i would like to protect a 3 mosfets and 3 leds connecting to this mosfet from over voltage or transient voltage spikes.

Attached is a scheme with a capacitor in paralel prior each mosfet.

vin = 12v from regulated wall adapter, vload =12v, 2A each load, load is a led. mosfet i am using is FDMA430NZ.

Mcu to gate 5v, 25ma max.

Apreciate for any advice for determining the capacitors values so they will be efective in protecting? or is a 0.1uf is the right value base on these details?

What If i will use a tvs diode instead of the diode in beginning of the circuite, will it cover all this circuite protection for over voltage spikes?

Thank you

enter image description here


2 Answers 2


No, the capacitors across the MOSFETs will not do much, I do not think it is usefull to include them.

I think you're being overconcerned with protecting your MOSFETs. The most sensitive part of the MOSFETs is their gate. These MOSFETs already have build-in gate protection. You included resistors in series with the gates (good!) but these are only 10 ohms. I would make them 1k ohm so that no damaging current can flow.

You have to ask yourself what you want to protect against specifically. Just "voltage spikes and transients" is too vague. Do you want to protect against the 12V wall adapter failing ? Although it failing and supplying too much voltage is a very small chance you could protect against that with a fuse at the input and a 15 V zener diode across the supply after the fuse. When the supply goes above 15 V, the zener will conduct and blow the fuse interrupting the current.

I do not see a reason to include more protection, that just seems paranoid.

  • \$\begingroup\$ Hi FakeMoustache, thank you for your enlighting reply, i burned two circuites in the minuet i connected the adapter to the AC, and there was a spark when i did that so i assume it is for a voltage kick and thats why i think this circuit needs voltage spikes protectction. do you think that only the 1k resistor in series with the mcu/Gate will be enough? and rest will do the mosfet inner diode? \$\endgroup\$
    – barg
    Commented Mar 7, 2016 at 10:40
  • \$\begingroup\$ " i burned two circuites in the minute ...there was a spark when i did that" Then you have a different problem ! Maybe a mains isolation problem. You need to investigate that. You are focusing on the design of the circuit shown above but it is not the problem. Maybe there is something wrong with your AC adapter. \$\endgroup\$ Commented Mar 7, 2016 at 11:01
  • \$\begingroup\$ maybee the focus needs to be on the ac side, i will check again this issue. \$\endgroup\$
    – barg
    Commented Mar 7, 2016 at 12:28
  • \$\begingroup\$ As for the gate/mcu pin resistor of 1k you noted, is it not too high resistance that will block big part of the current which designated to the mosf gate? \$\endgroup\$
    – barg
    Commented Mar 7, 2016 at 12:28
  • \$\begingroup\$ No 1k will be fine, a MOSFET gate has a very high input impedance, it behaves like a small capacitor. Only if you need to switch the gate on/off very fast (faster than a couple of MHz) a 1 k resistor be absolutely fine. MOSFETs are different from Bipolar transistors in this respect, they do not need any input current ! \$\endgroup\$ Commented Mar 7, 2016 at 12:34

Capacitors will not protect you from overvoltage except in very specific circumstances where the surge energy is finite, e.g. discharge of another capacitor. That would not seem to be the situation here.

What overvoltage are you concerned about? Are you worried about VIN going too high? About the loads being non-resistive and causing inductive kickback when turning off? If so, a reverse-biased diode to catch the inductor's current would be the classic solution:


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Thank you william, well noted for the incorrect use of capacitors in this case as i am just learning. as i noted to FakeMoustache, my concern is for the circuit components will not get over voltage spikes when turning on or turning off, i am not using inductor in the circuit so a parallel diode is still relevant? \$\endgroup\$
    – barg
    Commented Mar 7, 2016 at 10:50
  • \$\begingroup\$ Not so much, the MOSFETs also have a reverse protection diode already so an extra diode is only needed if your load is very inductive, for example: a motor, a selenoid or a relay. For example for a lightbulb or a heating element: no extra diodes needed. \$\endgroup\$ Commented Mar 7, 2016 at 11:04
  • \$\begingroup\$ If your load is purely resistive, then no diode is required. However, common things like relay coils are very inductive, and something as simple as a long cable can have a lot of parasitic inductance. There might be more inductance than you expect, depending on the applications. \$\endgroup\$ Commented Mar 8, 2016 at 11:31
  • \$\begingroup\$ secondly, the body diodes in a FET will not provide protection in this situation. An inductive load causes the drain voltage on the FET to go extremely high, not low/negative. The body diode will not be forward biased but will likely enter avalanche breakdown mode, which, unless you are super-careful with your FET selection vs the quantity of energy stored in the load, WILL destroy the FET. \$\endgroup\$ Commented Mar 8, 2016 at 11:33

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