switching mosfet fully

I think you want to make the positive feedback AC only (capacitor in series with a resistor) and low pass the negative feedback (capacitor from - input to ground). The positive feedback can be from the comparator output, but the negative feedback should be from the load, to measure the average of the pulses. I would also drive the variation with a series resistor to the - input, and bias the positive input with a voltage divider at 50% across the supply.

kell wrote: (snip)

Have you installed LTspice simulator on your computer? I threw together a slight variation of your circuit that does pretty close to what you ask for, except that it has an inverted control voltage (12 volts in gives zero volts out and zero volts in gives 12 volts out). This keeps the overshoot at the + input from the positive feedback between the rails. Here is the text of the ASC schematic file for LTspice (you can copy and past into notepad and save as DCdimmer.asc and run the simulation with LTspice).

Version 4 SHEET 1 880 680 WIRE -16 -80 -128 -80 WIRE 304 -80 64 -80 WIRE -64 16 -80 16 WIRE 32 16 16 16 WIRE 304 48 304 -80 WIRE 336 48 336 16 WIRE 336 48 304 48 WIRE 336 80 336 48 WIRE 288 160 224 160 WIRE 224 176 224 160 WIRE -240 240 -304 240 WIRE -128 240 -128 -80 WIRE -128 240 -160 240 WIRE -16 240 -128 240 WIRE 96 256 96 16 WIRE 96 256 48 256 WIRE 128 256 96 256 WIRE 176 256 128 256 WIRE -80 272 -80 16 WIRE -48 272 -48 176 WIRE -48 272 -80 272 WIRE -16 272 -48 272 WIRE -48 304 -48 272 WIRE -128 320 -128 240 FLAG 432 288 0 FLAG 432 208 p12 FLAG 336 176 0 FLAG 224 272 0 FLAG 16 288 0 FLAG -48 384 0 FLAG 336 -64 p12 FLAG -48 96 p12 FLAG 16 224 p12 FLAG 128 176 p12 FLAG 224 80 p12 FLAG -304 320 0 FLAG -128 384 0 FLAG 336 48 Out SYMBOL voltage 432 192 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 12 SYMBOL Comparators\\\\LT1017 16 256 R0 SYMATTR InstName U1 SYMBOL nmos 176 176 R0 SYMATTR InstName M1 SYMATTR Value BSS145 SYMBOL nmos 288 80 R0 SYMATTR InstName M2 SYMATTR Value FDS6680A SYMBOL res 208 64 R0 SYMATTR InstName R1 SYMATTR Value 1k SYMBOL res 320 -80 R0 SYMATTR InstName R2 SYMATTR Value 1 SYMBOL res -64 288 R0 SYMATTR InstName R3 SYMATTR Value 100k SYMBOL res -64 80 R0 SYMATTR InstName R4 SYMATTR Value 100k SYMBOL res 112 160 R0 SYMATTR InstName R5 SYMATTR Value 10k SYMBOL cap -144 320 R0 SYMATTR InstName C1 SYMATTR Value 220n SYMBOL cap 96 0 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C3 SYMATTR Value 10n SYMBOL res 32 0 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R6 SYMATTR Value 100k SYMBOL res 80 -96 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R7 SYMATTR Value 100k SYMBOL res -144 224 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R8 SYMATTR Value 100k SYMBOL voltage -304 224 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value PULSE(0 12 0 1) TEXT 184 360 Left 0 !.tran 1

Oops. Zero volts in gives 12 volts out, which, for a 12 volt connected load is zero power. The neat thing about averaging the voltage of the pulses and comparing that to the DC control input, you get a linear power control (50% input voltage gives 50% power for a resistive load. Thats the average of 100% of the power, half the time.) I just built something very close to this as the output stage of a precision temperature controller for a very thermally sensitive magnetic field sensor. Mine is a little smaller... a 1 watt heater.

I have been procrastinating about it, but as soon as I can I'll get LTspice and load the simulation you wrote, then I'll get back to you. Thanks, Kell

I did run that sim you made. Looks good. But I came up with a circuit that I like even more... I think it's quite elegant.

With R2 100k it runs at something on the order of 100 Hz. But it will run much, much faster. Even with my quickie breadboard prototype and alligator clip jumpers, it will go up around 100 kHz by adjusting R2. The mosfet keeps cool as a cucumber. I bunged in the 555 as an inverting buffer/fet driver. I didn't want to use a pullup resistor; I'm planning on using this circuit to drive a very heavy load, about 25 amps. I feel pretty confident it's up to the job. The variation in the brightness of the bulb seems to vary in a fairly linear relationship with the pot position, just from the looks of it. I haven't done a simulation...

Cool beans!