low ESR or low impedance?

[Blackthorn]

What is the real difference between a low impedance capacitor and a low ESR capacitor? The reason I am asking is when I go searching on Mouser, they categorize them differently. I would think that a low impedance capacitor would also have a low ESR. I guess it would depend on the circuit the capacitor is used in. I have been away from electronics theory for way too long.

Maybe someone could explain this to me. I would also think that a low ESR capacitor would be preferred over others in a power supply.

Any help here would be a great help, since I am working on both a motherboard and its power supply. Bad caps on both...

[Pentium4]

Re: low ESR or low impedance?

ESR stands for Equivalent Series Resistance. So low impedance capacitors means the same thing as Low ESR capacitors. Low ESR is crucial in circuits such as switch mode power supplies and motherboard VRM's. It's not so important with general purpose electronics.

So for your projects you definitely will want low ESR caps. Are you preventatively recapping, or fixing them because they are broken?

[mariushm]

Re: low ESR or low impedance?

Impedance is one of the things that form the ESR.

At high frequencies (like you have in power supplies and motherboards and so on), the other things that are squeezed into that single value called ESR are basically insignifiant, so you can pretty much say "impedance" in datasheets is the esr value.

You can read a long article about ESR here : http://www.pa4tim.nl/?p=3775

[PeteS in CA]

Re: low ESR or low impedance?

mariushm accidentally reversed the order of his first sentence. ESR is the DC resistance of the inner parts of an electrolytic capacitor - the lead wires, the aluminum foil, the electrolyte (the electrolyte is not the dielectric). Impedance is the overall resistance of the part to AC current, of which ESR is a part. Impedance is the vector sum of the ESR (DC) and the reactance (capacitive reactance minus the parasitic inductive reactance of the leads and foils). At low frequencies (e.g. 100Hz or 120Hz) the capacitive reactance dominates; in the 10s of KHz the ESR dominates; in the 100s of KHz the inductive reactance becomes relevant and begins to dominate. You can see this effect in the impedance vs. frequency charts in the datasheets of many/most low impedance electrolytic capacitors.

In a switching P/S having a low impedance output capacitors is pretty much a must (maybe excepting .5A-1A outputs).

[mariushm]

Re: low ESR or low impedance?

Absolutely correct. Thanks for correcting me.

That's what happens when posting after a sleepless night, I should know better than that.

[PeteS in CA]

Re: low ESR or low impedance?

I've been known to get my tang tonguelled up in my beykoard on occasion.

[Blackthorn]

Re: low ESR or low impedance?

Thanks, I remember that Impedance is the total opposition to current flow, but of course as was stated here the frequency of the circuit is an important factor when determining the proper capacitor.

I really appreciate the explanations and am really enjoying my review of electronic theory. Much of which I have forgotten.

I am needing several 2.2 uF 50V capacitors for my power supply, since most of them have failed and have a high ESR; anywhere from 4.4 ohms, 5.5 ohms, and even one that is off the scale. I am using the Blue ESR meter for these readings. The capacitors are made by Elite and most seem to be leaning, though it is hard to see if they were just soldered to the board poorly or that they have other issues. I will know more when I remove some of them.

To me it would seem that the Elite capacitors are cheap capacitors and that I should replace them all. I found them on the bad list on this site:
http://www.capacitorlab.com/low-esr-...manufacturers/

I also found a few pictures of them failing here:
https://www.badcaps.net/forum/showth...lite#post47665

I found the following capacitor, and would like your opinion of how well it would work in my power supply. It seems by the data sheet that it is made just for that use.

http://www.mouser.com/ProductDetail/...A0lXP81VwKs%3d

Again thanks PeteS in CA, mariushm, and Pentium4 for your explanations.

It looks like I live within 100 miles of you PeteS in CA.

Any help is greatly appreciated.

[Pentium4]

Re: low ESR or low impedance?

Yes, Elite makes low quality capacitors. When the capacitance goes down, ESR goes up. So it's normal to have higher ESR for those small caps. Although at ~5 ohms ESR, they are possibly starting to fail. nichicon PW is a great replacement for those. What capacitors are on the secondary side of the PSU? Could you post some pictures of it?

[Blackthorn]

Re: low ESR or low impedance?

