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
in the power supplyrails? Perhaps the tolerans isn't that critical?
Best Regards,
Christian V
Film or electrolytic?
Would not go above 75% working voltage on an electrolytic. Much lower on a tantalum.
Jocko
Would not go above 75% working voltage on an electrolytic. Much lower on a tantalum.
Jocko
On my experience, no. As jocko said, don't get over %75 of the rated voltage. This also helps the cap to last longer.
I can confirm it as well. I was running caps very close to their rated voltage. After few years they were completely "dead".
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.
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.
>> 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
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
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?
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?
Christian said:
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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