How big of a resistor do I need to drain a capacitor?

[zaph1]

I have 1280uf 200v capacitor in a circuit. The input is variable between 1vdc and 108vdc, 1vac and 120vac. Amperage is less than 1A. When I turn the variable transformer down, it takes a long time to drop the voltage. I have a 100w 100ohm resistor that drains the power. It is currently on a momentary switch that I push to drain the cap. Can I user a smaller resistor and have it on all the time to both give a small load(around 250ma) and drain the capacitor so I can change voltage quicker?

1. Caps don't care whether AC or DC, correct? Or am I gonna blow this thing up?
2. How big of a resistor do I need to drain the voltage in a couple seconds?

C. How do I find a list of my previous posts and/or threads?

 

[karesz]

Hi,
1,
Yes, you will blow up your electrolitics_or you must rectify the input voltages_everytime...
3,
write your name into the searcher of EdaBoard...
K.

 

[zaph1]

If I drain the capacitor first, can't I recharge it with either AC or DC?

 

[IanP]

Depending on frequency (Zc=1/wC) a capacitor is like a short circuit for AC ..
It can only be “charged” by DC ..

Have a look at the attached discharge graph ..
It shows how to estimate the value of a discharge resistor ..
More details are here:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capdis.html

IanP

 

[zaph1]

OK. NO AC current in the caps. Got it. However, I have no clue about the equations on the link. Would a 5w resistor work for a load, or should I just plug in a light bulb?