[wab]

I am in the process of gathering parts for a Cmoy amp. Capacitors C1 and C2 in the powersupply need to be 220uF, 25V electrolytic capacitors. For those who don't know which capacitors I am talking about:

Problem is I can only find these in 16V or 35V. Can I also use one of these? What would be the consequence of using a capacitor with a different voltage?

Thanks!
wab

 

[Videoshielded]

Make sure the voltage of the batteries you intend to use will not exceed the voltage rating of the two capacitors you're talking about.

If you're only going to use one 9V battery, the 16V caps are fine and should be smaller than the 35V caps. But, if you intend to use 2 9V batteries (makes this circuit sound much better IMHO), the 16V caps won't be big enough, and they could rupture. Ask Michael for pictures of what it looks like when this happens!

I'd advise you to use the 35V if there's any way they'll fit in your case. That way, if you want to go all the way up to 24 volts of batteries (using AAA or AA in battery holders) to get the maximum performance out of the amp, you won't have any problems. I have actually had to go back and upgrade the capacitors in a couple of CMOY's I've built when I decided they needed more voltage.

 

[wab]

Thanks Videoshielded! That helps a lot.

Now if only I could find those op-amps! They are really hard to find here in the Netherlands

 

[thomas]

Nope, 16 volt caps are fine...

the voltage each capacitor sees is only half of the input voltage because of the resistor divider. So you could put in 32V which becomes +/-16V and your amp will still work properly. And electrolytic voltages are working voltages, which means it can take a peak several volts higher before the electrolyte breaks down.

The maximum voltage your opamps can take is about +/-18 volts, before they start blowing up, so chances are they would get fried before 16V caps do...

 

[Possum]

I would definitely get a capacitor that's rated at or higher than what you'd ever want to use. I've had ~40V fry a 35V capacitor in a few minutes (quite a nasty-smelling steam and oily liquid spewed). Better safe than sorry.

It can potentially save you a lot of trouble to replace the capacitor and clean the surrounding components later.

 

[thomas]

Well, high voltage caps are larger and more expensive, and most importantly, if they're not available, then you might as well go for the minimal voltage needed.

With your 40V blowing a 35WVDC cap, i'm assuming its comming out of rectified AC, which would be pulsating DC with peaks at 58V. That is way over the rating of a cap.

But with batteries, you will never get peaks over the voltage range, and 16V is already more than you should use with these opamps...

But yeah, it is better to be safe than be sorry, might as well get 25v caps if it is available.

 

[Possum]

I'm saying he shouldn't buy caps that are under the voltage he intends to use and pray that it will work.

 

[thomas]

No, you DO NOT USE more than 15V on an OPA opamp. Any more will risk damaging your opamps, which are much more expensive and eisier to blow up than any cap. 16WV caps won't blow before the opamps are smoking...

 

[ppl]

No one is looking at the fact tthat Electrolyttic capacitors have a working voltage that is the max DC voltage that should be applyed. thay also have a surge rating that when the Working voltage and DC potential are matched this serge rating takes care of Peaks. If you apply half the rated working voltage to an electrolytic then you will get about half the rated capacitence. so select a working Voltage the next standard rating Higher than your DC voltage like 16 volts for a 12 volt supply 25 volt for a 15 volt supply. and if operating on batteries then select a working voltage the same as the Max battery Voltage For the above application i would use 10 volt caps. But that's me.

 

[Possum]

Plenty of people use 18V (+-9V) with their OPA's. Unless you're talking about +-15V, not 15V.

 

[thomas]

yeah, each cap only sees half of the input voltage, so if each cap is getting 16V, then you would have +/-16V, which is just about the maximum for an opa

 

[Videoshielded]

D'OH! Should've looked at the schematic for a sec.

The voltage divider is before the caps.

Yep, each cap is only going to see half the source voltage.

I've been working with too many CRC PS ripple filters lately, sorry.

 

[wab]

thanks for your help all! I got some nice small 16V ones so I won't have to upgrade them should I decide to upgrade the powersupply later.