandescent light bulb. Rather, they are filled with a gas that becomes a conductor when exposed to a high voltage electrical charge.
If the bulb tests good, some further checks on the lamp socket are necessary. See the second photo. Set the meter to the 400 volt AC selection. Touch one probe to the side of the bulb socket. Touch the other to the tip at the bottom of the socket. The meter should read about 120 volts. (Note: I live in the USA where most electrical outlets operate on 120 volts. Heavy duty appliance circuits [electric stoves, clothes dryers] use 230 volts. In many other parts of the world the standard voltage in household electrical systems is 230 to 240 volts. Adjust the readings you expect according to the normal voltage provided in your locale.)
You may find the expected voltage in the bulb socket and also the bulb are good, but the lamp still does not light. If the socket is older, the contact tip at the bottom of the socket may have lost its springiness and it may not make dependable contact with the center tip of the bulb. Use a popsicle stick to lift the tip. If you can be sure there is no electrical power to the socket, you may use a screwdriver. Then screw the bulb into the lamp socket and it should work. In time you may want to have the lamp socket replaced, but it can work this way for a long, long time.
If the bulb tests good, some further checks on the lamp socket are necessary. See the second photo. Set the meter to the 400 volt AC selection. Touch one probe to the side of the bulb socket. Touch the other to the tip at the bottom of the socket. The meter should read about 120 volts. (Note: I live in the USA where most electrical outlets operate on 120 volts. Heavy duty appliance circuits [electric stoves, clothes dryers] use 230 volts. In many other parts of the world the standard voltage in household electrical systems is 230 to 240 volts. Adjust the readings you expect according to the normal voltage provided in your locale.)
You may find the expected voltage in the bulb socket and also the bulb are good, but the lamp still does not light. If the socket is older, the contact tip at the bottom of the socket may have lost its springiness and it may not make dependable contact with the center tip of the bulb. Use a popsicle stick to lift the tip. If you can be sure there is no electrical power to the socket, you may use a screwdriver. Then screw the bulb into the lamp socket and it should work. In time you may want to have the lamp socket replaced, but it can work this way for a long, long time.
Fuses
Many devices use fuses, from your automobile to your household air conditioning system. A fuse is an electrical conductor designed to fail when a pre-determined threshold of current is present in the circuit. Their purpose is protect the rest of the circuit from a current load that would destroy it.
Sometimes fuses are out in the open where they are visible. Sometimes they are under a cover marked "Fuses." Sometimes they are inside an appliance with no notice of their presence. This is true of some microwave ovens and some television sets. Open the case and look for a small cartridge fuse like the one shown in the photo. The fuse will usually be near where the electrical cord comes into the appliance. You may have thought you need a new microwave, but the real problem may be only a small fuse costing less than a dollar. Sometimes there is a more serious problem that will cause a new fuse to "blow," too. But, more often, the old fuse simply became weak or "blew" because of a momentary electrical surge. If your multi-meter saves you from replacing an appliance that failed due to a blown fuse, your meter will have paid for itself many times over.
It is always good if you can have extra fuses available for whenever one needs replacement. When you need to buy a fuse, take the old fuse with you to the store so it can be matched. Fuses come in a variety of physical sizes and types. It is important to replace a fuse with one exactly like it. Some fuses are available at your local hardware or building supply store. Some, like small cartridge fuses similar to the one shown in the photo, are available only where electronics parts are sold, or on-line. Identifying numbers are imprinted on the shiny metal end caps.
The fine wire in cartridge fuses is often so fine that it is difficult to see with the unaided eye. Some fuses have a solid body that blocks vision. When testing fuses set the multi-meter to CONT. for continuity. Touch the probes to the ends of a good fuse when on this setting and you will hear a shrill tone indicating there is a continuous circuit through the fuse. No sound means a bad fuse. The words "continuity" and "continuous" are purposely related.
WARNING: If you open the case on a television or a microwave, be careful not to touch components other than the fuseholder. There are parts that retain a high voltage electrical charge. It is not likely you would touch these, anyway; but, if you did, they can kill.
