I'm sure that the Peak unit (and others) discussed recently are very nice, but could anyone point me to an open source ESR tester, or at least the principles required to build one.
For one, I want to save money but at the same time, I really like to make my own kit where practical.
Yes, I could go Google, but sometimes it's nice to get real recommendations from real people ;-)
Just hook a scope to the cap and hit it with the fastest rise step you can muster. Keep it below half a volt amplitude so you don't turn on junctions. Stated another way, build your own TDR.
If you know the source voltage and source impedance, you can easily calculate the ESR from the size of the initial step. You can also calculate ESL from the overshoot and capacitance from the time constant.
Using this method, you know exactly what you're dealing with. You're not relying on some readout number that may or may not reflect what's really going on.
mike
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Do you have access to Television & Consumer Electronics magazine? (This used to be Practical Television back in the old days). In Dec 04
- Jan 05 issues they described an analog reading ESR meter designed by Alan Wilcox. This design operates in a similar manner to Capacitor Wizard but is not nearly so complicated and it is built on Vero stripboard. The only problem is that it specifies a Monacor PM-2/100uA meter movement
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which is not available in Australia. According to my research this meter is almost physically identical to the SEW
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ST645 and this meter may be available in Australia, and hopefully the meter scale drawing in the article will fit without alteration.
Contact me in private if you need more info. (remove SPAMEX)
In "PopTronics" (USA) July 2001, pages 25-28 there is a build it yourself design that uses a few common parts and has full protection. It claims to test in circuit from 1 uF on up.
You may find copies at larger libraries under "Popular Electronics", "PopTronics" was a later name.
What do you actually use it for? Seems that every time I want to know ESR it's in a power supply or on a motherboard with a dozen hooked in parallel thru ground/power planes. I've had zero success finding bad caps.
Out of circut, yes, but if you're gonna go to the hassle and board risk of removing the cap, might as well replace it. Half the time, have to pull the leads out and remove 'em one at a time from the ground plane. Holes are just too tiny to suck out the solder while the lead is in there.
Over the decades, I've tried 35pS TDR to try to isolate caps. Tried spectrum analyzer with tracking generator to sweep power/ground planes while shorting caps to look for change.
It's rare that there's only one cap, and with those, you can just look for ripple while it's operating.
So what's the magic method for finding high ESR caps in-circuit in a typical situation of parallel caps? mike
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I used a shunt ohmmeter cct. with the meter signal rectified and driven by a twin-T osc at 33KHz driving an LM386. Experiment with series resistor to get the meter range, and calibrate with resistors. Mine reads 0 to 75 ohms at about two-thirds deflection.
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