mosfetnot-gate
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)
Try
simulate this circuit – Schematic created using CircuitLab
The switches don't carry any signifigant current. Closing either of them turns off the output, at the cost of 50uA current from the source (trivial unless you're trying to run off a small battery for a long time).
I've not done the work of picking a suitable P-channel MOSFET. You should choose one with a low Rdson.
A floating gate has a voltage. Any wire has a voltage. Until you put it in reference to something else, that voltage can be anything. Don't expect it to be 0V or any other known voltage.
That gate also has a capacitance. Your body is a capacitance. Every time you touch that gate, you're connecting two capacitors and a charge balance will occur. This will alter the voltage on the gate. Ergo, you've created a simple transistor capacitor memory scheme not terribly different than DRAM. Pretty nifty huh?
I'm surprised a little that the brightness remains the same between power cycles, but maybe that's attributable to having a high quality mosfet with a high gate impedance?
Lastly, although this is a great lesson, don't design a circuit like this with a floating node expecting it to be a specific voltage.
Best Answer
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.
Summary:
1. Remove NMOS from circuit.
2. Decrease your pull down resistance to 1-10k ohm.