Capacitors in DC circuit

John mckenney's picture

Offline

Joined: May 12, 2019

Replies: 1
Capacitors in DC circuit
May 23, 2019 - 8:04am

If a capacitor blocks dc from flowing till removed from power, why on earth would you ever use it in a battery run circuit? And yet I see capacitors in schematics all the time for projects like simple audio amplifiers using only transistors all the time. Am I missing something? I get it evens the flow of ac while filtering out dc “noise”, but that would seem to be negative if your power source is a battery 

Aswinth Raj's picture

Offline

Joined: Aug 16, 2016

Replies: 923
Think of them being in parallel
May 24, 2019 - 7:15am

This is one common question which most people consider. It is true that when a capacitor is placed in series with a DC power output it gets charged up and they stop outputing any more current acting like a open switch. But a capacitor is mostly never used like this in a DC circuit. Think of it being used in parallel. One example that I would give you de-coupling or transient suppressor capacitors. 

You can commonly find a capacitor in parallel to the output side of any voltage regulators like LM7805 etc. These capacitors are there as de-coupling capacitor to avoid any output voltage spikes. Say when a 7805 regulating 5V gives out 8V spike due to some reason, this 8V spike should not reach the output as it might damage the 5V sensitive electronics. To prevent this from hapening a capacitor is used in parallel. This parallel cap will provide a low impedance path for the 8V spike voltage to reach the ground rather than get out through the high impedance output, thus filtering the spike voltage before reaching the output.

To know how they are used in amplifiers you have to gid in a bit more deep, but to give an idea they are charged and discharged acting a small power source to contorl a switch like transistor. You can check about the multi vibrator circuits to get an idea about this. 

It is also true that if the device is battery powered (pure DC) you will not need any filtering capacitors in a DC circuit since batteries do not provide voltage spikes.