Should an electrolytic capacitor have measurable resistance? - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

stuarth · 4th Dec 2017, 8:20 pm

Most analogue meters with a non linear ohms scale (i.e. Most analogue meters!) have an equivalent circuit which is the ohms battery in series with a resistance equal to the centre scale value of the chosen resistance range, and the test terminals, i.e. the unknown resistance in series.

Passive meters (such as an AVO 8) show the current flowing in this circuit, so an open circuit at the test terminals shows zero deflection, a short circuit shows full scale deflection (with an adjustment pot to make it so), and a resistor across the terminals equal to the half scale value gives half scale deflection.

The voltage actor the unknown resistance (an electrolytic in the OP's case) is proportional to the meter deflection, with the battery voltage (15v) at full scale. When you watch the "resistance" of the capacitor rise with time, you are actually seeing the voltage across the cap increase as it charges up.

Stuart

Mooly · 4th Dec 2017, 9:10 pm

Can I also make another point... the high (15 volt) terminal voltage the AVO uses on its high ohms ranges can in certain circumstance degrade the noise figure of certain sensitive RF devices if applied in such a way that it causes reverse breakdown.

It's non destructive and the device will appear 'normal' but possibly with an impaired noise figure.

I have no experience of the 'magic box' testers that check all and everything, but again, depending on what appears at the test terminals the user should be aware of possible device degradation if certain conditions are met.

4th Dec 2017, 8:20 pm	#21
stuarth Heptode Join Date: Feb 2010 Location: Heysham, Lancashire, UK. Posts: 669	Re: Should an electrolytic capacitor have measurable resistance? Most analogue meters with a non linear ohms scale (i.e. Most analogue meters!) have an equivalent circuit which is the ohms battery in series with a resistance equal to the centre scale value of the chosen resistance range, and the test terminals, i.e. the unknown resistance in series. Passive meters (such as an AVO 8) show the current flowing in this circuit, so an open circuit at the test terminals shows zero deflection, a short circuit shows full scale deflection (with an adjustment pot to make it so), and a resistor across the terminals equal to the half scale value gives half scale deflection. The voltage actor the unknown resistance (an electrolytic in the OP's case) is proportional to the meter deflection, with the battery voltage (15v) at full scale. When you watch the "resistance" of the capacitor rise with time, you are actually seeing the voltage across the cap increase as it charges up. Stuart

4th Dec 2017, 9:10 pm	#22
Mooly Octode Join Date: May 2011 Location: Lancashire, UK. Posts: 1,351	Re: Should an electrolytic capacitor have measurable resistance? Can I also make another point... the high (15 volt) terminal voltage the AVO uses on its high ohms ranges can in certain circumstance degrade the noise figure of certain sensitive RF devices if applied in such a way that it causes reverse breakdown. It's non destructive and the device will appear 'normal' but possibly with an impaired noise figure. I have no experience of the 'magic box' testers that check all and everything, but again, depending on what appears at the test terminals the user should be aware of possible device degradation if certain conditions are met.