Switch

If you could find a mosfet with a gate threshold of 13v, it wouldn't be fully off at 13v and it wouldn't be fully on at 14v. A Zener in series with the base of a bipolar transistor or even a Darlington might be better.

It's a strange request.. what's the controlling device that uses such a threshold?

It's easier to help answer better, if you provide more information.

If your specifications are critical you might find that a small microprocessor is needed and use that to control a MOSFET or a comparator chip like an LM311 with a 1 volt hysteresis that again controls a MOSFET to turn the load on and off.

John

Now I think about it (since we don't have much to go on) from the username sailorman2 maybe it is boat related : )

Maybe it's related to automatically cutting off lights if a generator is switched off or something.. but I know nothing about sailing : ( so maybe it's a completely different use-case..

I noticed that this question was asked 11 months ago. It is a great question and I wonder why it suddenly popped up on the forum now. My apologies to Hardin for how long he has had to wait for someone to notice the question. It seemed like a reasonable challenge for me and a great project for an afternoon. I am not sure I would have done the work if I had noticed the date before hand though. Anyway, here is a schematic of what I came up with:

Some of the things that I learned along the way were that the power for the 1 amp load was best supplied from a different source than the line that is being sensed by the LM311 comparator. If the supply rail was very solid this may not be needed but in my case on the bread board the 1 Amp load pulled the rail low enough to shut off the MOSFET. In my schematic the calibration potentiometer allows the start and stop voltages to be adjusted up or down. The difference between the start and stop voltages are the hysteresis of the circuit and this is controlled by the value of R6. If I were to build this circuit on a board I would make R6 a trimmer as it was a challenge to get just the right resistance to produce a hysteresis of 1 Volt. Here is a video of the Circuit in action:

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Once again my apology to Hardin if he is still around and also a thank you for the pretext to spend the afternoon playing in my electronic sand box.

John

Hi John,

That works really well!