Capacitor Voltage Rating

[Chad]

Is there anything wrong with using 250volt caps on a 70 volt rail? My local electronics shop has four surplus 26,000uf 250v caps for the the same price as four 10,000uf 125volt caps. I'd like to use the bigger caps but i'm not sure if the voltage ratings will cause problems.

Thanks in advance, Chad

[hifiZen]

certainly, there's nothing wrong with using an overspec'd cap. In fact, I was thinking that Grey's example of 32.5 volts being within the range of a 35V cap was probably cutting it a little close for my comfort. For anyone who's had a cap overvoltage and blow on them, you know what i mean ...they make an extremely loud bang (like an explosion, really) and spray all kinds of lovely fluid into a cloud of mist which settles on everything. I typically add a minimum of 25% to the supply voltage to get the cap rating.

If I were you Chad (wait a minute, I *am* Chad... hmmm... ), I'd snap up those caps. 25,000uF caps with a 250V rating are rather hard to come by, even the 10,000/100V caps are probably a steal. But, I'd be careful to check their age first! Electrolytics degrade slowly, and the tolerance may be high because it covers the spec'd lifetime of the cap (MTBF usually).

Chad.

[AuroraB]

Hmmm....

Clearly,- in some of the posts, there must be some misunderstandings.

The tolerance of +/-20 % relates only to the capacitance value. There is as such, no tolerance stated for the voltage rating, - but usually in "pro" circles, we put a finger in the wind, and expects a cap to forgive us some overvoltage spikes, and to tolerate some 10% overvoltage on an intermittent basis. Cap's should of course NEVER be designed in to run on permanent over voltage. Giving room for a 15-20 % overvoltage is concidered good design practice.

ESR does not diminish by increasing voltage, it increases.
An example,- a RIFA comp. grade, - 22.000 uF/ 25V has an ESR of 5 momh, and the 100V version has 15 mohm.
The relative value of ESR also, quite logically, increases with lower capacitance. In this light, the practice of some manufacturers to parallell "many" smaller value caps, relies totally on the idea of parallelling the ESR of each individual cap, which in turn really depends of the resistance of your parallelling connection,- copper plates or "thin" wires, anyone???

Running a capacitor on relatively lower voltage than rated does not lower its capacitance or lifetime. Where does this come from ?? High temperature though, reduces the lifetime, and often conversely also the capacitance, by aging.

Somebody mentioned MOVs, and this is of course a very good idea, and commonly used in pro gear of all kinds. This is the remedy for high over voltage spikes, but not for marginal capacitor voltage rating. If you live in middle latitudes and/or have air born power lines to your house, OV spikes from discharge are rather common.

Even if some of you US residents have rock steady line voltage in your houses, I do feel quite sure that US power companies also have tolerance values of at least +/- 5%, as this is also a part of their mechanism to protect themselves from claims of damage resulting from varying line voltage, among other things also from load variations. Designing a power grid without allowing for load variations is impossible.

Just to sum up,- you have to allow both for line voltage variations, and the load variations you create by e.g. turning your amp up or down, reflecting to voltage variations from the amp's own transformer.
Allowing for up to 20% variations of the voltage on your caps is indeed a good idea.

BTW, - there is an excellent book available on the net -
the "Rectifier Applications Handbook", from Motorola. Try a search, - my net connection is somewhat clogged at the moment. But beware,---it's huge !! (some 350 pages....)



[Edited by AuroraB on 09-21-2001 at 04:32 AM]

[GRollins]

Two ideas for those who are afflicted by power fluctuations:
--Build a regulator circuit into the line from the rectifier to the main bulk caps...but don't design it for the target voltage, set it for, say, 3 or 4 volts higher. Under normal AC line conditions it will be in full conduction and add minimal resistance to the power supply as a whole. In overvoltage conditions, it would clamp the voltage at whatever level it was designed for. Power dissipation in the circuit would be minimal. Obviously, it would always carry full current, but the voltage drop under normal AC line values need only be the normal drop across the pass transistor, so heat would be limited to a few watts at most. In an overvoltage condition, you would need somewhat more power dissipation capability, as the circuit would then be producing more like 10 or 20 watts. The circuit could be built for a few dollars worth of parts (excluding a heatsink for the pass transistor) and would surely be much cheaper than buying caps with higher voltage ratings.
--Purely by coincidence, I ran across a blurb last night on a new power box from Monster Cable. It doesn't work like the usual box with filtering, etc. Instead, it has a variac inside with a motor drive. It detects the incoming voltage and corrects it via the variac. Conceptually simple. Implementation might be a bit more difficult, but I can't see that it would be *that* bad. The flow chart would probably go something like this:
AC->variac->rectifier->comparison of rectified DC to reference voltage->correction via motor
What kind of motor to use to drive the variac, I'm not sure, but the monitoring circuit would be trivial. Needless to say, they gussied it up with LED readouts on the front and such, but you need not worry about that. The variac alone would cost perhaps $75 to $200 depending on current rating and whether you were able to source it surplus or new, so it wouldn't be a cheap project. Still, it'd be something to consider.
Model: AVS2000
List price: $1499 (surely it could be built for $200 or so)
Web: http://www.monstercable.com

Grey

[Super_BQ]

Great discussion on capacitors.

Here's a link for the caps i've bought for my next project (warning it's a large jpg); Pass's Balance Line Stage pre-amp (4 channel).

http://www.geocities.com/super_bq/Hi...ectrolytic.jpg

Notice the 105°C rating. Capacitors may be an overkill but I just could not pass up on the great price. Bought 14 of them @ $7cdn each (includes taxes). Also Rubycons (which make the Blackgate audio capacitors) are great for audio.