> > However you may also find that a better way to deal with the same
> > problem of reducing the coil current is to have a resistor from the
> > supply to the relay coil, and a capacitor (say 1uF) from the
> > resistor/coil junction to ground. Then you do not use PWM to reduce
> > the relay current, but size the resistor to an appropriate value.
> > When the relay is off, the capacitor is charged to full rail
> > voltage, and supplies the initial peak pull in current to the relay
> > when the transistor turns on. As the relay draws current from the
> > capacitor, and the voltage drops, then the current will decrease to
> > a value set by the resistor. You will still need a diode across the
> > relay to clip the back emf on turn off though.
>
> 1uF?? I've been using this trick for about 20 years,
> in most cases 470uF is a minimum and 1000 or 2200
> much better.

Well thanks guys but it's all academic really. I just wanted a better
understanding of what was happening during diode turn off. In
practice I would now know how to do proper PWM control of relay if
required, with a suitably fast diode (probably 1N4148 for small
relays). The RC idea is good for controlling on/hold current, PWM
with voltage or current feedback even better if the supply was not a
fixed value. Even then a small RC circuit to isolate current pulses
from the main supply might be a good idea.

--
Brent Brown, Electronic Design Solutions
16 English Street, Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/txt: 025 334 069
eMail:  brent.brown@clear.net.nz

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