Re: emitter followers / Darlington - need help

mrdarrett@xxxxxxxxx wrote:
John Popelish wrote:
mrdarrett@xxxxxxxxx wrote:
(snip)
Basically just trying to learn. I'm using a lamp for now... once it
works properly, I'll use my motor, with capacitors to smooth out the
power.
Not a good idea to smooth the output from a mosfet switch
with a capacitor. The capacitor charges up to almost the
same voltage, regardless of the pulse width. Better to put
a diode across the motor, and let the motor's inductance
average the pulses. This may require a higher pulse
frequency to get a smooth rotation. If you want to try a
capacitor across the motor, you will need some additional
inductor between the mosfet and the parallel motor and
capacitor, with the catch diode moved to the drain, instead
of being across the motor.


This is actually what I'd had in mind:

http://www.cpemma.co.uk/555pwm.html

(with the parallel port and my DOS software taking the part of the 555
and variable resistor.)

Yes, I was planning on sticking a diode across the motor.

Are 100nF and 100uF caps between +12VDC and ground still a bad idea...?
Not at all. They should connect as closely as possible between the motor diode node and the source of the mosfet (or emitter if you use a junction transistor). This provides a very close source of charge that keeps the pulses out of the supply lines.
.

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