Build a Dual 15V Power Supply Using Off the Shelf Modules for Under $50

Introduction:

If you're a hobbyist that deals with audio, you'll be familiar with dual rail power supplies. Most low power audio boards such as pre-amps require anywhere from +/- 5V to +/- 15V. Having a dual voltage power supply makes it just that much easier when prototyping designs or just general repairs.

This power supply is easy to put together as it generally uses off the shelf module boards with the exception of the regulator board, which you'll have to build yourself. However there is a reason behind that which I'll come to later on.

The regulator board used boasts voltages from +/- 1.25V to 37V (depending on your input voltage). I only need +/- 15V, so an input power supply a couple of volts above that (around 19V) is fine. LM317 and LM337 voltage regulators can also pump out around 1.5A ea (depending on how much voltage they're dropping), so the input power supply's current rating also needs to be higher than this. That's why I chose two laptop power supplies to supply the input voltages. They output 19V and around 3.4A, which is more than enough to supply the regulator board. Not to mention they're cheap as chips.

I also wanted a linear power supply as they generally have less DC ripple on the output (although not as efficient as a full switch-mode power supply). Using a switch-mode input power supply to drop the 240VAC to 19V is cheap and effective. Their switching is also generally above the audio band so doesn't affect power supply noise going into your test pieces. The linear regulators will filter out most of the residual DC ripple. So, you pretty much get the best of both worlds.

The meters used can measure voltage and current (0-100V and 0-10A), are dual colour for easy reading.

With a few modifications, you can turn a bunch of parts into a very useful bench top power supply.

Note: One thing this power supply doesn't have and that is a constant current regulating control. The LM317/337 regulators themselves have some over current protection, however I wouldn't run them too long this way. That's why the load switch was placed in this project. So if that's import, you could use a different regulator board to suit your needs.

240V Wiring & Laptop power Supplies:

As this project uses high voltages (240V), they can be quite lethal if you get it wrong. If you're unsure on how to hook up high voltage components, or are not comfortable working on live equipment, my suggestion would be not to attempt this. I take no responsibility if you kill yourself. I don't want to hear from you after you've died saying Pete, I electrocuted myself and now I'm dead - OK??

Now with that being said, you have a couple of other options:

1. Just use the laptop power supplies in their supplied form and use some DC power connectors on the back of the box. It just means you have to plug in two laptop power supplies - but it's a much safer option. However, you will need to find another solution to powering the LED meters as they require separate supplies as well.

2. You could mount the laptop supplies in the case, and just cut the 240V plugs off and wire them directly to an IEC socket on the back. However, you will need a bigger case than what I've used and again, it's got live connections so still not that safe..

LED Panel Meters + Supply Voltages:

There are several types of LED meters on the market. They all essentially do the same thing, however their connections are not always the same. Going off the gauge of wire is not always guaranteed. When ordering, try and get their wiring diagram. Generally the two thick wires will be the current shunt meter. The other three will be meter power (to power the display which is red/black) and a yellow voltage sense wire to measure the voltage.

What you will notice with the meters is that they have a common earth or 0V point (the black wires are connected together internally). For this particular project that is no good. This is why the meters are powered separately via two small power supply boards (240VAC to 12VDC module board). You also have to use two boards to power, otherwise you'll be shorting outputs when using the power supply. One other essential reason is that the LED meters require a minimum or 4.5V to run. So if you turn your output down to 1.25V from the regulator board, the meters won't power on.

This is what you'll need. You can buy all of this off Ebay, Amazon or Aliexpress. I bought it all off Ebay

- Plastic Case (I've used a plastic instrument case) - $12-15

- 1x LM317/337 Regulator Kit Board - $10

- 2x 19V 3.42A Laptop Power Supplies - $6.75 ea

- 2x 240VAC to 12VDC 450mA switch-mode step down transformer boards - $1.50 ea

- 2x Voltage/Current Panel Meters 0-100V/0-10A - $3.50 ea (cheaper in bulk and available in different colours)

- 2x 10K ohm multi turn pots + knobs to suit - $2 ea (you can use the supplied pots, but multi-turn are easier to set)

- Miscellaneous and general hardware: 240VAC switch (I used one with a 12VDC LED light), binding terminal posts (6), IEC socket, fuses and fuse holders (3), small off cut of aluminium angle (2), stand offs (6), general lengths of wire and heat shrink - probably another $5-10

Note 1: The fuses to use will depend on how much current you intend to use. I'd suggest 1-1.5A for the two regulator boards and 0.5A for the 240V supply. You could go lower as well as you won't be drawing 7A from both supplies.

Note 2: The most expensive part of the build is the case. So if you can find one cheaper or want to roll your own it will save you a few bucks.

Note 3: There are a few brands of multi-turn or precision pots available The one sent was a Bochen branded pot, that has specific knobs available and doesn't use standard rough knurl knobs. It doesn't particular matter which type you use, only that you're able to get knobs to suit.

Note 4: I bought these laptop power supplies as they were only about $6ea. Save a few bucks again if you happen to have a few old ones laying about.

