ESR Meters

I was wondering what the groups experience is with ESR meters? I just went through a bunch of large PSU filter caps with the Electronic Design Specialists 88A cap analyzer. One value was 3550uF and the other one was 2200uF. All of them tested good for capacitance value and ESR.

Over the weekend I replaced the motor run cap on my Lyrec lathe turntable motor. The cap wouldn't keep it's charge If I shut the motor off. Today I checked the known bad cap with the Peak LCR and the 88A and they both said it was good. I remember in the past using a different brand of ESR meter and getting tons of false positives.

Are there any good ESR meters?

Maybe those meters aren't measuring leakage?

JR

Neither meter is measuring leakage current.  I asked a friend of mine about  this.  He said to measure leakage current  apply a DC voltage  at 100%  rating.  Put a resistor  in series  and measure the voltage drop.

By the time I do that I might as well just replace them.

Your friend is right, but you could also just use a sensitive VOM on current scale. Good caps should be in the range of uA leakage.

JR

He thought testing them at 100-150% voltage rating was a better test than a low voltage test. A DMM is only using a volt or two right? He also said capacitors could behave differently under load. So it seems even if everything tests good out of circuit it could still not be up to spec in circuit.

I was trying to be thorough and valiant by testing first. It seems there is no valor in trying to keep old aluminum electrolytics. I'll just change them.

The VOM in current mode is dropping very modest voltage. Connect the + lead of VOM to rated voltage,connect - lead to top of cap (disconnected from rest of circuit). In current mode the VOM should directly read cap leakage current.

JR

I just tried it with the VOM on uA and mA scales. It showed no leakage...

Gold said:

I just tried it with the VOM on uA and mA scales. It showed no leakage...

Click to expand...

I forgot to mention that of course the bottom of the cap needs to be connected to ground to complete the path.

No leakage is good.

In your first post you say the cap doesn't hold charge when you shut the motor off. Are there some caps that do?

JR

JohnRoberts said:

I forgot to mention that of course the bottom of the cap needs to be connected to ground to complete the path.

Click to expand...

One side was not connected to ground. So connect one side to ground and put the ammeter in parallel across the cap?

No leakage is good.

Click to expand...

Except when the cap is known to be bad.

In your first post you say the cap doesn't hold charge when you shut the motor off. Are there some caps that do?

Click to expand...

Normal behavior for the motor is to maintain speed after applying power and spinning up. The bad cap had to have the motor hand rotated until there was enough of a charge to turn the turntable. Sounds leaky to me.



JR
[/quote]

Gold said:

So connect one side to ground and put the ammeter in parallel across the cap?

Click to expand...

No, you measure voltage *across* something and current *through* something, so the ammeter must go in series with the cap. You might just blow up your ammeter the other way!

Gold said:

JohnRoberts said:

I forgot to mention that of course the bottom of the cap needs to be connected to ground to complete the path.

Click to expand...

One side was not connected to ground. So connect one side to ground and put the ammeter in parallel across the cap?

Click to expand...

No no... Current meter is in "series" with cap.

- lead of cap to ground
+ lead of cap to - lead of current meter.
+ lead of current meter to voltage source.

And of course the voltage source - lead connected to ground to complete the circuit.

No leakage is good.

Click to expand...

Except when the cap is known to be bad.

In your first post you say the cap doesn't hold charge when you shut the motor off. Are there some caps that do?

Click to expand...

Normal behavior for the motor is to maintain speed after applying power and spinning up. The bad cap had to have the motor hand rotated until there was enough of a charge to turn the turntable. Sounds leaky to me.



JR

Click to expand...

[/quote]

Oh, so the meter is a substitute for the series resistor in the other setup. It just directly measures current instead of measuring voltage drop and plugging in the numbers. Still need a high voltage power source to test under operating conditions.

Paul,

Have you looked at the Sencore meters? (used)

Cheers,
Ruairi

ruairioflaherty said:

Paul,

Have you looked at the Sencore meters? (used)

Cheers,
Ruairi

Click to expand...

I haven't. I remember there was a Sencore cap analyzer at the lathe shop. I think there was a Sprague one too. Now that you mention it I think they both did leakage current. I think they were tossed. Figures. I couldn't take everything.  Thanks for the reminder.