By looking at the graph on the ESR meter, I saw that a 2.2uF capacitor rated at 50V would still be within limits at close to 40 Ohms...? One had such a high reading that the meter just showed a - which means that it is out of range for the meter which is good for up to 99 Ohms. So at least one is definitely bad.

I thought since the capacitors were cheap, and the readings so varied that they might all be starting to fail.

I will try to get some pictures of the power supply later. My phone has a lousy camera on it, so I will have to use another camera to take good pictures of it.

I will go ahead and order the capacitors anyway. There are also two 470uF capacitors rated for 200V that read .07 ohms & .10 ohms. It looks like they are approaching their limits of about .11 Ohms if I read the graph correctly. I don't recognize the manufacturer's symbol on the capacitor, but since they read so differently, I suspect they are starting to fail also.

Anyway, I will try to get some pictures on here soon.

Thanks for the help!

[mariushm]

Re: low ESR or low impedance?

If their tops are not swollen, if they're not leaking from the leads, then there's no need to replace the high voltage capacitors, those will be fine even with 0.3-0.4 ohm esr. They're working at low frequency (100-120 Hz) so the ESR is not that important.

For low capacitance capacitance, the ESR depends on various factors, including the type of electrolytic used, series, lifetime etc.
I would consider up to 2-4 ohm esr OK for a 2.2uF 50v capacitor... in fact I just measured the smallest capacitor I have close to me (well, without looking through boxes of parts) , a new, never used Nichicon PW 10uF@35v, and it measured 9.76uF @ 1.62 ohm ESR. So a 2.2uF it's possible to go over 2ohm esr easily.

I've had barely used phone chargers with 4.7uF 450v capacitors which had esr of 6-8 ohm..

ps. The graphs you got with Blue ESR, the ones on the label, are very generic and outdated... Keep in mind Blue ESR is a design made.. I don't know.. 10-15 years ago? Modern series of capacitors don't usually have ESR over 10-15 ohm (and that's maybe for something like 2.2uF 16v miniature electrolytic).

[Kiriakos GR]

Re: low ESR or low impedance?

What is the real difference between a low impedance capacitor and a low ESR capacitor?

Both those parameters in the form of a measurement are available by the instrument which is called as LCR bridge.

The manufacturers of capacitors are not obligated to follow one specific parameter of the two in their product catalogs.

If you own an extremely cheap ESR meter then you are unable to be aware of the impedance of one capacitor simply because it is unable to measure it.

The impedance is the sum of three electrical parameters, therefore there is no way to use some sort of math converter so to get just the ESR parameter out of the total impedance.

You may have a look in this product review which is an analysis of what the LCR meter is all about.
http://www.ittsb.eu/de-5000.html

[tom66]

Re: low ESR or low impedance?

The impedance is a function of resistance, inductance and capacitance.

Capacitance is already known and can be measured, at high frequencies (>100kHz) it is essentially providing no addition to the impedance for all capacitors beyond about 100uF.

Inductance of the capacitor can play a role but is generally not important until you reach over 200 to 300kHz. Any such contribution is likely to be negligible until these frequencies, at least for the average ESR tester (with an accuracy circa ±5%.)

Resistance is by far the most important factor and contributes the most, so one can safely say for the frequency of 50kHz to 150kHz that impedance ~= ESR (within about ±5%.) Therefore, in this case, impedance and ESR may well be equivalent for most general uses. It is certainly a bit extreme to suggest that the OP buy an LCR bridge to accurately separate the ESR, when the average capacitor would be lucky to have a ±20% spec on the ESR!

[PeteS in CA]

Re: low ESR or low impedance?

It looks like I live within 100 miles of you PeteS in CA.

If you're somewhat familiar with Silicon Valley, I live near where State Highways 17 and 85 intersect, and can see Good Samaritan Hospital from my front or back yard. UraBahn lives not too far away, in Woodland, where I grew up. Even CA can be a "small" place, sometimes.

There are series with much lower impedance, but Nichicon's PW series is a really good bead-and-butter low impedance, long life, series. It's a series that would also do well for decoupling or for the start-up cap for UC384X-based power supplies (where cheap crap caps are frequent friers). United Chemicon's LXZ series and Panasonic's FC series are pretty much drop-in equivalents to the PW.