First image is the original schematic for a stock regulator board, with input caps and rectifier included, using an AC 12V-0V-12V transformer to power the board (for this power supply we don't use)

Second image is the wiring diagram for all individual boards to connect together

Third and Fourth images are stock wiring diagrams for the panel meters (I used) showing different configurations to power and measure. Essentially in this project, we're using the fourth diagram.

Why 19V Laptop Power Supplies?

The reason for this is that the regulator board originally was designed to run off a dual 12V AC transformer (12V-0-12V). However, if you look at the cost of one of these either from ebay or in you local electronics shop - they're around $30AU. Two laptop supplies come in at half that.

If you want a higher voltage out of the regulators, just use a higher input power supply. Remember the regulator boards will output +/- 37V, so the input can be a few volts above that. Just remember though, the higher the voltage differential (input to output), the more heat that is produced by the regulators. For example: if input voltage is 35V and output is 5V, there's going to be a lot of heat developed and you might need bigger heat sinks and/or a fan.

Prepping The Laptop Supplies

For my build, I took the supplies out of their cases as I needed them to fit in the instrument case. If you're just going to use the laptop supplies as is and use DC connectors you can skip this step.

What you'll need to do is crack the plastic case. Use a flat screw driver and carefully pry the edge until the top comes off. Then remove the circuit board assembly.

In the 2nd photo, I've drilled a piece of angle aluminium and drilled some holes into the side of the supply (I believe I used the existing holes in the supply). Be careful not to damage any components while you're doing this. I've also drilled some extra holes to screw mounting posts to it and attach the assembly to the bottom of the plastic case. Using the angle made it a little more sturdy than just using mounting posts.

The wires coming out of the board looked a little light, so I've changed them over to heavier gauged wire. De-solder the old wires, insert the new wires through the top of the board and solder them in place on the bottom of the board (in hindsight I should have used a lighter gauge but longer lengths as it was difficult having so many wires connect to the same points).

The laptop supplies also have a power LED. They're not needed, however you can keep them in if you want confirmation that each supply is effectively working (they'll die out if there's an issue with the supply or the amount being drawn). I kept them in for easier fault finding.

Note: You should use the same type of laptop power supply. The reason being that if the voltages are out by a little, they can tend to sink current into themselves and run away and then blow. Generally, it shouldn't be a problem if you use the same supplies. However, if you're worried or want extra protection, you could place a couple of power diodes (such as IN4004 or IN5404) reverse biased across the outputs of each supply (so cathode to positive, anode to negative). This will stop each supply from sinking any current from the voltages being slightly off or if one supply powers up before the other.

The regulator board comes in kit form, meaning you have to solder it up yourself. There are a few suppliers that will sell them pre-assembled for a few extra dollars. Sometimes, removing components from these types of boards can accidentally rip tracks off. You'll need to remove some components anyway, so it's just as easy to built them in the first place without them.

The first picture shows a completed board (which is what it should look like if you made it stock). The second photo however shows the modifications with the input caps and rectifier removed. I've added links instead to change the input terminal block to accept the +/-19V and direct it to the input of the regulators. You could keep the input caps if you want but they're not necessary as the laptop supplies are pretty good.

You'll also note I've put in terminals for the LED power light and also the pots just to make it easy to remove boards if necessary.

So just assemble the board as in their instructions except for the modifications above.

Once completed, hook it up to a working power supply and verify the output of each regulator stage. Remember, if using a single input supply to test, +/- in (on the +/0V terminals) +/0V out of regulator board. +/- in (on 0V/- terminals), 0V/- out of regulator board. Make sure you can adjust the output voltage (last picture showing external test power supply).

Measure out how you want your components to sit on the rear of the front and back panels. Remember, it's going to be back to front (I made that mistake myself). Actually, I wanted the mirror image on the front panel. But luckily, I hadn't done the back panel yet, so I just made it fit to the front (or I might have just turned it around 180 deg).

Drill holes using small drill bits first. Then enlarge with a bigger drill bit. If you don't have a large enough drill bits (as I have), you can use a reamer to enlarge the holes (very handy tool).

Once all the holes are drilled, pop the cut outs for the meter panels and file down just enough so the meter and IEC socket fit.

I've also added some labels to the front (using letter sheets). You can get these online, or you can print your own on clear printer paper. Then I just sprayed some protective lacquer over the top.

Once the front and back panels have had time to dry, mount all of the hardware on the front and back panels.

The two laptop power supplies can be mounted to the bottom of the case. Remember to leave room for the IEC socket, fuse and wires to run to the switch at the front. Alternatively, you could mount a switch to the back if you prefer.

Mount the regulator board.

Last but not least, as the 240V/12V power supplies for the meter panels doesn't have anywhere for them to be screw mounted, I've used a blob of silicon to hold them in place. Just make sure you've added input and output wires first!

Start by wiring up the 240V wiring from the IEC plug to the switch and also the input fuse holder. Then connect all 240V wiring to the two laptop power supplies and two meter board supplies. Insert a fuse and at this stage, it's probably a good idea to check your wiring and power up, just to make sure all voltages coming out of the laptop supplies are correct (should be 19V each)

Connect the pots and LED to the front panel controls from the regulator board. I've used 2-pin sockets and pins to make disassembly easier at the regulator board.