Sometimes fuses are out in the open where they are visible. Sometimes they are under a cover marked "Fuses." Sometimes they are inside an appliance with no notice of their presence. This is true of some microwave ovens and some television sets. Open the case and look for a small cartridge fuse like the one shown in the photo. The fuse will usually be near where the electrical cord comes into the appliance. You may have thought you need a new microwave, but the real problem may be only a small fuse costing less than a dollar. Sometimes there is a more serious problem that will cause a new fuse to "blow," too. But, more often, the old fuse simply became weak or "blew" because of a momentary electrical surge. If your multi-meter saves you from replacing an appliance that failed due to a blown fuse, your meter will have paid for itself many times over.
It is always good if you can have extra fuses available for whenever one needs replacement. When you need to buy a fuse, take the old fuse with you to the store so it can be matched. Fuses come in a variety of physical sizes and types. It is important to replace a fuse with one exactly like it. Some fuses are available at your local hardware or building supply store. Some, like small cartridge fuses similar to the one shown in the photo, are available only where electronics parts are sold, or on-line. Identifying numbers are imprinted on the shiny metal end caps.
The fine wire in cartridge fuses is often so fine that it is difficult to see with the unaided eye. Some fuses have a solid body that blocks vision. When testing fuses set the multi-meter to CONT. for continuity. Touch the probes to the ends of a good fuse when on this setting and you will hear a shrill tone indicating there is a continuous circuit through the fuse. No sound means a bad fuse. The words "continuity" and "continuous" are purposely related.
WARNING: If you open the case on a television or a microwave, be careful not to touch components other than the fuseholder. There are parts that retain a high voltage electrical charge. It is not likely you would touch these, anyway; but, if you did, they can kill.
Your Air Conditioner Unit
Air conditioning systems always seem to fail during the hottest, most miserable weather. No one wants to spend a muggy night trying to sleep with no air conditioning. If the problem is only a fuse, your meter can save you the cost of a service call by a technician, and you can have your system up and running again before the house has even warmed up inside.
Your air conditioning system may have more than one set of fuses protecting it. There may be a set of circuit breakers in your main electrical panel. Check to see that the circuit breaker toggles have not moved to the "off" position due to a sudden overload. Go to the air conditioner unit outside your house. Look for any cable conduits (metal pipes, some flexible). Follow them with your eye and look for any metal boxes that might contain fuses. The fuses will likely be mounted in a fuse block that can be pulled from the box. Use the continuity setting to check the fuses. The first photo shows a control box near our heat pump/air conditioner. It looks like it might contain fuses, but it contains only a disconnect switch.
See the second photo. It shows the inside of the fuse box that controls our air conditioner. The yellow text box contains information on the ON/OFF switch. I overlaid the photo with capital letters as markers. Those in red make a "hot" circuit even when the switch is "off." Keep your hands safely away from these terminals.
Set the meter to the 400 volt AC setting. If you place one probe on A and the other on B, the meter should read about 230 volts. That is true whether the switch in the box is "on" or "off." Place one probe on A and the other on C. The meter should read about 115 volts. Place one probe on B and the other on C. The meter should again read about 115 volts. Readings between F and C or G and C should also each give a reading of about 115 volts when the switch is "on." These readings indicate the fuses are good. With the switch in the "off" position, readings between F and C or G and C should be zero volts.
If you wish to check the fuses without them being electrically charged, move the switch lever to the "off" position. Set the meter to CONT. Touch one probe to D and the other to F. You should hear the meter's shrill chime tone. Now place one probe on E and the other on G. You should hear the tone again. This also indicates the fuses are good. If one of the fuses does not test good, be certain the switch is in the "off" position. With your fingers or a pair of pliers or a wooden stick to pry, remove the bad fuse. Take it with you to a hardware or building supply store and get a replacement. Use a fuse. Do not use a piece of metal, a piece of copper tubing, or a wire with alligator clips to bridge the space for the fuse. In the days when most homes had screw-in plug fuses some people sometimes put a copper penny behind a blown fuse when they did not have a new fuse. Then they often forgot about the penny. Pennies do not burn away like a fuse does when there is an overload. Pennies used this way often caused house fires. There was a frequent saying that, "The words 'In God We Trust' were placed on pennies for the benefit of those who use them as fuses."
