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.

Supplies

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

Step 1: Circuit Diagram

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.

Comments

[merlinxl5]

Thanks for this great circuit! What is the current-carrying capacity of the circuit as is. And what values would need to be changed in order to handle 2amps continuous? Thanks!

[sncarter]

This depends on the rating of the transistor you use. The one I used will certainly support 2 amps. With a heat sink it will support 10’s Of amps. Look up the spec for this mosfet. You can use any logic level mosfet you like. I.e modder designed to work with 0-5v logic level on the gate. Good luck with it! Simon

[galaxy1997]

hi
non of MOSFET and TRIAC cannot be found in my country and i can't buy it from international online store, can You please, tell me equivalent popular Mosfet and Triac can be found everywhere.
thanks

[sncarter]

E.g NTP360N80S3ZWhat you need is a logic level n-channel mosfet such as FQP30N06L or with a low RDS(on) rating, there literally thousands out there, just choose one with the voltage and current range appropriate for you application.

As far as the SCR you just need one with a sensitive gate e.g https://www.digikey.ca/en/products/detail/littelfuse-inc/2N6073BG/918264. Again loads of them out there.

[THX 1138]

If the circuit was accidentally connected to 12 V instead of 5 V, would this cause ~30 mA to flow through the red LED? In that case, should R3 be doubled?

[sncarter]

Don’t know for sure, but I’d guess you’d need to double the values of R1, R3 and R4. Try that and see if it works. In this case R1 will need to be 1/2 watt absolute minimum, probably better to use 1 watt.

[THX 1138]

I'm using R1 910 ohm, R2 300 ohm (1 W) and R4 300 ohm. With these values, while the voltage is close to triggering Q2, the red LED illuminates weakly. The same thing happens with R2 270 ohm. It doesn't happen with R2 150 ohm or R2 240 ohm. Could 240 ohm be okay? Mine are rated 0.6 W and get pretty hot at 12 V but I could get some 1 W ones, not that I'd be likely to accidentally leave my project connected to 12 V for very long.

[THX 1138]

Is a BT131-600 a suitable alternative for Q2? The MAC97A4 seems to be discontinued and kind of expensive because of that.

[sncarter]

The MAC97A4 is a sensitive gate type triac.The data sheet for the BT131-600 say is a very sensitive gate, so it will likely work also. As long as its voltage and current carrying capabilities are what you need for your application you should be okay. This stuff is cheap, buy one and try it :-)

[THX 1138]

Thanks for the instructable. Would this operate quickly enough to protect WS2812 addressable LEDs if they were connected to 12 V? Would a zener diode operate more quickly?

[sncarter]

It should work. I’m not sure if a zener would be any faster. For this circuit you’d need to manually reset it though. With a zener the diode will just conduct during the overvoltage condition and stop by itself once the condition is cleared. Both should work and both should be very very fast. With a zener you’d need an appropriate series resistor or else your zener will burn out pretty quickly if power is applied for any length of time. With the circuit there’s no chance of burn out because it essentially turns of Q1 and removes power to the load.

[THX 1138]

I was actually thinking of combining the two if that makes sense, using a zener of, say, 6.3 V (just what I happen to have knocking around), so Q2 would still get the voltage it needs to activate. I will order the components and try it but don't have the equipment to test the speed.

[jacobtohahn]

What're the purposes of R1, R2 and C1? Also, if C2 is a decoupling cap, why not put it near the load?

[sncarter]

Hi Jacobtohahn,
Thanks for your question.

R1 is there to provide enough current for Q2 to turn on when triggered. If LED 1 is not conducting then R3 cannot supply this current.

R2 is there to turn off Q1 by pulling its gate to ground when power is removed from the gate. Look up MOSFET theory when you have time. Once Q1 has been turned on, the gate needs to pulled firmly to ground to make sure it turns off.

The purpose of C2 is to prevent high frequency noise on the power rails. Typically in digital circuits we place these near the power rails on each IC. In this case the only IC that could possibly generating noise is the Arduino IC. It will have decoupling caps on the Uno board anyway, but as a matter of good design you should include a decoupling capacitor across the power rail. Location will be very specific to the particular circuit of course.

Regards,
Simon

[jacobtohahn]

Thanks for the thorough reply. So, as for R1, does a triac not turn on without current applied to its collector? That would make sense. It would also make sense to use a resistor so you’re not just shorting VCC to GND when the triac switches. As for R2, that makes more sense now. I had forgotten that MOSFETs can switch when floating due to capacitive coupling. Same reason why CMOS IC inputs are always pulled low, right. C2 tripped me up because of its seemingly random location on the schematic. I’m more used to seeing them separately connected to VCC and GND flags.
I’m still confused about C1, though. It doesn’t seem like it does anything. Why is it there?

[sncarter]

Somewhere in the spec you’ll find there’s a minimum current required for the triac. Also need R1 to keep Q1 gate high and supply current to Led 2 during normal operation.

You are partly correct about C1, you could get away without it. This just there to stabilize the circuit against very momentary over voltage spikes. So if you went over voltage for say 1 nanosecond only, you would not want the circuit to trip. C1 should slow down and prevent the trip response of Q1 in these cases.

Try build it and play around with it. This circuit is somewhat experimental and you can probably vary the resistor and capacitor values somewhat depending on your application. For example if you wanted to use a higher voltage range, you’d need to change the value of R1 but I haven’t tried it.

keep building and experimenting!
cheers,
Simon