Do you want your capacitor to have lower or higher tolerance? (10% vs 20%)

[TimLaw]

Hi,

I looked around, couldn't find much.

I am finalizing my part selection for my power supply build. Now this sounds stupid that I don't know but I never could seem to figure this out.

Does the tolerance rating measure how much voltage/amp it can take out of its design specification before it fails?

OR

Does the tolerance rating measure how different the output voltage/amp could be from the design specification.?

OR

Does it not even matter in a power supply. (But which would be the best in theory?)

**broken link removed**

I am looking for the 100 uF caps on Mouser right now. Trying to determine between a couple different types.

Thanks a lot.

-Tim

 

[Aussie Susan]

Generally, the tolerance refers to how close the actual capacitance is vs that stated. In this case, the 100uF capacitor with +/- 10% could have a capacitance from 90uF to 110uF.

This is quite separate from the voltage the capacitor is rated at - this measures (roughly) at what voltage the dielectric that separates the plates will break down and let current flow directly them.

Therefore, you need to understand the reason you have a capacitor in that place in your circuit. If it were part of a tuned circuit (for example), then the resonant frequency will be directly related to the capacitance and so you may want the smallest tolerance possible. In your case it looks as though these are only being used as smoothing capacitors in the output of the PSU so the exact value is of lesser concern. On the other hand, you had better get the voltage tolerance well above the output voltage or you will be in big trouble.

Susan

 

[TimLaw]

Thank you Susan. So generally smaller tolerance percentage would be better as you would have more control over the flucuations. That is all I really needed to know. The caps I will be using in the supply will probably rated from 50-100V just in case.

I suppose that should be sufficient.

 

[Aussie Susan]

That's not quite what I said. In your case you are not interested in the actual capacitance and so the tolerance is not really an issue. There are other factors such as the size of the capacitor, how easy it is to mount, the cost etc that would play a bigger part in selecting the one you want to use.

In your case, these capacitors are used to stop the large voltage swings when there is a sudden change in the current being drawn (such as when the device being powered is turned on or off). Therefore you want a large capacitance so that it holds a lot of charge that can be released (or absorbed) quite quickly - but exactly how much is not really important.

There are other uses for capacitors in relation to the supply of power to a circuit: for example in a digital circuit where the devices are turning on and of very quickly, there can be very large high frequency spikes on top of the DC level. In this case you use capacitors in the nF to low uF range as these have a low impedance at high frequency (the spikes see them as an easy path and so prefer to go that way) than at low frequency. These capacitors are usually mounted close to the circuit elements as possible. Again, as you can see by the range I've just mentioned (3 orders of magnitude) the actual value (and hence the tolerance) is not an issue.

It is only when you are looking for very precise frequency-dependent operation of a capacitor (such as in a filter) that you want to start getting into the tolerance of the capacitor so that the actual device value is close to that you have used in your circuit design calculations.

Susan

 

[TimLaw]

Thank you once again.

So it matters naught if my capacitor can hold more charge than the circuit design calls for?

This is what I understood:

If it says 3300uF, it would be safe to use a 3700uF cap. I can also have that cap rated for much higher voltages than you would anticipate being in the circuit to make up for varying power input/draws. Tolerance is only an issue when trying to make an extremely stable power source for things like audio.

I appreciate your contributions.

 

[Diver300]

Even in a very stable power source, larger smoothing capacitors would be better.

Where you need a tight tolerance capacitor is in a tuned circuit. If you combine an inductor with a capacitor, you get a tuned circuit that resonates. The resonant frequency depends on the value of the capacitor and the inductor. The usual reason to construct a tuned circuit is to work at that frequency, so tolerance of components matters there.

In audio, the capacitors used in crossovers would need to be tighter tolerance, and the circuit would not perform as well if some capacitors were much larger than expected.