# Why is resistor in parallel with capacitor needed in capacitor bank?

Capacitor bank in power supply compensates for when input voltage is shortly disconnected (50ms lets say). Do capacitors in this setup (between input and the output of power supply), get initially charged and then remain charged until input power gets disconnected? After that, do capacitors discharge their energy at the output load. If the output load is there to discharge capacitors, why would resistors in parallel to capacitor bank be needed?

A typical DC power supply (like you might find in your phone charger) will look something like this (probably with more protection devices and other extras). The AC will be rectified by a bridge rectifier and smoothed by a capacitor to create high voltage DC. Then a power supply IC will quickly switch the current through a transformer on and off to step down the voltage to say 5V. The output is once again rectified and smoothed, then fed to your phone:

simulate this circuit – Schematic created using CircuitLab

The resistors across the capacitors (especially R1/C1 because they are at high voltage) are there to discharge the capacitors when the device is unplugged, so that if someone touches the output they won't get shocked. Because the capacitors are there only for smoothing, and not to allow the device to function for a while after power is removed, having the capacitors discharge quickly when the device is unplugged does not hinder their operation and improves safety, so they're included almost everywhere.

Note: I have omitted several things from the schematic, such as a power rail for the converter chip, feedback windings, or flyback configuration, as they do not help to illustrate the functioning of the capacitors and discharge resistors.

• For series capacitors, particularly super/ultracaps, sometimes a parallel string resistors is used to form a voltage divider across the caps and ensure they share voltage and live up to their ratings too. @C_Elegans for the discharge resistors, is there any standard drain time used for sizing these resistors in electronics? In electrical we're limited to 1 min for up to 750V and 5 mins for above 750V, but our capacitors are basically never intended to store charge after a device is off. A rule of thumb even would help greatly.
– K H
Aug 30 '18 at 2:20
• If capacitor bank - capacitor C2 in your schematic is connected to a load, do we actually need the discharge resistor R2? When whole unit is turned off, capacitor C2 will keep the load alive for some time until it gets discharged. My point is, if C2 will get discharged at the load, so why is then R2 needed?
– DenR
Sep 1 '18 at 1:11
• R2 is needed for when the load isn’t connected to anything. If you can guarantee that there will always be a load then the resistor isn’t needed, but many power supplies can’t make that guarantee Sep 1 '18 at 12:30
• @C_Elegans, Thanks. That is what I was wondering about and wanted to confirm.
– DenR
Sep 2 '18 at 12:51