Capacitor type and polarity? - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Uncle Bulgaria · 11th Oct 2025, 2:35 pm

Actually I think I've got that wrong and it'll be rather less - more like 30mA maximum since the metering starts on the 20mA scale with 10k in circuit after the capacitor under test, that puts around 20k between the DC supply and ground with the capacitor in the middle if the capacitor were short, and the voltage up at 630V. I doubt it could supply such current!

I think the 5W ones before the capacitor are to protect the power supply more than limit current to the capacitor. If there is mA leakage in a capacitor the reported voltage creeps up gradually as it can't cope at full whack, so I have not seen the display report more than a few mA; I don't think double figures even, so probably it remains well below the Philips recommendation you quote.

Maarten · 12th Oct 2025, 1:39 am

The punch throughs are very short in duration and I doubt they could be seen on any meter other than a recording oscilloscope.

On the other hand, it might be argued that the entire energy stored in the capacitor is available to damage the foil around the punch through, so maybe the current limiting is indeed only relevant for the charge current.

Uncle Bulgaria · 12th Oct 2025, 7:56 pm

Slightly off-topic...I have some capacitors here that are possible replacement candidates, and no bridge or capacitance meter to hand. They have only lost a couple of volts over 24hrs of being charged to working voltage, so self-leakage is low. Can they behave like this even if their capacitance value has changed irrevocably from age-related drying out, or does charge-retention imply they're OK in other parameters?

11th Oct 2025, 2:35 pm	#21
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,936	Re: Capacitor type and polarity? Actually I think I've got that wrong and it'll be rather less - more like 30mA maximum since the metering starts on the 20mA scale with 10k in circuit after the capacitor under test, that puts around 20k between the DC supply and ground with the capacitor in the middle if the capacitor were short, and the voltage up at 630V. I doubt it could supply such current! I think the 5W ones before the capacitor are to protect the power supply more than limit current to the capacitor. If there is mA leakage in a capacitor the reported voltage creeps up gradually as it can't cope at full whack, so I have not seen the display report more than a few mA; I don't think double figures even, so probably it remains well below the Philips recommendation you quote.

12th Oct 2025, 1:39 am	#22
Maarten Dekatron Join Date: Dec 2007 Location: Haarlem, Netherlands Posts: 4,703	Re: Capacitor type and polarity? The punch throughs are very short in duration and I doubt they could be seen on any meter other than a recording oscilloscope. On the other hand, it might be argued that the entire energy stored in the capacitor is available to damage the foil around the punch through, so maybe the current limiting is indeed only relevant for the charge current.

12th Oct 2025, 7:56 pm	#23
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,936	Re: Capacitor type and polarity? Slightly off-topic...I have some capacitors here that are possible replacement candidates, and no bridge or capacitance meter to hand. They have only lost a couple of volts over 24hrs of being charged to working voltage, so self-leakage is low. Can they behave like this even if their capacitance value has changed irrevocably from age-related drying out, or does charge-retention imply they're OK in other parameters?