# Decoupling capactor for DC voltmeter

I have a DC measuring digital voltmeter. The power for the voltmeter is coming out of a charging rectifier. The frequency out of the charging rectifier is anywhere between 20 and 60Hz. Operating voltage of the voltmeter is about 12V.

The voltmeter consumes max 30mA. What capacity is required for the decoupling capacitor to supply the voltmeter to continue working during power dropouts?

• As a minimum, you need to specify the voltage requirement for your voltmeter as well as how long do you want it to work after a power outage. This will allow the calculation of how much energy must be stored in the capacitor. Combining the energy requirement with the voltage would then allow the calculation of the needed capacitance. – Barry Aug 21 at 19:21
• How is the 'about 12V' derived from the 'charging rectifier', and what is it charging? – Bruce Abbott Aug 22 at 7:31
• @bruceabbott About 12v is because if the generaror which delivers the ac is running not fast enough, voltage will be below 12v. The charging rectifier tops voltage at 13.8V for charging a lead acid battery. That's also the reason why frequeny can vary. – JohnDoe Aug 22 at 16:06
• Are you trying to power the voltmeter from the charger output? Will a 12V battery be connected at the same time? – Bruce Abbott Aug 22 at 23:04
• @bruceabbott I see where you are aiming at. When a battery to charge is connected, the volt meter reading is fine as there are no power spikes. Unfortunately most of the time no car battery will be attached that's why I want to get rid of the flickering / malfunction of the volt meter reading. – JohnDoe Aug 23 at 4:10