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Old 18th May 2002, 12:17 PM   #1
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Default Capacitor tolerans

Is it possible to use caps rated a few volts lower than necessary
in the power supplyrails? Perhaps the tolerans isn't that critical?

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Christian V
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Old 18th May 2002, 04:56 PM   #2
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Film or electrolytic?

Would not go above 75% working voltage on an electrolytic. Much lower on a tantalum.

Jocko
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Old 18th May 2002, 09:20 PM   #3
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On my experience, no. As jocko said, don't get over %75 of the rated voltage. This also helps the cap to last longer.
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Old 18th May 2002, 09:35 PM   #4
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I can confirm it as well. I was running caps very close to their rated voltage. After few years they were completely "dead".
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Old 19th May 2002, 11:12 AM   #5
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to me, this is a bit confusing, so i'll try to make it a bit clearer:

Voltage rating on electrolytic caps needs to be AT LEAST 25% higher than the voltage that's going to be passed across them. However, it doesn't matter HOW MUCH higher the voltage rating is - they'll probably just last a bit longer.

For example, if you need to pass 75V, then you need 100+ volt caps.

I don't know much about tantalum and other caps.
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Old 19th May 2002, 03:18 PM   #6
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>> However, it doesn't matter HOW MUCH higher the voltage rating is - they'll probably just last a bit longer.

Might not be that simple as other parameters are affected as for example ESR which may vary in either direction depending on voltage - sometimes it goes up and soemtimes it goes down. Prices are also a big factor in choosing the overrated (!) value.

(Not to be nitpicky, but not to exceed 75% of rated voltage is not the same as choosing a capacitor with the 25 % higher rating than the voltage needed but choosing one with 33 % higher rating)

/UrSv
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Old 21st May 2002, 05:06 AM   #7
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Yes, i suppose you're right. And yes, price is a big one, too.

about the 75% thing: are you saying that your estimated maximum peak voltage + 22% (normal safety margin) should be no more than 75% of the rated voltage of the cap you're going to use?

In other words, if i have an application where the RMS voltage under normal circumstances will be, say, 48V, but i'm expecting peaks of 52VRMS, then i should choose a cap by:

52VRMS=73.528V peak + 22% = 89.704V +25% = 112.13V =approximately 120V capacitor?
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Old 21st May 2002, 12:04 PM   #8
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Default Re: Capacitor tolerans

Quote:
Originally posted by Christian
Is it possible to use caps rated a few volts lower than necessary
in the power supplyrails? Perhaps the tolerans isn't that critical?

Best Regards,
Christian V
YES, but only if you know what you're doing!

My old RT75 power amp from 1975 have Siemens 10000µF/40V and 47 volts across them, since 1977! The amp has been switched on 24/7 since 1988!

If you are a professional you would never do this but as an amateur it can be OK. An easy way to test the max voltage is to apply DC-voltage and then measure the leakage current. When the current exceeds 1-10 mA then you have reach the max voltage. This test takes a few hours because the isolation (leakage) gets better and better if DC is applied.

Suitable equipment is a DVM with 10k or 100k across the terminals of the voltmeter. (= high impedance current meter).
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