# Capacitors

Discussion in 'Homework Help' started by Manmeet Singh, May 25, 2009.

1. ### Manmeet Singh Thread Starter Active Member

May 21, 2008
37
0
Hey guys I decided to look through the lectures posted on this site to expand my knowledge and understanding of electronics during my time off from school and so far ive come across a few questions pertaining to capacitors

In one of the safety lectures it was saying that to reset a capacitor (discharge it) you would connect the terminals together using a wire. I understand theoretically that by doing this we cause the potential difference between the terminals to be zero and therefore cause the electric field to dissipate and therefore release its stored energy but where would this charge actually go? wouldnt this be like putting a wire between the terminals of a battery assuming the capacitor was charged which is daaangerous?

Second question has todo with a capacitors application with power source voltage spikes. I remmeber reading somewhere that capacitors are used to keep voltage values constant is this done by using a capacitor with a large time constant so that it respnds to the voltage change very slowly and therefore keeps the voltage at that point steady even if a spike occurs? Just want to make sure thats along the right lines

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
If the energy stored in an undischarged capacitor is small then the effect of placing a short circuit across the terminals will usually be uneventful.

On the other hand if the stored energy is large the result can be quite spectacular - typically a healthy spark and possibly some molten wire spraying outwards.

A better approach is to use some resistance in the "shorting" link - the energy discharge proceeds more slowly and with the right choice of resistance without any potential damage or injury.

Remember stored energy = (1/2)xCxV^2

If you have a 100uF capacitor charged to 250 Volts the stored energy is about 3 Joules - enough to be a problem if you short it out with a wire.

If you discharge the capacitor through a 1000Ω resistor is will take about half a second to fully discharge in a safe manner - just don't touch the capacitor terminals when it's fully charged to 250V!

It's often common practice to place "bleeder" resistors across high voltage capacitors in equipment to ensure they are discharged when the power is disconnected.