Your air conditioning system may have more than one set of fuses protecting it. There may be a set of circuit breakers in your main electrical panel. Check to see that the circuit breaker toggles have not moved to the "off" position due to a sudden overload. Go to the air conditioner unit outside your house. Look for any cable conduits (metal pipes, some flexible). Follow them with your eye and look for any metal boxes that might contain fuses. The fuses will likely be mounted in a fuse block that can be pulled from the box. Use the continuity setting to check the fuses. The first photo shows a control box near our heat pump/air conditioner. It looks like it might contain fuses, but it contains only a disconnect switch.
See the second photo. It shows the inside of the fuse box that controls our air conditioner. The yellow text box contains information on the ON/OFF switch. I overlaid the photo with capital letters as markers. Those in red make a "hot" circuit even when the switch is "off." Keep your hands safely away from these terminals.
Set the meter to the 400 volt AC setting. If you place one probe on A and the other on B, the meter should read about 230 volts. That is true whether the switch in the box is "on" or "off." Place one probe on A and the other on C. The meter should read about 115 volts. Place one probe on B and the other on C. The meter should again read about 115 volts. Readings between F and C or G and C should also each give a reading of about 115 volts when the switch is "on." These readings indicate the fuses are good. With the switch in the "off" position, readings between F and C or G and C should be zero volts.
If you wish to check the fuses without them being electrically charged, move the switch lever to the "off" position. Set the meter to CONT. Touch one probe to D and the other to F. You should hear the meter's shrill chime tone. Now place one probe on E and the other on G. You should hear the tone again. This also indicates the fuses are good. If one of the fuses does not test good, be certain the switch is in the "off" position. With your fingers or a pair of pliers or a wooden stick to pry, remove the bad fuse. Take it with you to a hardware or building supply store and get a replacement. Use a fuse. Do not use a piece of metal, a piece of copper tubing, or a wire with alligator clips to bridge the space for the fuse. In the days when most homes had screw-in plug fuses some people sometimes put a copper penny behind a blown fuse when they did not have a new fuse. Then they often forgot about the penny. Pennies do not burn away like a fuse does when there is an overload. Pennies used this way often caused house fires. There was a frequent saying that, "The words 'In God We Trust' were placed on pennies for the benefit of those who use them as fuses."
The Electric Clothes Dryer Does Not Dry.
The wet clothes in your dryer are still wet after running for the full cycle. The drum turned. The dryer went full cycle. What could be wrong? First check for a clogged filter or vent pipe. If those things are good, the problem may be electrical, and a multi-meter can help you find and fix it.
Dryers operate on two different circuits at two different voltages. The motor that turns the drum works on 115 volts. The heating element works on 230 volts. The electrical outlet behind your dryer that powers the dryer looks like this.
Set your meter selector to 400 volts AC. Put one probe into the "A" slot. Do not touch the other probe, but insert it into the "B" slot. The fit may not be tight. You may need to wiggle the probes to touch the metal inside and get a reading on the meter. The meter should show 230 volts. "C" is the neutral wire. "D" is the ground wire. Place a probe into "A" and the other probe into "C" or "D" and the reading on the meter should be 115 volts. The same should be true of a reading between "B" and "C" or "D." If you do not get these readings, check to be sure one circuit breaker or one fuse is not blown. There is the possibility that the dryer's heating element could be burned out, but that is not as likely. You can access it from the back of the dryer. Remove a cover over its terminals. Be sure the power to the dryer is disconnected. Use an Ohms setting on your meter and check for an open circuit. Testing an oven element in your stove is very similar. Remove the screws that hold the element in place and pull it from the oven. Do a resistance check on the two terminals.
(The photo is from Bing Images.)
Dryers operate on two different circuits at two different voltages. The motor that turns the drum works on 115 volts. The heating element works on 230 volts. The electrical outlet behind your dryer that powers the dryer looks like this.
Set your meter selector to 400 volts AC. Put one probe into the "A" slot. Do not touch the other probe, but insert it into the "B" slot. The fit may not be tight. You may need to wiggle the probes to touch the metal inside and get a reading on the meter. The meter should show 230 volts. "C" is the neutral wire. "D" is the ground wire. Place a probe into "A" and the other probe into "C" or "D" and the reading on the meter should be 115 volts. The same should be true of a reading between "B" and "C" or "D." If you do not get these readings, check to be sure one circuit breaker or one fuse is not blown. There is the possibility that the dryer's heating element could be burned out, but that is not as likely. You can access it from the back of the dryer. Remove a cover over its terminals. Be sure the power to the dryer is disconnected. Use an Ohms setting on your meter and check for an open circuit. Testing an oven element in your stove is very similar. Remove the screws that hold the element in place and pull it from the oven. Do a resistance check on the two terminals.
