measurement of capacitors

[axell16]

Hi!
I played around with a little electronics as a child. Know many years after thinking of playing around with it again.
I just bought myself a multi meter which has the ability to measure capacitance. Not a Fluke. Just a cheap ut139. I have some old capacitors that I salvaged from electronics back then and a few unused. Most of them do not have the value that the MM shows. Some above and some way under the value written on them. Like one showing 26uf while it should be 22 or one at 470uf is measuring 514uf. So I am worried that my new dmm is faulty. All of them cant be bad, I think, or can they?

 

[Warpspeed]

If they are all electrolytics, and fairly old, they will definitely degrade over time, especially if they are never powered up.
Even when brand new, the measured values of electrolytics can vary considerably from the specified face value.

If you want to check your meter with high value capacitors, a good quality plastic film capacitor of close tolerance would be about the only type of capacitor you could rely on.

 

[j33pn]

I think electrolytics can have a tolerance of +80% / -20%, so your readings may actually be right on.

 

[axell16]

Ok, glad to hear that me dmm seems to be reading fine. I am surprised that they can be up to 20% off. That is a lot. So you dont have to be that picky when it comes to ecaps. Like when a parts list says 100uf you can use an 80 or 120uf without any problem?

 

[Audioguru]

If a parts list says 100uF and you use an 80uF that is 20% low (64uF) then it might not be enough. If you use 120uF that is +80% (216uF) then the circuit might power up in more than double the normal time.

 

[KlausST]

Hi,

Just to explain: -20% .... +80% on a 100uF capacitor means: 80uF....180uF.

Like when a parts list says 100uf you can use an 80 or 120uf without any problem?

That's not generally true.
100uF in a partlist says nothing about tolerance.
But if additionally the tolerance is given to be +/-10%...this means the capacitance may be in the range of 90uF...110uF
But it is not safe to use a capacitor with the measured value of 92uF.

-->
* your capacitance meter has a tolerance, too. So showing 92uF does not mean it really is 92uF.
* capacitance may change with applied (DC) voltage.
* capacitance may change with time
* capacitance may change with temperature
* capacitance may change with frequency
....

Klaus

 

[Warpspeed]

Ok, glad to hear that me dmm seems to be reading fine. I am surprised that they can be up to 20% off. That is a lot. So you dont have to be that picky when it comes to ecaps. Like when a parts list says 100uf you can use an 80 or 120uf without any problem?

It depends what the electrolytic does.
If its just part of the typical power supply decoupling or ripple filtering, the value is usually not terribly critical.

But if its part of a timing circuit, perhaps a low frequency oscillator or a filter, then you might be in trouble if the value were off by a half, or double.

If it is a highly critical circuit, chances are the designer avoided the electrolytic type of capacitor altogether and used a plastic film capacitor, which would be much physically larger and more expensive.

 

[axell16]

So if using a 92uf the tolerence of 10% would be up to 101.2uf and might not be enough for the curcuit. Unless using one with 20% tolerence?

 

[KlausST]

Hi,

So if using a 92uf the tolerence of 10% would be up to 101.2uf and might not be enough for the curcuit. Unless using one with 20% tolerence?

No, it´s the other way round.

But if additionally the tolerance is given to be +/-10%...this means the capacitance may be in the range of 90uF...110uF

If you meter shows 92%, it seems to be safe, but one can not be sure.

If the meter has a tolerance of +/-5%...Then showing 92uF means it actually can be anywhere in between: 87.4uF and 96.6uF. The valid range is 90uF ..110uF, so 87.4uF is out of this range.

Klaus

 

[FvM]

May be there are electrolytic capacitors with a -20/+80% specification. I see +/- 20% with most recent electrolytic capacitors. Your measurement is still well within the range.