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Kizmo

Mon Apr 07 2008, 08:05PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

I just got my ESR meter together and it seems to be working fine. But how much ESR is too much if we are for example PC motherboard or smps filter capacitors?

HV Enthusiast

Tue Apr 08 2008, 12:24AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

What kind of meter are you using to do this?

Simply measuring the resistance using an LCR meter or multimeter isn't going to give you a correct value. You need an impedance analyzer (or similar method across frequency) to accurately measure what the ESR is.

You're better off just looking up the datasheet for the capacitor you have in question. The capacitors usually have vendor information on them.

Kizmo

Tue Apr 08 2008, 04:18AM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

Im using EVB ESR/Low ohms meter

Dr. Shark

Tue Apr 08 2008, 08:25AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Hi Tuomas,
if you are looking at big filter electrolytics, e.g. 3000uF 350V, they should usually be in the range of 30mOhm. Smaller caps have significantly higher ESR, as high as 1Ohm for a 100uF photo flash capacitor.
Lower voltage ones probably have significantly less ESR, but I suppose you are mostly interested in the 300 - 400V ones.

EDIT: Found this

in the archives, I wonder if anyone has tried it?

Proud Mary

Tue Apr 08 2008, 10:24AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Kizmo wrote ...
how much ESR is too much if we are for example PC motherboard or smps filter capacitors?

"Too much" ESR in an SMPS output filter capacitor is an ESR which causes an unacceptable temperature rise in the capacitor due to ripple current dissipation.

GeordieBoy

Tue Apr 08 2008, 10:46AM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

...or more commonly too much ESR is that which causes the inductor ripple-current to result in an unacceptably large voltage ripple at the output of the supply.

The reason far paralleling lots of electrolytics at the output of a SMPS is often to reduce the effective ESR at the switching frequency _NOT_ to increase the actual capacitance value.

On the flip-side too little ESR can also have implications for closed-loop stability!

-Richie,

Sulaiman

Tue Apr 08 2008, 11:25AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Most ESR meters actually measure IMPEDANCE at a high frequency, typically 100 kHz
so the 'ideal' esr reading would be R=1/(2xPIxFxC) where F is the ESR meter frequency.
In practice at 100 kHz we use the following guidelines where I work;
uF/Ohms
1/4
2.2/3
4.7/2
10/1.5
22/1
47/ 1
100/.5
220/.3
470/.1
1000/.05

For a given capacitance value, increasing the voltage rating above 35V gives higher ESR.

If you compare the above figures to 1/2xPIxFxC you will see how non-ideal electrolytics are.

P.S. These are just the figures that we use - others may use different pass/fail figures.
In any case we routinely change all electrolytics up to 3300uF when we repair equipment since we give 12 months warranty and electrolytics aren't rated for 12 months continuous use!
Tantalum and solid aluminium rarely give problems, the little round aluminium can SMT electrolytics are almost always dry when we get equipment in to repair.

Proud Mary

Tue Apr 08 2008, 11:46AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

There is a very good article - Capacitor Values: Don't believe the label- which discusses changing characteristics, including ESR, in power supply capacitors, here:

GeordieBoy

Tue Apr 08 2008, 12:27PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

> electrolytics aren't rated for 12 months continuous use!

Not at their maximum ratings, ...but any decent power supply would run them well below 105'C and well below their maximum rated ripple current to extend component lifetime. Low cost flyback power supplies with their high ripple current and electrolytics sandwiched in between heatsinks are about as bad as it gets!

In general electrolytic ESR decreases with increasing temperature. So a SMPSU will often have highest ripple and be most stable when switched on in a freezing cold environment, and will have lowest output ripple when warmed up and operating in its maximum ambient temperature. This temperature coefficient of ESR, combined with manufacturing spread and drift of C can often cause badly designed SMPSU's to go unstable at temperature extremes or later in the product's life.

-Richie,

Steve Conner

Tue Apr 08 2008, 12:46PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hi all

I measure ESR using an audio power amp, 8 ohm resistor, square wave generator and oscilloscope. I'll leave it as a brain teaser as to how you actually measure ESR with these things

The life of capacitors does indeed depend on a lot of things. I've owned tube equipment from the 1960s that was still running on its original electrolytics! However, I've always noticed an improvement in performance after changing them in cases like this.

The worst thing you can do (apart from boiling them dry inside a badly designed SMPS, I guess) is leave electrolytics unused for years. They are perhaps the only electronic component that actually goes stale if you don't use it! If you ever use old electrolytics, or buy an old tube amp from a charity shop or whatever, you're supposed to bring the power up gradually the first time, to "reform" them.

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