Now connect up the outputs of the laptop supplies and connect to the input of regulator board. You can also connect the power to the meters. Remember, that the positive of one supply goes to the negative of the other to create a virtual zero voltage point. Again, power up and make sure voltages are as expected - you should have 38V between the input voltages, +/- 19V between 0V at the inputs and some nominal voltage on the output of the regulator board (depending on where the pot is set).

Connect the output of the regulator board to the output fuses and the load switch. Connect the meter current lines (as per the wiring diagram) and then the voltage sense lines from the meter. Insert some fuses and again, test and see if the meters are reading a voltage. Fingers crossed, you haven't let the magic smoke escape!

Note: The meters are probably the hardest thing to get running. Just remember that the current part of the meters runs from positive to negative. Same will happen with the negative voltage - it flows from 0v to negative voltage!

Once you've verified that smokes not escaping, hook up a reliable meter and check the output voltages on both the positive and negative outputs. You'll most likely find that the LED meters are slightly out (as in photos 2 + 4). As these meters can be slightly out at either end of the spectrum, calibrate them to the voltage you're generally going to use the most or in the middle of a range of voltages. For example if you use 12V a lot, calibrate them to 12V. If you go between 5V and 15V regularly then calibrate at 10V.

If you have two multimeters you can do the voltage and current adjustments together. Otherwise, connect a nominal load to the output, adjust the voltage, then disconnect the meter and put in series with the power supply and swap the multimeter lead (if your meter has separate voltage and current terminals) to measure the current.

On the back of the LED panel meters, there will be two small trim pots to adjust voltage (v-adj) and current (i-adj) (see photo one). It's generally a good idea to load the output with a resistor when calibrating as the output voltage may move a little when loaded.

So adjust the v-adj until the voltage reads the same as the meter. The trimmers are a little sensitive and a small turn can go past where you want it. Just persevere until it's correct

For the current adjustment, I'd recommend using a large heat sinked resistor to calibrate (photo 6). Just make sure it's not lower than what the supply can put out. Each side of the regulator board can supply 1.5A. Calibrating it at around 1A should be sufficient.

Using ohms law V=IxR - so (V/I=R) 15V/1A=15ohms. 15 ohm resistors are a little hard to come by so 2x 8ohm resistors in series will give 16ohms. Measure the resistors - the two I've got measure 8.3 and 8.1 ohms = Total 16.4 ohms.

So, plug the numbers in again (V/R=I) 15V/16.4ohms = 0.914634A - that's the number we'll calibrate to. You should find the meter should display this as well as a double check of your meter.

You'll also have to calculate the power being put into the resistors as you don't want them frying! So, ohms law again P=VxI - 15Vx0.91463=13.72W. Make sure your resistors are each bigger than this value - 25W is good. I've used a couple of 100W which is gold (see photo 6). You can get these off ebay for about $8 for two.

To measure current out of the supply, you'll need to put your meter in series with the power supply and the load resistors. Doesn't matter if the meter is first or the resistors, just make sure the current flowing through the meter is positive to negative (so positive & 0V terminals - positive/negative on the multimeter current terminals). The negative side of the supply should be measured from 0V to negative with positive of the meter going to the 0V and negative of the meter going to the negative of the power supply. If that just confused you - look at the last photo.

Once connected you should see both a voltage and current on the front panel meter. Adjust the current pot on the back of the panel meter until it's reading the same as your multimeter. If you have two meters, have one to measure current (in series) and one to measure the voltage (in parallel).

Now you're good to go.

While everything fit in the case, I could have played around with the internal layout a little and perhaps moved the IEC socket over to allow for the two laptop supplies to sit 90 deg to where they currently are. The layout should have been mirrored also, as I generally like everything to go from left to right. I've included a sketch of what potentially I should have done.

I used 7.5A 240VAC wiring from a mains cord (because that's what I had laying around). Being that this is such a confined space, I probably should have used lighter gauge 240V wire as the project doesn't draw a lot of current.

I also didn't notice that one of the case screws went straight through where the 240V switch was. In retrospect I should have moved the switch over slightly and probably should have installed the 240V fuse holder on the front panel as well to avoid unnecessary wiring. With a little shuffling, I probably could have put the output fuse holders on the front panel as well, but the front panel was already fairly crowded.

At the end of the day, it supplies the +/- 15V that I require, easy to adjust, is reliable and uses readily available parts.

Future projects

I've also got another dual 0-30V/3A power supply in the works, although this might end up as two separate power supplies (again depending on spacing). This one does have constant current features. I bought these boards at the same time as I couldn't make my mind up which one I wanted so I got both!

There's also going to be the mother of all power supplies - a dual low/high voltage power supply using two regulator boards per side (4). It will switch from a low range 0-30V to a high range 30-90V and 5A! This will be used for testing dual voltage power amplifier boards. Again, it might end up as two separate power supplies depending on spacing.