.25 mfd capacitor

rj1939 · Aug 19, 2021

Having trouble getting a .25 mfd dc capacitor to replicate a Cincinnati schematic...................it goes across the dc leads coming off the rectifier bridge, to convert 120 ac into dc.
Any suggestions

Just a Sparky · Aug 19, 2021

More information. Voltage ratings, type, continuous current/power rating, original manufacturer...

if 25,000uF is not available, why not 30,000 or 40,000, etc.?

DigiKey

mksj · Aug 20, 2021

In current terms 0.25 mFd = 0.25 uF, in year past it was milliFarad which is an order of magnitude greater, I am assuming you are looking for a standard film or an RC snubber which are more common. Maybe a copy of the schematic and some more information as to a power supply for what?
RC Snubber
https://www.mouser.com/ProductDetai...EpiMZZMsh%2B1woXyUXj9yQU%2Bz0G7ZzAsfvWMH8VFY=
https://www.mouser.com/ProductDetail/Cornell-Dubilier-CDE/254M06QD100?qs=kwWuCeDmeqEwNMM%2BE2N9GQ==

Film capacitor
https://www.mouser.com/ProductDetail/Cornell-Dubilier-CDE/418P25496MB3?qs=M5E3J2MuaV4mvv9V91NTMQ==

jim rozen · Aug 20, 2021

mfd = microfarad = 1 e -6 farad

mmfd = picofarad = 1 e -12 farad

kc = kHz = 1 e3 sec*-1

mc = MHz = 1 e6 sec*-1

Old school stuff.

Tony Quiring · Aug 20, 2021

What you describe is just basic power supply.

Value seems to small for ripple filtering but may be for noise filtering.

Either way, what matters more is voltage rating, can be higher but not less.

Actual value can be anything close.

For ripple filter bigger usually better upto a point as too large may cause current limit issue on rectifier.

Sent from my SM-G781V using Tapatalk

TDegenhart · Aug 20, 2021

That's just a switching spike (noise) filter. Often used with a series resistor to limit peak filter current.

Tom

Just a Sparky · Aug 20, 2021

0.25mFd. Not 25mFd. Whoops.

0.25 millifarad = 250 microfarad. :dunce:

Then again, what 'brilliant' engineer came up with the idea of expressing 250uF as .25mFd? That expression defies convention.

Frankly using 'mFd' at all is and was a terrible idea from the very start of the electronic age. Just do as every sane person & manufacturer does and skip over the milifarad range of notation entirely unless spelling it out completely. pF, uF, F. That's all you'll ever need.

250uF across + and - of a bridge rectifier is a basic electrolytic filter cap. Available anywhere.

SomeoneSomewhere · Aug 20, 2021

If capitalised as MFD, it almost always refers to microfarads. I think this may date from periods when all text was capitals and things were not very standard, plus capacitors in the milifarads were simply impractical.

The 'modern' symbol for a Farad is simply 'F' - no d.

rj1939 · Aug 21, 2021

Am trying to upload a diagram, in the file it says 10k, when I try to upload it, it says it is 9.9M........................

rj1939 · Aug 22, 2021

Here it is

JST · Aug 22, 2021

Looks like they are being used for suppression of spikes from the magnetic coils. Yeah, 600V seems fine, won't likely get an overvoltage with that if they have selected the capacitance correctly.

rj1939 · Aug 23, 2021

OK...........how do you read the capacity in modern terminology?

Why couldn't I go oversize on capacitor...........if it is more expedient from a cost/supply standpoint?

jim rozen · Aug 23, 2021

thermite said:
Snubbers. The clutch & brake generate a sharp back-spike similar to breaker-points to an ignition coil for firing a sparking plug when the power to their electromagnetic actuating coils is de-energized.

These function to pass the fastest-rising portion of that spike to protect the control contacts and extend their life. Radiated electrical "noise" of said spike is also reduced, so it is less of a rude neighbour to any nearby sensitive electronics goods.

Especially so as they're dc-operated. Also note the 600 volt rating.

In modern times this would be a snubber diode and a MOV varistor.

rj1939 · Aug 23, 2021

thermite said:
Snubbers. The clutch & brake generate a sharp back-spike similar to breaker-points to an ignition coil for firing a sparking plug when the power to their electromagnetic actuating coils is de-energized.

These function to pass the fastest-rising portion of that spike to protect the control contacts and extend their life. Radiated electrical "noise" of said spike is also reduced, so it is less of a rude neighbour to any nearby sensitive electronics goods.

Seems like an old fashioned automotive condenser would be a workable substitute

Just a Sparky · Aug 23, 2021

As shown, your diagram depicts a 0.25uF snubber capacitor.

A larger substitution will work just fine as long as it's intended for that use. (Snubbers are designed to have low inductance to perform better at their unique job - going from zero to 'a bunch' of amps in a hurry.)

jim rozen · Aug 23, 2021

Catching diodes are pretty common in this application - sized (prv and current rating) correctly they do a good job of taking the ringdown away fast.

JST · Aug 23, 2021

Capacitors are very reliable, and are effective. Diodes are tough, but not as reliable. And they have some other issues as well

The size is more critical than one might think.... you do not want to end up with an RCL circuit that "rings" and keeps sparking on the contacts. Ideally you get "one and done" on transients, with the "one" blunted severely by the capacitor.

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