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I have to replace a smoothing capacitor in the "power supply" of a mains operated device. The stabilization is done with two 12 V Zener diodes in series. The supply uses a capacitor and resistor to step down the 230 V and then clamps the voltage using the Zeners. The estimated current draw of the whole circuit is less than 50mA.

The old capacitor was rated 220µF / 50V. I don't have specifications on the old capacitor. I can't even find the manufacturer "GTH". It died, losing almost all of its capacitance.

Is it acceptable to replace the old capacitor with a (low ESR) capacitor rated for 35V?

Using a 35V capacitor (for a short test) the device was functional. The expected voltage is 24 V. The reading matched that expectation.

Here is the relevant part of the schematic. The capacitor of interest is C2:

Schematic (the power supply section)

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  • \$\begingroup\$ You don't by any chance know the exact part number of the old capacitor, do you? \$\endgroup\$ – mkeith Mar 26 '17 at 16:52
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    \$\begingroup\$ Nope. I've only the PCB - thankfully with component value or model designators printed on the top. The old cap has only these markings: "-40 - 105°C 50V 220µF GTH". I doubt that the old one (older than 10 years) is even a low ESR type. @mkeith \$\endgroup\$ – try-catch-finally Mar 26 '17 at 17:00
  • \$\begingroup\$ I reverse engineered the circuit, the schematic is drawn by me. And yes, the 24V label is actually an output, not an input. \$\endgroup\$ – try-catch-finally Mar 26 '17 at 17:19
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In this case (a Zener-only stabilized mains supply) capacitor cyclic charge and discharge.

Charge through D3 when a half-wave voltage is positive. Discharge on load when a half-wave voltage is negative. Сurrent through the capacitor is essential(appreciable).

Usually, Equivalent series resistance (ESR) is inversely proportional to the voltage rate of the capacitor at constant capacity. ESR a 220µF/35V capacitor has greater value as a 220µF/50V capacitor.

Cyclic current and ESR greater value lead to greater heating of the capacitor and this reduces durable.

My opinion is replacing permissible but not justified. I do not recommend.

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  • \$\begingroup\$ Can you please explain or elaborate a bit more on "Equivalent series resistance (ESR) 220µF/35V capacitor has greater value as 220µF/50V capacitor."? \$\endgroup\$ – try-catch-finally Mar 26 '17 at 14:34
  • \$\begingroup\$ @try-catch-finally , ESR is inversely proportional to the voltage rate of the capacitor at constant capacity. It follows from the datasheet a capacitors or if you look at the table for ESR-metr​. \$\endgroup\$ – AltAir Mar 26 '17 at 16:04

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