(The photo is from Bing Images.)
Electrical Outlets
Sometimes you need to check an electrical outlet, or replace it. See the text boxes in the photo. Set the meter to 400 volts AC. Place a probe in the "hot" slot and another in the neutral slot. The meter should read about 120 volts. Place a probe in the "hot" slot and in the ground slot. The meter should read 120 volts. Place a probe in the neutral slot and in the ground slot. The meter should read zero volts. Check outlets with your voltmeter before removing the cover and beginning to work on them. You want to be certain the power is "off."
Cords and Other Things
I have added alligator clip attachments to the probe ends on my meter. They are a very handy accessory I got at Radio Shack. They are like an extra hand.
This toaster works well, and the meter shows a resistance reading due to the heating element inside the toaster. This is normal and good. This test could indicate there is a break in the circuit within the toaster. Further testing would be necessary to determine which part of the circuit is defective. Then a decision would need to be made as to whether the appliance can be repaired and whether it is worth the effort.
Whether it is a toaster, or a phone charger, or a power cord for a computer; cords often fray internally within an inch or two of the plug. This is very understandable. That is the area of the cord that is frequently flexed back and forth. If something, like a phone charger, works some of the time, but not at other times; attach the meter with a resistance scale setting. Gently flex the cord back and forth near the plug end. Make fairly sharp bends. Watch the meter reading to see if it fluctuates between a normal reading and no connection. If it does, ask someone with a soldering iron to cut the cord back and reattach the plug end for you.
This toaster works well, and the meter shows a resistance reading due to the heating element inside the toaster. This is normal and good. This test could indicate there is a break in the circuit within the toaster. Further testing would be necessary to determine which part of the circuit is defective. Then a decision would need to be made as to whether the appliance can be repaired and whether it is worth the effort.
Whether it is a toaster, or a phone charger, or a power cord for a computer; cords often fray internally within an inch or two of the plug. This is very understandable. That is the area of the cord that is frequently flexed back and forth. If something, like a phone charger, works some of the time, but not at other times; attach the meter with a resistance scale setting. Gently flex the cord back and forth near the plug end. Make fairly sharp bends. Watch the meter reading to see if it fluctuates between a normal reading and no connection. If it does, ask someone with a soldering iron to cut the cord back and reattach the plug end for you.
Lightning and Your Telephone
This is an illustration of what is possible with a meter. It is not intended that most users would ever do this. Yet, many on Instructables experiment with LEDs (light emitting diodes), and it would pertain to that.
During the 1980s I was the pastor of a church in a rural setting. Lightning sometimes struck the utility company's power distribution lines and jumped to telephone lines running into our building. Once our telephone would not work. Twice the phone answering machine would not work. I was able to use my meter to rescue the telephone. I was also able to rescue the answering machine once, but not the second time. After the second strike we added a surge protector and had no problems after that.
See the first photo. The selector is set to the diode checker. Diodes are one way electrical valves and are very sensitive to surges of electrical current, especially with things like lightning. The arrow with a line across the arrow point is the standard symbol for a diode. Not every meter has a diode checker setting. Because diodes are very sensitive to current overloads, the diode checker limits the current in amperes that flows through a diode while testing it.
The second photo shows the circuit board from a telephone. The yellow text box identifies a bank of four diodes. Although diodes can come in many sizes and shapes, those commonly used on circuit boards like this one are small black cylinders about 5/16 inch long and about 5/32 inch in diameter. At one end is a gray band. A wire runs out of each end of the diode.
You may notice the probes do not look anything like the probes you have seen before. This is another accessory I got at Radio Shack. It is a set of clip on probes designed to grasp the small wires of electronic parts to make getting an accurate reading easier.
When testing a diode, you are looking for a relatively high current reading when the red and black leads are attached one way, but a relatively lower reading when the connections are reversed. Note that the red probe is above the black probe in this photo and the reading is 1.585. See the third photo. The black probe is now above the red probe and the reading is one-third the previous reading, which is considerably lower. These readings are the sign of a good diode.
