30 Apr
2000
30 Apr
'00
6:49 a.m.
From: ard(a)p850ug1.demon.co.uk (Tony Duell)
Subject: Re: Cap. leakage current Was:Re: PDP-5 Rescued!
To: classiccmp(a)classiccmp.org
Date: Sun, 30 Apr 2000 19:34:08 +0100 (BST)
Reply-to: classiccmp(a)classiccmp.org
Hi all,
Another thing that's hard to detect with simple
instruments, but rather
important, is the effective series resistance I mentioned above. When an
electrolytic dries up, the ESR goes high, but often the capacitance, as
measured by a capacitance meter, doesn't change that much. A capacitor
with high ESR is a very poor filter, though, so your PSU ripple
increases. And some switch-mode PSUs do _very_ strange things when they
contain high-ESR capacitors, including puting 10V spikes on the output
rails (don't ask how I found that out...)
Correct, this is what I had to rebuild a peecee PSU switcher with
new caps. Now good as new.
An ESR meter, or some other way to measure it, is a
very useful
instrument. There are various designs out there -- I've heard good things
about an Australian design (I beleive it's called the 'Dick Smith
Electronics ESR meter), but I've never actually used one.
Very good meter, but it also can measure .01 to 99 ohms
resistors. Which is very useful especially when one wants to check
low current contacts is low resistance enough and works reliably.
Usually greater than 6ohms in contacts makes the device behave
strangely. Even 10 ohms on nicked 10cm long trace is enough to
cripple the peecee motherboard, had to splice it to decrease this
resistance.
And this is great device for checking fusiable resistors in circuit
usually for opens or partial opens because it's voltage output is
lower than junction therhold.
I just fixed a Apple ][e PSU, 220uF 10V ESR in PWM circuit went
skyhigh and I didn't pull any caps to check thanks to the ESR meter
and job took 30 minutes start to end. That was last week, that
faulty cap looks same as other caps in same PSU visually.
Dick Smith Electronics sells this ESR meter and is designed by Bob
Parker.
My other kit-built DMM can't zero'ed out when I need to mesaure
very low resistances <3 Ohms, the ESR meter comes out for this.
-tony
Wizard