I have a 680uF capacitor that reads 640uF on my meter is this a bad capacitor, or does it have a range that is considered functional? I pulled it from a LCD monitor.

 

Hello,

Electrolitic capacitors can have tollerances upto ± 20 %.
A capacitor of 680 μF that read 640 μF can be fine.

Bertus

 

Usually caps that measure a drop in capacitance also simultaneously measure an increase in ESR. Depending on the application and brand/series of capacitor, it may be on the way to the big e-cycling center in the sky.

 

I'm with bertus. The tolerance on aluminium electrolytic capacitors can be as much as ± 20% so your 680uF capacitor is probably OK.

 

I tend to agree with Evil Lurker, and suggest to OP to use an ESR meter to ascertain the CAP. has not exceeded its ESR value.

 

640 on 680 is not quite 6% low, or "within expected tolerance."

 

An electolytic can be +/- 20% from the day it is manufactured. ESR is not an issue unless the capacitance falls since manufacturing date.

Without knowing the initial capacitance, one cannot assume the ESR is rising over time. Try it in your application and see if it works.

If it doesn't, then try a different one.

 

Most larger modern electros read low as the "bean counters" in the capacitor manufacturing factory can save 5% of material costs by making caps 5% low, and still "in spec".

On larger caps the savings to them are much larger, which is why you'll measure 1uF caps from 0.9 to 1.1uF, but 680uF caps are usually a few percent low and you won't see any high value ones.

 

Ha! That's nothing. I replaced a 100uF cap the other day that read 3540uF with the meter! Explain that one! I checked it again once I desoldered it to make sure I wasn't seeing things and it still read the same. The meter is a Fluke 87V with fresh battery too!

 

So even if it should be good, I think it's the weak link in my repair. I am fixing a monitor that shuts off after 5 seconds or so. I read it should likely be the capacitors, so I changed all but that one and a big 450v 100uf one. I cannot find these two at any local electronics store..can I put 2 340uF in parallel to replace the 680uF?

 

Good luck finding a 340μF one. You'll probably end up with two 330μF ones, so your nominal combined capacitance will start out at 660μF and could range anywhere from 530μF to 790μF and still be in spec.

Whether you can replace the single cap with two caps depends on the specifics of the circuit, but is likely okay in this situation. Be sure to use caps with at least the same working voltage rating and be sure to note the polarity when you take the old one out and keep it the same when you put the new ones in.

 

Ha! That's nothing. I replaced a 100uF cap the other day that read 3540uF with the meter! Explain that one!
...

I've seen that type of thing. The cap measurement in the multimeter can give real high values if the cap has a lot of leakage (short cct or partial short). It measures the time to charge the cap, and if the cap has a lot of leakage that time is large, so it thinks the cap is large in value.