Cap value for timing circuit

[DaveC]

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>

Thanks,
Dave

 

[Jon Slaughter]

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).

 

[Jon Slaughter]

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.

BTW, if ac is on the cap and not rectified you'll either want to add
rectification or go with a triac/scr version instead. Mosfets only work well
one way. I'm assuming the relay is DC since you mentioned an electrolytic
cap.

 

[DaveC]

What was the existing cap?

It's a Mallory FP119A.

Any clear indications as to why or how it failed?

No. Guesses: Old age? Heat?

 

[DaveC]

A motor start cap should be ok. They're meant for intermittent use under

heavy load so they don't accept heavy ripple currents for lomg, but this one
won't have to, with a few tens of kilohms between it and the mains.

A single rectifier in series with 50 ohms feeds the charge current to the
cap. Seems that 150v is a bit overkill, no? (The 25K pot is on the discharge
side of the circuit which is switched in at the appropriate time.)

I think I'm going to go with the 270mfd / 125v start cap. It's sourceable
locally, fits the space, and although a bit low on the capacitance it will
fit the bill better than what's there.

 

[DaveC]

peak = RMS * sqrt(2) = 110 * 1.414 ~ 155V

JF

This is for half-wave rect.?

Thanks.

 

[Mike Paff]

This is for half-wave rect.?

Thanks.

Half-wave and full-wave rectification result in the same peak voltage.
It's just that for full-wave, the peaks occur twice as often.

 

[DaveC]

Sorry for the confusion, and you're right, an unloaded cap will charge

to 1.414 times the RMS value of the unrectified sine wave, no matter
whether the output of the rectifier is full-wave or half-wave.

No worries, mate! I'm learned(er).

Chances are that a loaded one will also, since most power supplies are
designed to have the cap charge fully and then discharge, between peaks,
to yield an acceptable ripple.

Wow. That's probably why the original 150v cap is no good after 40 years. It
should have been spec'd higher. Is 160 good enough for this application?

Thanks.

 

[whisky-dave]

I agree. Was just saying that length of time is like the length of a piece
of
string. Like someone else here said, some electrolytic caps work fine in
gear
tens of years old. Valve/tube gear even, where you have enough heat to
accerate drying. It sort of suggests that looking at lengths of time is
missing plenty, there's a lot of difference between electrolytics, they're
not at all consistent.

And there's how hard the user pushes them as regards the voltage and
temperature
ratings.