switching mosfet fully

[kell]

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 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.

 

[John Popelish]

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.

 

[John Popelish]

kell wrote:
(snip)

This
dimmer should switch pretty slow, between 10 Hz and 1 kHz. I'd
appreciate any ideas how to slow it down.

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

 

[John Popelish]

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).

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.

 

[kell]

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

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

 

[kell]

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 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.



12v
|
,-------------+----------------------------+
| | |
| | Rload
| | (taillight
| R1 bulb)
| 10k ,-------, |
/ | | | |
\ |\ | | _______ +---,
/<-----|+\ | | |8 7 6 5| | |
\ | >---+----+ | |NE555 | |
/ ,---|-/ | |1 2 3 4| | R2
| | |/ | |_______| | 100k
| | LM2903 | | | |-' |
| | '-----' '----Rg------|| |
| | 10 |-, |
| | RFP30N06LE| |
| | | |
| '---------------------------------------|---+
| | |
| | = 100n
| | |
| | |
'------------------------------------------+---'
|
gnd

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...

 

[John Popelish]

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.



12v
|
,-------------+----------------------------+
| | |
| | Rload
| | (taillight
| R1 bulb)
| 10k ,-------, |
/ | | | |
\ |\ | | _______ +---,
/<-----|+\ | | |8 7 6 5| | |
\ | >---+----+ | |NE555 | |
/ ,---|-/ | |1 2 3 4| | R2
| | |/ | |_______| | 100k
| | LM2903 | | | |-' |
| | '-----' '----Rg------|| |
| | 10 |-, |
| | RFP30N06LE| |
| | | |
| '---------------------------------------|---+
| | |
| | = 100n
| | |
| | |
'------------------------------------------+---'
|
gnd

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!