Rich D.
Active Member
...that's supposed to be sarcastic. Theoretical capacitors are simple. Real-world capacitors are just nuts. I'm posting today about Electrolytics.
Recently, I've been worried that my capacitor meter was going out of calibration. Values seemed to read much too high. Smaller values measured as expected, but the larger values read 2X up to 5X or more too high. Two different meters show the same reading issues. I soon discovered the difference turned out to be whether they were electrolytic or not. Ceramic, Mylar, Poly caps all measured as expected, and accurately reflect the values from my capacitor decade box. "Some day I'll get this meter fixed" I thought.
The other day I tested a few big caps before using them in a circuit, only to discover that they measured nearly perfectly. So I started measuring a lot of the various capacitors in my collection. My collection consists of brand new parts for more critical and high-end circuits, and also a rather large collection of older "harvested" parts from old TV's, stereos, microwaves, VCRs, etc. By and large, all the new stock parts measured good, and the old ones bad.
At first I was getting worried that all of my older parts were faulty - either damaged by overheating when removing the solder or damaged in-circuit that caused the device it was harvested from to be discarded in the first place, or just plain too old. I assumed of course that they were very leaky, causing the excessively high readings. Only a few very old ones measure good. 90% bad caps? Can't be. High-ohm measurements from the DMM showed no significant leakage - typically measured in the MegΩ range.
So I took a couple of 33μF caps that were known to be much older and measured maybe 100 or 200μF (the measurement drifts down slowly from say 150...120 depending on how long I observe), and soaked them for a while (10-20 min) at 15 volts DC (they were rated 16V). The power supply didn't show any leakage current (<10mA), and they didn't get warm at all. After that and a discharge via shorting they still measured too high. When I removed the 15-volt power, they seemed to hold the charge for minutes. With no DMM attached, they sat for a good hour and held perhaps 90% of their charge. For all practical purposes, these capacitors are good.
So barring the typical guesses of leaky, damaged, high ESR or high inductance-type excuses, or me mixing up the polarity (I didn't), does anybody know what is going on with these older capacitors? I'm pretty sure they are mostly good caps, or are they? What could cause them all to measure too high? If they indeed are good, what will it take to automatically measure them properly? And if bad, exactly what is wrong with them? Could it be only because I have cheaper hobby-grade meters? It just doesn't add up. Baffled.
Recently, I've been worried that my capacitor meter was going out of calibration. Values seemed to read much too high. Smaller values measured as expected, but the larger values read 2X up to 5X or more too high. Two different meters show the same reading issues. I soon discovered the difference turned out to be whether they were electrolytic or not. Ceramic, Mylar, Poly caps all measured as expected, and accurately reflect the values from my capacitor decade box. "Some day I'll get this meter fixed" I thought.
The other day I tested a few big caps before using them in a circuit, only to discover that they measured nearly perfectly. So I started measuring a lot of the various capacitors in my collection. My collection consists of brand new parts for more critical and high-end circuits, and also a rather large collection of older "harvested" parts from old TV's, stereos, microwaves, VCRs, etc. By and large, all the new stock parts measured good, and the old ones bad.
At first I was getting worried that all of my older parts were faulty - either damaged by overheating when removing the solder or damaged in-circuit that caused the device it was harvested from to be discarded in the first place, or just plain too old. I assumed of course that they were very leaky, causing the excessively high readings. Only a few very old ones measure good. 90% bad caps? Can't be. High-ohm measurements from the DMM showed no significant leakage - typically measured in the MegΩ range.
So I took a couple of 33μF caps that were known to be much older and measured maybe 100 or 200μF (the measurement drifts down slowly from say 150...120 depending on how long I observe), and soaked them for a while (10-20 min) at 15 volts DC (they were rated 16V). The power supply didn't show any leakage current (<10mA), and they didn't get warm at all. After that and a discharge via shorting they still measured too high. When I removed the 15-volt power, they seemed to hold the charge for minutes. With no DMM attached, they sat for a good hour and held perhaps 90% of their charge. For all practical purposes, these capacitors are good.
So barring the typical guesses of leaky, damaged, high ESR or high inductance-type excuses, or me mixing up the polarity (I didn't), does anybody know what is going on with these older capacitors? I'm pretty sure they are mostly good caps, or are they? What could cause them all to measure too high? If they indeed are good, what will it take to automatically measure them properly? And if bad, exactly what is wrong with them? Could it be only because I have cheaper hobby-grade meters? It just doesn't add up. Baffled.