Question about Capacitors

[Craig]

If a schematic calls out for a capacitor of a value of 47uF 16V, would it be ok to use a 47uF 25V capacitor instead? I am specifically refering to what is called for in the PSU project in the March N&V issue.

In general, is the capacitor voltage rating on a schematic a minimum value or a specific value?

[GoingFastTurningLeft]

Yes. That is the minimum value.

47uF is the capacitance. 16V or 25V is the rated value.

A higher voltage means it will be bigger, that's pretty much the only difference. There is not a huge difference in size between a 16V cap and a 25V cap, I think the legs might still be the same distance apart so no fitment problems there.

I've built circuits that work with 1V pk-pk through 12V and almost always use caps rated 25V or 50V because that's just what I have laying around. I'm pretty sure those Radio Shack electrolytic capacitor assortments are pretty much all 25V or higher.

I'm looking to build this project as well. Have you sourced the transformer?

[Chris Smith]

The main value of the voltage rating is its safety rating. That is the maximum amount of voltage it can handle with out piecing or internally jumping, arcing, or failing. The higher the voltage value it is, the better it is for the most part.

[Robert Reed]

Run of the mill 'Lytics have at least a +/- 10% value rating. 'Lytics in the past had a voltage coefficient - The lower the DC voltage across them, the lower the actual capacitance will be. I don't know if that still holds true with the bulk of todays capacitors. In any case its usually not that big of a deal. Application will determine how critical the value is. For instance in coupling or decoupling or power supply filtering the designer will give a lot of leeway to cover value error, aging etc. The value given will be higher than required due to this. For filter(signal) and other special applications, the value is much more critical. As to voltage rating vs. size for the same value - the packages go in steps covering value and voltage. For example a 10 mfd value may be the same size as an 100 mfd value. A 16 V rating may come in the same package as a 25 volt rating. You have to look at manufacturers specs if minimal size is important yo your project. Most retailer catologs will show this with the capacitors listing. To sum up, most of the time you can grab whatever is at hand thats in the ballpark of listed value, voltage and it will do the job adequately.

[Craig]

Thanks for the info guys!

As far as the transformers go, I have found a place called Hammond Manufacturing (http://www.hammondmfg.com) that has a 48V 2A unit (http://www.hammondmfg.com/266.htm). In this case, is it ok to use a 48V rather than 44V?

[Robert Reed]

Craig
If you are referring to the cover issue power supply - PM me.