Using an N channel MOSFET to interrupt Ground

Thread Starter

waldobenton

Joined May 19, 2023
2
I currently use a SPST switch to interrupt ground to the headlight on my motorcycle. I want to replace that with a switch that includes a LED to indicate it is on. The switch I have obtained is intended to switch the high side, but I want to switch the low side, as the switch I am using right now does.

I could use a NC relay, but I'd rather save space and power and go with an N channel MOSFET. How can I achieve the desired Normally Closed operation? I understand that Normally Open operation would be fairly simple to achieve, but I'd like the MOSFET to be powered only when the headlight is off. (Most of the time, the headlight will be lit. Ordinarily I only turn it off for starting the bike.)

Or should I just try to locate a small relay? Maybe I'm way off-base and I actually need a P channel device?

The current in this circuit will be less than 10 amps at 12vdc.

Thanks in advance for any assistance!

-Waldo
 

sghioto

Joined Dec 31, 2017
5,379
From what I'm reading the switch he wants to use with the built-in LED is high side only so that should activate a N mosfet directly grounding the headlight.
When the LED in the switch is on the mosfet should be activated indicating the headlight is on. Don't see anything else required other than a 10K resistor from the gate of the mosfet to ground.
Two scenarios?
Headlight ON when LED is lit:
1684527142362.png


Headlight OFF when LED is lit.
1684527397297.png
 
Last edited:

crutschow

Joined Mar 14, 2008
34,282
Is this what you want (sim below)?

When the S1 switch is closed (green trace high) then the headlight bulb is turned off (bulb current-yellow trace)

1684528684427.png
 

Thread Starter

waldobenton

Joined May 19, 2023
2
Powering the mosfet when the headlight is off? That doesn't make sense , what's the reason for that ?
From what I'm reading the switch he wants to use with the built-in LED is high side only so that should activate a N mosfet directly grounding the headlight.
When the LED in the switch is on the mosfet should be activated indicating the headlight is on. Don't see anything else required other than a 10K resistor from the gate of the mosfet to ground.
Two scenarios?
Headlight ON when LED is lit:
View attachment 294539


Headlight OFF when LED is lit.
View attachment 294540
Thank you very much! I tried to explain clearly, but apparently I confused at least a couple of people here. You got my meaning precisely, and your second circuit will do what I want to accomplish. One of my main desires was that the MOSFET not be turned on as it's default state. As I said, I knew how to do this with a NC relay, but preferred a solid state solution. That being said, the additional parts count and circuit complexity is making me reconsider the relay solution. I was hoping to do everything with something that looked more like your first drawing, LOL.

Thank you again for the effort you put into not just answering my question, but also for making it perfectly clear with your diagrams. I really appreciate the help. -Waldo
 
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