MOSFET drain-source voltage drop

It's connected wrong.

Connect the source to ground, drain to +12 thru 1k.

Ed

do you drive it low actively? That will be a problem if they have a decoupling cap internally.

Oh sorry I had it hooked up a bit different, Gate to +12, Source to ground, 50 ohms from +12 to Drain. You want to keep the gate voltage high at all times.

George H.

Oh ... ! It never occurred to me that it would make a difference. Thank goodness this is s.e.BASICS or I would be laughed off the group.

Thank you, Bob

Ed just pointed out that I had the load wrong.

If I had posted a circuit, I'm sure that it would have been obvious to everyone. My worded description could be read the way people expected it to be.

Thanks, Bob

Well, that's better! Vds is immeasurable with my Fluke 8000A DMM on 200 mv range. It's only a 12ma load, but clearly things are OK.

Thanks again, Bob

I knew it! The "duh thing" was that I was high side switching. I just didn't know any better.

Why does it make a difference?

Bob

The thing that turns a N channel MOSFET on is the voltage difference between source and gate.

When you're switching the high side, the source voltage goes up when you turn it on, (so you need a higher voltage than the supply to turn it all the way on)

when you're switching low side the source stays at 0V so it's easy to create the voltage difference needed to turn it on.

--
  \_(?)_

Because a MOSFET doesn't turn on fully until you have a hefty voltage from gate to source -- this is why at least two of us were asking about the voltage supply used for the gate driver (without thinking that you were using the thing high side).

When you hold the gate at the drain voltage, and pull current from the source, the source voltage has to fall enough to develop the necessary voltage from gate to source for the FET to conduct.

There are high-side driver chips out there (that you can drive from a

0-5V PWM, even), but if you can rewire it so that the MOSFET is working on the low side that'll be easier.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

I don't see where anyone answered your question.

Look at the potential difference between your G and S when low side switching vs. high side switching.

Mikek