'Standard' ESR capacitors??!

[johnm]

I've read quite a bit recently concerning the use of capacitors before/after a regulator.

I've read that low ESR caps are not a good idea AFTER the regulator, and 'standard' ESR type caps are better here.

Trouble is, I don't know of any standard ESR caps - just about all the ones from the usual audio parts vendors are marketed as low or super-low ESR types.

Can anyone recommend a 'standard' level ESR cap, but one that's good quality (a contradiction in terms?) please?

Many thanks.

- John


P.S. In my case, the caps are before/after a LM7808 regulator...

[Conrad Hoffman]

Figure plastic film caps will be very low esr compared to everything else. You won't get into trouble with almost any common electrolytic as the esr isn't low enough to get into ringing problems in most circuits. I don't like tantalums, but one thing they have going for them is very consistent and moderate esr; I think that's why people have been so successful with them. Unfortunately they tend to be bad for audio and they fail short circuited.

[TheLaw117]

Panasonic FC/FM will be fine in almost every DIY application. PSU and Amp alike. I think it is only when you start using REAALY low-ESR caps, like Nichicon HZ/HN that you have problems. These are PC motherboard grade. They have to be really low.

[jcx]

it may be more reliable to use a low esr cap and add the required damping as an explicit series R - there is a optimum R for minimum noise peaking

the "boiler plate" op amp datasheet bypass rec of tant or 10x value Al electros for the right esr damping is past its "use by" date given the evolution of low esr Al electros you can't rely on just any electolytic C to supply useful levels of damping esr

[peufeu]

Originally Posted by TheLaw117

REAALY low-ESR caps, like Nichicon HZ/HN.

I have used Nichicon R7 and LF series caps, on the output of DC-DC converters. A tiny 270 µF 16V takes 1 amp of ripple current at 250 kHz without breaking a sweat. 7-9 mOhm ESR, 5 amps ripple current rating, it's about the size of a half cigarette butt, lol.

When using that stuff at the output of linear regs (or using ceramic caps which have very low ESR), the datasheets need to be read extremely carefully.

[gootee]

I think Conrad is right and just about any electrolytic should have a high-enough ESR to prevent ringing, etc. But jcx makes a very good point, that using a more stable, longer-lifespan type of cap, such as a film type with very low ESR, but adding a series R, would make the long-term reliability much better, and would also enable you to optimize he "ESR" much more readily.

[johnm]

Thanks for all the replies folks - much appreciated

I guess my next question is, how do I calculate the value of a series resistor, to be used between the output of LM7808, and the output cap? (1000uF Rubycon ZLH at the present). I have the same cap at the input of LM7808 too. This is for the popular 'Shigaclone' CD mechanism. The Cap-Reg-Cap are located right next to the transport.

Thanks,

- John

[gootee]

You probably won't need additional resistance in series with a 1000uF cap, since it has to be an electrolytic cap, at that value (unless you go crazy and parallel ten 100uF polypropylenes <grin>), and probably has sufficient ESR built in.

If you have an oscilloscope, you could check to see if there's any ringing on the power rail, and could possibly add a snubber (series R and C to ground) or a very small R in series with your existing cap. I have some links to snubber design/calculation procedures, if you need them.

[johnm]

Well all the caps I have in my parts box (i.e. various Rubycon ZL series, Panasonic FC and FM and assorted Silmics and Cerafines) are all billed as low ESR I believe.

I don't have an oscilloscope alas. In leau of that, what's a tell-take sign that a regulator is oscillating? I have to say the Shigaclone CD transport appears to be working fine, and nothing is over-heating. So perhaps this is all a huge moot point? I just like to know things have been engineered properly though

- John

[Conrad Hoffman]

All a moot point. Your caps are low esr in comparison to high esr caps of similar types. In the grand scheme of things they're nowhere near low enough to cause problems. Even with truly low esr film caps one rarely has a problem. Don't worry, put on some music!