I have put together this circuit from parts I had lying around, It is to invert a normally closed momentary switch to behave like a normally open. The switch is for a car and toggles a tail light.

Even though the operation is correct the N-Channel is getting very hot, (too hot to touch) worst when the switch is open.

Can you help with this ? Thank you


simulate this circuit – Schematic created using CircuitLab

(R1 is to act as pulldown)

  • \$\begingroup\$ Why even have the NMOSFET there? \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 21 '15 at 3:24
  • \$\begingroup\$ I read and tested that by having to it reduces current consumption greatly. \$\endgroup\$ – Hayden Thring Mar 21 '15 at 3:26
  • \$\begingroup\$ are you sure you've connected your P-FET correctly? \$\endgroup\$ – brhans Mar 21 '15 at 3:36
  • \$\begingroup\$ @brhans in that schematic or in real life ? \$\endgroup\$ – Hayden Thring Mar 21 '15 at 3:37
  • \$\begingroup\$ in real life. the schematic looks correct, but I also wonder why the N-FET is there. \$\endgroup\$ – brhans Mar 21 '15 at 3:39

Remove the N-channel, you have no use for it. They're describing digital logic circuits where you are driving cascades of logic elements and the input impedance/resistance of the next stage is very high. In your case, you have a relatively low impedance of the next stage (tail light) probably being in the 100s of ohms tops. So your output will drop to zero automatically and rapidly without the NMOS at all.

Another potential issue with your circuit is the 3 Mohm. This is going to cause a very slow drop in the gate voltage causing very slow turn-on time for the PMOS. With the redundant NMOS also turning off really slow at the same time, you have a good chance that you've burned both your PMOS and NMOS and that's why you're seeing the 120 ohms across your pulldown rather than the 3 Mohm. Your NMOS is partially shorted now so you definitely want to chuck that part.

Your circuit as it stands would probably work if you had a much lower (say 1-10k pulldown) which would prevent the PMOS and NMOS from turning on too long at the same time. You allowed them to turn on "forever" with the 3 Mohm which causes a direct short from the top of the PMOS to the bottom of the NMOS as they transition from one state to the next.

1. Remove NMOS from circuit.
2. Decrease your pull down resistance to 1-10k ohm.

  • \$\begingroup\$ ok then, see here at this image, they use a resistor between ground and F, do i need that ? \$\endgroup\$ – Hayden Thring Mar 21 '15 at 6:58
  • \$\begingroup\$ en.wikipedia.org/wiki/File:PMOS_NOT.png \$\endgroup\$ – Hayden Thring Mar 21 '15 at 7:13
  • \$\begingroup\$ @HaydenThring Nope, not as a separate resistor. Replace that resistor with the load (tail light). \$\endgroup\$ – horta Mar 21 '15 at 14:34
  • \$\begingroup\$ great, simple. thanks. I think it was that resistor that i thought i needed which was giving me trouble when i first made that circuit which led my to try the dual mos version. \$\endgroup\$ – Hayden Thring Mar 21 '15 at 14:43
  • \$\begingroup\$ Just about that wiki schematic, what is "F" then ? \$\endgroup\$ – Hayden Thring Mar 21 '15 at 22:49

I agrre with horta.. Just discard NMOS ...Find the wattage of the tail lamp bulb assembly.. if it is filament it would be around around 15 to 25 watts and normal current would be around 1 to 2 Amp. The on state resistance of PMOS would be 0.1 to 0.2 ohms device dessipation will be around 0.5 watts. If the tail lamp wattage is still higher you may have to fix the the device on a hetsink. pl. let me know.Connect 22K(0.5 watt carbon ) resitance between gate of the PMOS device and negative supply and 1 K res. between gate and positive. Best wishes. V T Ingole

  • \$\begingroup\$ it drives 2x 21w filament globes, so 42w in total \$\endgroup\$ – Hayden Thring Mar 21 '15 at 7:04
  • \$\begingroup\$ Vijay, welcome to SE.EE. Your answer and knowledge are very appreciated and I'm sure it will be useful to the OP. The Stackexchange community prides itself in a high level of presentation quality in everything that is posted here. Please take the time to use spell check, proper grammar, and avoid unnecessary abbreviations. \$\endgroup\$ – Dan Laks Mar 21 '15 at 8:15
  • \$\begingroup\$ @vijay ingole , are you able to respond. \$\endgroup\$ – Hayden Thring Mar 21 '15 at 22:50

The other answers have the correct idea for what the problem is, however there is an alternative solution where you remove the PMOSFET and keep the NMOSFET. This assumes you can use a low-side switch.


simulate this circuit – Schematic created using CircuitLab

This has a slight advantage that NMOSFET's typically have lower on resistance. However, this is difficult if the the tail light is grounded to the chassis.

  • \$\begingroup\$ yes it would be common ground chassis, battery and globe \$\endgroup\$ – Hayden Thring Mar 21 '15 at 9:21
  • \$\begingroup\$ Then this solution will not work for you. However, I will keep this answer here for others who do not find themselves in this situation. \$\endgroup\$ – helloworld922 Mar 21 '15 at 9:25

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