Capacitor substitution values?

[ChadTower]

I've looked high and low for this info over the weekend... no luck...

...when working on old boards, and you have to substitute modern values for some old dead caps... how far is acceptable in either direction for capacitance?  I know voltage can go up just fine but never down... but the capacitance rating?  Say you have to replace a 500mfd cap but you only have 470mfd caps on hand... or would you be better off swinging up?  If so, how far up?

[Level42]

The problem is that there is no universal answer to this question. I guess it depends on what circuit the caps are used.
And are we talking electrolytic caps only or also regular caps ?

If I had to  choose, I'd stay as close as possible, but choose the one with the higher value instead of the lower (Farrads).

Voltage can be anything as long as it's equal or higher.

[SirPeale]

Stay as close as possible.  If we're talking electrolytic caps here, 500 to 470 isn't that bad.  Remember that values drop anyway as they get older.  In order to be really bad, they'd drop way down before they're bad.

I'd meter the cap anyway to see what kind of shape it's in.  Even though it's older, they made them better those days.  It may be just fine.

[ChadTower]

I'd meter the cap anyway to see what kind of shape it's in.  Even though it's older, they made them better those days.  It may be just fine.

Don't have an ESR meter... everything I read everywhere says the capacitor checker on a DMM isn't all that useful in proving anything.

[sarge]

Usually + or - 20% of the rated capacitance is considered acceptable when dealing with electrolyitics.  If you were too concerned with matching the original you could always connect a 470 and a 22 or 47 uf capacitor in parallel and that will bring the total capacitance up.  Electrolytics rarely fail abrubtly but will instead gradually die out due to capacitor drying up.

[Fozzy The Bear]

All of this really depends on the circuit it's in. If it's a timing critical circuit (as in some parts of a monitor), then you must use the exact value or you'll get some odd results.

If you can't get an exact value like 500mfd  you can make up the value by putting 2 X 250mfd caps in parallel. Or putting a 470mfd and a 30mfd in parallel to make 500mfd. Exact value is always best! but watch the tolerances as well they can be anything from 5% to 30% try to match the original tolerance range or be inside of it.

Best Regards,
Julian (Fozzy The Bear)

[ChadTower]

Cool.  FWIW, I spent half the weekend emailing back and forth with Bob Roberts coming up with a replacement for an OOP multican capacitor... his advice was that going up a bit is acceptable but never go down.

I need to get an ESR meter... the least expensive new one I've seen so far is $200.   

[Fozzy The Bear]

I need to get an ESR meter... the least expensive new one I've seen so far is $200.   

Here's a trick that works if you don't have an ESR Meter......

For checking an electrolytic capacitor, you can get a good idea of its condition using a capacitance meter on a digital multimeter and a can of freezer spray.

Connect the cap under test to the meter. If the capacitance reading is well below the nominal value then it's faulty. If, however, the reading seems to be right, spray the capacitor and observe the reading.

With a faulty electro cap, you will see the reading decrease rapidly. Continue to apply freezer until it is approximately 75% of what it should be, if it goes that low it is definitely faulty.

Works every time for me when I don't have the ESR Meter to hand.

Best Regards,
Julian (Fozzy The Bear)