General Question Capacitor Pickiness

[Michale32086]

First a little bit of a back story..

Way way WAY back in high school, I was building a Van De Graff generator as a science project. The plans I was following called for a fiberglass tube, about 3 feet long, about 8 inch diameter.. Where was a high school kid going to find a fiberglass tube?? Thinking to myself that all that is really needed is something that is stiff and strong and not conductive, I hit upon using a stiff cardboard tube, the same length, same diameter and about a half inch thick..

Wanting to confirm my thinking, I asked my science teacher about the switch, telling him my reasoning.

He said, "If the plans call for a fiberglass tube, use a fiberglass tube. Always follow the plans!"

Well, I didn't like that answer very much. So I asked a different science teacher again, telling her my reasoning..

She said, "If the goals of the plans are met, then substitutions are acceptable"

Of course, this was the answer I wanted to hear.

So, my question...

When replacing capacitors, should I "follow the plans"?? I mean, if a board needs an 820uf 25v, USE an 820uf 25v...

Or, is there a little bit of leeway.. Say I only have an 820uf 50v.. Would that be a valid substitute??

If there IS leeway, how much?? I know that one shouldn't go DOWN.. IE Use an 820uf 10v to replace an 820uf 25v... But I would think a little leeway going up is permissible...

Totally off base here??

Thanx

[stj]

Re: General Question Capacitor Pickiness

good question.
most designers design well, but the product is then made with junk.
so you can use better parts to a degree.
but never worse.

btw, cardboard was a bad idea because it can trap moisture and become slightly conductive.
you didnt know why fiberglass was recommended so you couldnt sub it well.

glass would have been best.

[Michale32086]

Re: General Question Capacitor Pickiness

good question.
most designers design well, but the product is then made with junk.
so you can use better parts to a degree.
but never worse.

btw, cardboard was a bad idea because it can trap moisture and become slightly conductive.
you didnt know why fiberglass was recommended so you couldnt sub it well.

glass would have been best.

Good points, all...

Thanx for the reply...

[mariushm]

Re: General Question Capacitor Pickiness

How the capacitor is used in the circuit matters and it's important to understand the capacitors' role in the circuit before making replacements.

Capacitors have properties like ESR, they can only handle so much current ripple, etc etc.

Changing a capacitor with a higher voltage one can change the ESR (makes it lower usually) which can introduce oscillations/instabilities in the circuit ... if you're smart in such cases adding a single 0.01-0.03 ohm resistor in series with the capacitor could help correct the issue..

If that higher voltage capacitor is only rated for 85c instead of 105c, that could mean the electrolyte inside is based on another chemical formula which may make it unable to handle as high currents as the previous capacitor, so again you have to be careful what you use as replacement.

[budm]

Re: General Question Capacitor Pickiness

"When replacing capacitors, should I "follow the plans"?? I mean, if a board needs an 820uf 25v, USE an 820uf 25v...

Or, is there a little bit of leeway.. Say I only have an 820uf 50v.. Would that be a valid substitute??

If there IS leeway, how much?? I know that one shouldn't go DOWN.. IE Use an 820uf 10v to replace an 820uf 25v... But I would think a little leeway going up is permissible...

Totally off base here??"
That depends, when you substitute parts, you put the parts in and then you analyze and test the circuit to see if it still function as designed. The 820uF may have been chosen due to cost, but it may compromise the circuit with less margin for the circuit to work, you usually want the head room so that if there are changes in components, the change in environment the unit is still function with worst case scenario.
For example, you design the products to work in 120VAC, so does it mean it will not work at 125, 130, 115, 110, 100V when the other companies can make the same product that will work from 90V to 240V.

[Michale32086]

Re: General Question Capacitor Pickiness

Basically what I am seeing is that, unless I have indepth knowledge of what is going on within the circuit, substitutions are unwise...

Makes sense. Thanx for all the input..