"DaveC" wrote in message
...
Relay-logic controlled printing machine, 110 vac.

A 300mfd (150v) electrolytic cap is used together with 25K (25 W) ohm
wirewound pot and a 5K ohm relay coil to control a variable time delay
before
the relay opens.

The cap is shot (measures less than 1mfd). I'm having a hard time finding
a
replacement locally. (Yeah, I know: Digi-Key, et al) But I need to get
this
up and running today.

What other common types can I use in place of this can-type cap? (It has 2
wings that rivet to the sides of the opening in the chassis and solder
terms
make the connection below the chassis.) Maybe poly-somethings, two in
series
or parallel?

It's a Mallory FP119A.

Can I use a non-polar electro? (The wiring diagram shows a polarity
indicator.)

Or a motor-start cap? ::

http://tinyurl.com/y9d3ks6


You can potentially vary the values of the resistor and capacitor greatly
depending on how much drive is needed. As long as the time constant R*C is
the approximately the same it should approximately work. Obviously if it's
driving a relay then it needs enough current to turn it on. So your probably
not going to be able to vary R much unless it wasn't chosen well in the
first place.

The electrolytic caps were used because of the high voltage and high
capacitance. I doubt you can find anything but an electrolytic to replace it
with. Remember you can put them in parallel to increase the capacitance.
Just get you two, three, or how ever many you need to get approximately
300uF at 150V. I woudln't get any lower voltage and try to use them in
series because it is usually more trouble than it's worth.

Note, you can replace this simple delay circuit a mosfet who's gate is
charged up by a lot smaller capacitor and larger resistor.

Basically the gate is turned on in a similar time frame as the original but
requires virtually no current to do so. The mosfet then turns on which turns
on the relay.

All you need is a resistor(actually two), a cap, a suitable mosfet that can
handle the current and voltage(relatively easy to find for a few dollars),
and a diode for a snubber.

Since your input is 150V or so you'll have to reduce it for the mosfet's
gate voltage. I'll want to use a P ch but could probably put an Nch on the
low side of the coil.

R1
|
+-----+---mosfet Gate
| |
| C
R2 gnd
|
gnd

(use a fixed width font to view)

The R1 and R2 form a voltage divider. You'll want something about 12 to 1 so
that the mosfets gate only sees at most about 15V. A large voltage spike
could ruin the mosfet gate so technically it would be best to proect it. You
can do this by adding a 15-20V zener from the gate to ground.

Add the diode across the mosfet drain to source(look up diode snubber). Most
power mosfets actually have a diode built in to do this though.

In any case if you want to go this route then I'm sure someone can whip you
up a circuit. It's relatively simple and should do the trick better than the
large cap(And probably cheaper). Theres just a few small issues such as
getting hte polarity right and getting the voltage divider right(pch on the
high side would reverse the resistors and the cap would be to vcc rather
than to ground).