Often a diode can be checked without removing it from the circuit. Sometimes the readings are confusing. Chances are current is feeding through some other electronic component to cause an undependable reading. Then it becomes necessary to desolder one of the diode leads to isolate it from the circuit for accurate readings.
In the lightning incident I mentioned, there were only a couple of diodes. One of them failed its test. For less than a dollar I was able to replace the defective diode and the phone was good ever after. At the time a new phone was about $20.
See the fourth photo. It is a Maglite flashlight that now has one of their LED bulbs. It quit working and I needed to know if the LED was still good or not. I did a diode check with my meter and discovered the switch was the problem.
During the 1980s I was the pastor of a church in a rural setting. Lightning sometimes struck the utility company's power distribution lines and jumped to telephone lines running into our building. Once our telephone would not work. Twice the phone answering machine would not work. I was able to use my meter to rescue the telephone. I was also able to rescue the answering machine once, but not the second time. After the second strike we added a surge protector and had no problems after that.
See the first photo. The selector is set to the diode checker. Diodes are one way electrical valves and are very sensitive to surges of electrical current, especially with things like lightning. The arrow with a line across the arrow point is the standard symbol for a diode. Not every meter has a diode checker setting. Because diodes are very sensitive to current overloads, the diode checker limits the current in amperes that flows through a diode while testing it.
The second photo shows the circuit board from a telephone. The yellow text box identifies a bank of four diodes. Although diodes can come in many sizes and shapes, those commonly used on circuit boards like this one are small black cylinders about 5/16 inch long and about 5/32 inch in diameter. At one end is a gray band. A wire runs out of each end of the diode.
You may notice the probes do not look anything like the probes you have seen before. This is another accessory I got at Radio Shack. It is a set of clip on probes designed to grasp the small wires of electronic parts to make getting an accurate reading easier.
When testing a diode, you are looking for a relatively high current reading when the red and black leads are attached one way, but a relatively lower reading when the connections are reversed. Note that the red probe is above the black probe in this photo and the reading is 1.585. See the third photo. The black probe is now above the red probe and the reading is one-third the previous reading, which is considerably lower. These readings are the sign of a good diode.
Often a diode can be checked without removing it from the circuit. Sometimes the readings are confusing. Chances are current is feeding through some other electronic component to cause an undependable reading. Then it becomes necessary to desolder one of the diode leads to isolate it from the circuit for accurate readings.
In the lightning incident I mentioned, there were only a couple of diodes. One of them failed its test. For less than a dollar I was able to replace the defective diode and the phone was good ever after. At the time a new phone was about $20.
See the fourth photo. It is a Maglite flashlight that now has one of their LED bulbs. It quit working and I needed to know if the LED was still good or not. I did a diode check with my meter and discovered the switch was the problem.
Reading Amps. (current Draw)
Although often used interchangeably in this Instructable, and wrongly so, "current" and "voltage" are actually different from one another. Voltage concerns the pressure at which electrons flow, like water pressure in a pipe. Current (amps. or amperes) deals with the volume of electrons flowing at the operating voltage. A device may appear to work, but makes an unusual noise or quickly overheats. A check of the amps. drawn by the circuit can tell you if there is a problem, even though it will not identify the exact problem.
First, check the device specifications. Look for a plate or label on the back or bottom of the device. It may tell you the device is designed to draw (for example) 2.3 amps. at 120 volts. Or, it may tell you the device uses 276 Watts at 120 volts. (Watts equals volts multiplied by amps., so divide Watts by volts to determine the proper amps.) If this device were found to draw (for example) 4.5 amps., you would know immediately something is wrong.
Reading amps. is different from reading voltages. Voltage readings are the drop in electrical "pressure" across two points in a circuit, or a whole circuit. The meter is not part of the circuit, but reads what happens across or between two points in the circuit. When reading amps, the meter must become a link in the circuit, just like a link in a chain. See the graphic showing how you can make a sandwich with two conductors and a piece of plastic between them. This sandwich can be placed between two batteries in your device to see what the current draw is. The alternative is to break the circuit by cutting a conductor and connecting the meter to the ends of the cut conductor. You would need to reconnect the cut connector when you are finished. Set the selector for DC amps in the desired range.
You may need to change the holes into which the probes connect on the meter. See the second photo.
