Why does a 50uf 350v Capacitor only read 35pf on a Chinese tester?

[Old Sparky]

About a year ago I bought one of those multi-device testers advertised on ebay and have found it very useful, but the other day I tested a dual section Can capacitor of 100 + 50uf.  The first time the 100uf cap read ca. 105uf, but the 50uf only read about 35pf.  As the can in question was made in 1959, and the radio in which it was fitted I tought 'bad cap - get a new one', but then I checked it again, and checked a modern 32 + 32uf 375v can, only to find that in all cases a reading of only a few tens of pF was displayed.  All external connections to the capacitors have been removed, and they were discharged before being tested, so the only thing I can put it down to is that the tester works from nominally 9v (actually about 8.6v measured on the PP6 battery powering the tester by the device itself), and that I connected the capacitors to the tester via short flying leads.  Would I be correct in this assumption?   I have no other means of measuring capacitance.

[ppppenguin]

The usual problem with these testers is that they over read on leaky caps. Simply because they work by charging the cap and timing how long it takes to charge. A very low reading like this (essentially stray capacitance) suggests an open circuit capacitor. Of course all the caps you've tested could be o/c. Do a cross check by charging a cap from a power supply to some convenient voltage and watch it discharge slowly via a DVM. A 32uF cap and 10M DMM will have a time constant of 320 seconds. So if no leakage it would take over 5 minutes to drop to approx 30% of the voltage you charged it to. Leakage means it will discharge quite a lot quicker.

If you charge a cap beware of connecting it to a tester before it's fully discharged. Many of these cheap testers won't like it up em.

[AlanBeckett]

Just tried a recovered 50 + 32 + 32 and it measures fine.
I suspect there is some other problem as Jeffrey says.

Joe wrote his off by connecting it to a high voltage
Alan

[boater sam]

If by chance you connect a capacitor that is charged, even at quite a low voltage, the tester will fail. They have no protection against excess input volts.
Try with some caps in the 0.01 to 0.1 uf range and see if it is still working. At least they are not expensive to replace from China.

[Old Sparky]

Thanks, all for your replies. I did, however mention, that I had checked & discharged the caps. concerned, with an analogue meter on it's ohms ranges, before connecting them to the tester, but I will check that the tester is still working, and check the capacitors as Jeffrey suggested.

[ppppenguin]

Why are you discharging the caps with an analogue meter on ohms range? Good way to wreck a meter if the cap is charged to a high voltage. Screwdriver across the terminals is fine, or maybe with a 1K wirewound resistor if you're nervous.

Actually a meter on ohms range (analogue or digital) can give you a rough reading of a capacitor. Just by looking at how long it takes to get from zero ohms (discharged) to infinity (fully charged). Good for a quick and dirty comparison with a known good part. Can also tell you a bit about leakage but only at low voltage.

[Old Sparky]

Jeffrey, I did check that there was only a couple of volts DC present on the capacitors +ve terminals, and discharged this via the DC voltage ranges on the meter (must have had a 'senior' momnent when I typed 'ohms ranges' on my earlier post!! Neither cap. had been connected to any supply for some time, in one case for many months.

[Murphyv310]

Hi.
If you're still getting an odd reading with new capacitors I'd say your tester is duff. Don't forget its Chinese and you get what you pay for! New one on the cards I'd say

[Old Sparky]

At the moment I'm only getting the odd readings with a 55 years old dual can capacitor in a Pye Piper, and what I'd term a NOS modern dual 32uf can I'd fitted to aradio a few years ago, which hasn't been powered in a long time. I'l try a few new high voltage types (10u/400v or similar) to see what readings I get with them.

[AlanBeckett]

Just as a matter of interest which Piper is it OS?

Alan