Re: switching mosfet fully

kell wrote:
Dimmer circuit for dc?


12v
|
|
,---------------------+------------+-------+
| | | |
| 10K 1K Rload
10K | | |
| 0.1 | | |
| ,-----||-------|---10K------|-------+
| | | | |
/ | ,--1M------+ | |-
\ | | | ,--+----||
/ | | |\ | | |-
\<-----|---+--|+\ | /c |
1K/ | | >----+-------| npn |
\ +---+--|-/ \e |
/ | | |/LM2903 | |
| | | | |
| - 100K | |
| ^ | | |
| | | | |
| | | | |
| +---+--------------------|----------+
| | |
| | Rs
| | |
'-------------------------------+----------'
|
|
gnd


It does okay running a very light load (like a car taillight bulb), but
even driving a load of an amp or two, the mosfet gets warm. It
oscillates too fast, so the mosfet never turns full on or off. At
least I think so lol.

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.

I actually want to use this circuit topology to run a 25 amp load, so I
have somehow to get the circuit to do hard switching (also I have to
get rid of the transistor/pullup resistor combo and replace it with a
proper inverting fet driver).
I don't have any drivers on hand but I do have a pile of 555's. I can
make a 555 into an inverting driver, by tying pin 6 high and using pin
2 as the input for example. I only need very low frequency switching
so a 555 should do it.
Still figuring out how to make the circuit do hard switching. I
thought instituting a delay in the signal between the comparator and
the fet driver, or within the driver itself might accomplish it. This
dimmer should switch pretty slow, between 10 Hz and 1 kHz. I'd
appreciate any ideas how to slow it down.

.

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