First, check the device specifications. Look for a plate or label on the back or bottom of the device. It may tell you the device is designed to draw (for example) 2.3 amps. at 120 volts. Or, it may tell you the device uses 276 Watts at 120 volts. (Watts equals volts multiplied by amps., so divide Watts by volts to determine the proper amps.) If this device were found to draw (for example) 4.5 amps., you would know immediately something is wrong.
Reading amps. is different from reading voltages. Voltage readings are the drop in electrical "pressure" across two points in a circuit, or a whole circuit. The meter is not part of the circuit, but reads what happens across or between two points in the circuit. When reading amps, the meter must become a link in the circuit, just like a link in a chain. See the graphic showing how you can make a sandwich with two conductors and a piece of plastic between them. This sandwich can be placed between two batteries in your device to see what the current draw is. The alternative is to break the circuit by cutting a conductor and connecting the meter to the ends of the cut conductor. You would need to reconnect the cut connector when you are finished. Set the selector for DC amps in the desired range.
You may need to change the holes into which the probes connect on the meter. See the second photo.
Shocking, Simply Shocking!
My father had a small electrical business when I was in high school. I was often his helper. Many of his customers lived on farms. A frequent complaint was a tingle (mild electrical shock) when taking a shower bath. Invariably, there had been a lightning strike nearby recently, and the lightning surge had shorted an electric water heater element to ground through the water inside the water heater tank. Turn off the power to the water heater at the circuit breaker. Remove the cover plates over the heater elements and pull the fiberglass insulation back. Disconnect the wire from one of the terminal screws on the heater element to isolate it from any possible feedback through another part of the circuit. This is to avoid false readings. Set the Ohms scale to a high range. Look for this symbol: Ω to identify the Ohms or resistance scale. Touch one probe to one of the heater element terminal screws. Touch the other probe to bare metal on the side of the water heater tank. If the element is not shorted to the water inside the tank, the meter reading should indicate an infinite resistance (no current flow, an open circuit).
A few years ago we had a neighbor who avoided having grandchildren visit because anyone who touched the built-in kitchen stove received a mild electrical shock. I set my meter to an AC voltage setting in the 150 volt range and placed one probe on the chrome oven handle and one probe on a sink faucet. My meter told me the electrical current registered about 40 volts AC. It took me some time, guess work, and checking; but I found someone had disconnected the thin green wire that connects the metal frame of the stove to that home's grounding circuit. When I reconnected the wire, the stray voltage disappeared. The green ground wire was located on the back of the oven.
A few years ago we had a neighbor who avoided having grandchildren visit because anyone who touched the built-in kitchen stove received a mild electrical shock. I set my meter to an AC voltage setting in the 150 volt range and placed one probe on the chrome oven handle and one probe on a sink faucet. My meter told me the electrical current registered about 40 volts AC. It took me some time, guess work, and checking; but I found someone had disconnected the thin green wire that connects the metal frame of the stove to that home's grounding circuit. When I reconnected the wire, the stray voltage disappeared. The green ground wire was located on the back of the oven.
Another Use From the Past
The person to whom this Instructable is geared will not likely do this, but it is an example of yet another helpful use for a multi-meter. We parked our car in a store parking lot. When we left the store, it would not start. The battery had failed because the battery post had become disconnected from the internal circuitry of the battery. When I installed a new battery there were immediate signals that all was not well with the car's charging system. I did some reading in my auto manuals. I learned a poor battery connection can cause the rotating field of the alternator to burn out. The resistance of the field coil (rotor coil here) should be 1.5 Ohms. That is tiny. I set my meter for a very low Ohms reading, touched the probes to the slip rings, and found there was far less than 1.5 Ohms of resistance. The wires in the rotor coil had burned their insulation and shorted. This did not save any money, but it saved time for me. Suddenly I knew what the problem was and knew what to do to fix it. I also needed a new alternator. Without a good multi-meter, I would have only continued to guess about the problem.
(The photo is from Bing Images.)
Auto Ranging Meters and Reading the Meter
It is easy to forget to set the meter to the right range for the type of reading you will be making. With an analog (needle indicator) meter, that could be fatal to the meter. Some digital meters are "auto-ranging." That means the meter automatically makes the right setting. My meter has circuit protections built into the meter. Part of this is two fuses designed to "blow" before the circuitry can be harmed. I try to keep extra of these fuses on hand, just in case. See step 16 for a photo with the location of the fuses.
