Over Voltage Protection for 5V Circuits

In this instructable well explore building an over voltage protection circuit for 5V circuits. Anyone that's been playing around with electronics for a while has probably blown up a couple of components due to applying too much voltage accidently. I know I have!

So let's see if we can design and build a fairly simple circuit that will supply power to our circuit as long as the power supply voltage is below a certain pre-set threshold and then turn off very quickly once that limit is exceeded.

To do this I wanted to use an SCR (silicon Controlled Rectifier) and a MOSFET. An SCR can be set to trigger very precisely at a predetermined voltage point and a MOSFET with very low resistance from drain to source will deliver almost full power to our circuit until turned off.

I happened to have a MAC97A4 triac (a kind of bidirectional SCR) in my parts draw that seemed to fit the bill and an IRL540N N-Channel MOSFET with more than enough power handling capability for most small or large current draw.

1 - Breadboard
1 - MAC97A4 triac or equivalent SCR
1 - IRL540N or equivalent logic level N-Channel MOSFET
1 - LED Red
1 - LED Green
1 - Multi-turn 10K potentiometer
2 - 220 ohm 1/4W resistor
1 - 150 ohm 1/2W resistor
2 - 0.1uF capacitor
1 - 10K 1/4W resistor
1 - Normally closed pushbutton switch
1 - Suitable load resistor (similar to the load you plan to use with this circuit)
1 - Adjustable bench top power supply
Jumper wires

This is how the circuit works:

When the power supply is under the over-voltage limit, say set to a normal 5V level, the voltage applied to the gate of triac Q2 will be under the trigger threshold set by RP1 and Q2 is off. So the voltage applied to the gate of transistor Q1 will be approximately 4 to 5V and Q1 will be fully turned on and so full power will be applied to the load (Rload). The green LED will be illuminated due to having approximately 4 to 5V on its anode. The Red LED will be off.

Once the power supply voltage rises above a pre-set threshold set by potentiometer RP1, the triac Q2 is triggered and conducts fully. The voltage across the two main terminals of the triac will then be approximately 0.8V. So the gate of transistor Q1 is pulled down to just 0.8V above ground level and it turns off. For the same reason the green LED turns off and the red LED turns on. Until power is removed from the triac by pressing push button SW1 it will stay on. Also note: R2 will always pull the gate of Q1 to ground if power is removed from the gate.

The over voltage limit can be programed by first turning pot RP1 so that the wiper is at or close to ground potential. With this setting Q2 cannot trigger. Next turn up your power supply voltage to the over-voltage value you want to trigger at, in my case I set it to 5.3V. Next slowly turn the pot until the circuit triggers and the RED LED comes on. The circuit should now trigger everytime the power supplied exceeds 5.3V.

The component values for this circuit are optimized for operation at 5V. If you want to use another higher voltage, say 12V for example, you`ll need to change R1, R3 and R4 to more optimal values by experimenting. Be sure to use at least a 1/2 watt power rating on resistor R1 for the 5V circuit and at least 1W for a 12V circuit. (12/150 * 12 = 0.96W). You may also need to use higher wattage values for the LED current limiting resistors depending on what values you elect to use. Note that if you plan to build this circuit into a project, the LEDs are optional, they function mainly as indicators although they may affect voltages on the Q1 gate very slightly.

Also note that if you are using this circuit with high current levels, then where you sense the voltage level will be more critical. This depends on the point where the upper terminal of pot RP1 is connected. So connect it as close as possible to the part of the circuit you are trying to protect from over voltage. Also, try to keep circuit traces short.