On my meter the digit "3" indicates an open circuit (= no path for electrical current). If I am using the Ohms (resistance) scale, the digit "3" appears until I attach the probes to the device I am testing. Sometimes the digit "3" remains when attached to a device known to be good. There should be an actual reading, but the reason there is not is that I have the meter set to an incorrect range. The device I am checking may have (for example) a resistance value of 12,500 ohms. If the meter is set to a range too low, perhaps to the 4K range, the digit "3" will appear on the display. If I move the selector to the 40K range the meter will suddenly give the reading I am seeking. I try to remember to begin at a very high range that may not give me as many numbers in the reading as I hope to see. Then I move the selector to lower range settings until I see the number of digits in the reading for the accuracy I need.
Accessories and Features
I have already mentioned two accessories available for my meter: insulated alligator clips and spring clips for grasping electronics component wires. The photo shows a telephone pigtail that makes connecting to the telephone lines for a voltage check easy. I used my alligator clips to connect to the red and green wires on the pigtail. If all is well, the voltmeter should read about 50 volts DC. See my earlier Instructable. This can be a big help when you are reporting a service outage. The pigtail has two other conductors. I taped them so they would not touch anything and create a short. Note: 50 volts DC is low enough that you can touch the bare wires without a shock. Still, do not stand in water when doing this. Water multiplies the danger.
Sometimes you need an extra hand. Multi-meters come with a fold out stand on the back of the meter. These usually can also be flipped up to function as a hanger for the meter. This frees both hands for manipulating the probes, especially if the voltages could harm someone.
See the text box in the photo for details on the Hold button. Sometimes you may be able to reach terminals with the probes, but may not be able to read the meter. If you can manage to press the Hold button and release it, the meter will freeze the reading so you can move and see what the meter recorded.
User Serviceable Things
Very little on a multi-meter requires attention. There is a door or plate on the back that opens by removing a few small screws or with a snap that can be opened with a fingernail. Inside are the batteries that power the circuits in the meter. There may also be some fuses. Very cheap meters may not have fuse protection.
When the batteries grow weak there may be a weak battery indicator on the display screen. You will notice the display numbers grow more weak and difficult to see unless viewed at just the right angle. If the meter will not be used for very long periods of time, it is a good idea to remove the batteries so they do not begin to leak and ruin the metal tabs in the battery compartment. This meter uses three AA batteries. Many meters use one 9 volt battery. Notice that this meter has two fuses. One is a fast blow fuse designed to protect the meter from the wrong setting on the meter's selector switch. I have a few extra fuses in a small plastic bag. The bag fits into the battery compartment. Notice that the fuse specifications are also molded into the plastic case.
When the batteries grow weak there may be a weak battery indicator on the display screen. You will notice the display numbers grow more weak and difficult to see unless viewed at just the right angle. If the meter will not be used for very long periods of time, it is a good idea to remove the batteries so they do not begin to leak and ruin the metal tabs in the battery compartment. This meter uses three AA batteries. Many meters use one 9 volt battery. Notice that this meter has two fuses. One is a fast blow fuse designed to protect the meter from the wrong setting on the meter's selector switch. I have a few extra fuses in a small plastic bag. The bag fits into the battery compartment. Notice that the fuse specifications are also molded into the plastic case.
Additional Resources
If you want even more ideas for how to use a multi-meter to solve problems at home, get a copy of this book. A similar book appeared after this one. I bought this about 20 years ago and it is very helpful, although it tends to go into a next level beyond this Instructable with more applications and more detailed tests. And, a new edition of this book is available again at Radio Shack stores.
Do not be cavalier when working with electricity. Take many precautions. Check and double check to be certain a circuit is not live. Many important measurements that tell you about the health of a device can be made without power to the circuit, other than the small batteries inside your meter. Using a multi-meter can help you fix problems safely and quickly without waiting for an expensive service call by a technician.
Do not be cavalier when working with electricity. Take many precautions. Check and double check to be certain a circuit is not live. Many important measurements that tell you about the health of a device can be made without power to the circuit, other than the small batteries inside your meter. Using a multi-meter can help you fix problems safely and quickly without waiting for an expensive service call by a technician.