View Full Version : What size resistor to discharge capacitor?

04-19-2005, 03:50 PM
Been doing it with a scewdriver for years.
I knew about discharging through a resistor but didn't know why till recent. (sucks when your taught the wrong way arghhhh) Found out that if the capacitor has a leak, it can explode with the arc created.
They were talking about this during the NATE refresher

Anyways... I wanna do things right.. and SAFE

SO.. whats the correct resistor I need to discharge a capacitor.

On a side note...
I contacted Fieldpeice about it.
Wished I wouldda known the resistor size first.
I was wanting them to add a resistor in the meter to discharge the capacitor before checking it.
Someone got back with me (email) and said its already built into the meter and give me a couple phone numbers to contact in their service dept to get instructs on how to do this.

I just got off the phone with them and they said they never heard of discharging through a resistor, they always did it with a screwdriver.
He asked what size resistor... and I of course couldnt' say... I'm thinking its like 20,000 ohms but not sure.

HE said he'd make note of it and see if they can add it to a future meter.

How much you wanna bet the note is going to get poofed into oblivion.

Mr Bill
04-19-2005, 04:08 PM
I know when you discharge one by hand "that ain't right" I just use the old screwdriver method still but I am sure if enough a/c guys get interested they will come out with a new tool to do it if someone can make money hey maybe a contactor with two wires coming off the same pole would work I could put it in a real neat looking case with a push button on the top. :D

fat bob
04-19-2005, 04:20 PM
Do some of them discharge themselves when you cut power? Is that what a "bleeder" is?

i b cool
04-19-2005, 04:54 PM
Some capacitors come with a bleed resistor on it, usually when there is a potential relay in the circuit. Its to prevent relay contacts from arcing when start circuit is taken out

04-19-2005, 06:12 PM
I use a 6" long resistor, its made by Klein. LOL

04-19-2005, 06:30 PM
The bleed resistor that I just looked at, across a start cap. was a 15,000 uf

04-19-2005, 06:48 PM
Any resistor will do, just if you use a little bitty one it will only work once! ;) A 10,000 ohm or larger will work fine, the bigger it is the faster it will discharge. Be careful as that electrical energy is converted into heat energy you could get burnt. Just splice one into an old set of jumpers and don't touch the resistor while you are discharging.

04-19-2005, 06:53 PM
I'm sure it is suppose to be a 2 watt resistor.

04-19-2005, 06:53 PM
Don't need to be critical, but, larger resister = slower discharge time.

04-19-2005, 06:56 PM
Originally posted by bornriding
Don't need to be critical, but, larger resister = slower discharge time. oops, you're right.

Mr Bill
04-19-2005, 06:57 PM
Originally posted by beenthere
I use a 6" long resistor, its made by Klein.

Best tool for positive and quick discharge, I can see me now when I am trying to discharge that cap. were did I put the resistor? I know it's in my truck somewhere maybe I should give Sherlock Holmes a call.:D

04-19-2005, 11:14 PM
Who needs a silly resistor...what are you one of those girly men. A real man will short that thing with his bare fingers and take one for the team.

Mr Bill
04-19-2005, 11:16 PM
Originally posted by hackmaster
A real man will short that thing with his bare fingers and take one for the team.

Yea! and that will save you some money for that jump start cup of coffee in the morning, coffee don't hold a candle to the jolt you can get from a 55/440. :D

Edmund Forsthe
04-19-2005, 11:29 PM
cool mr bill def will give you a hardon in the morning

old educator
04-20-2005, 12:00 PM
We use to have our students make up the following for capacitor discharge.
20,000 ohm, 2 watt, carbon resistor with two soldered leads with booted aligator clips.
Works like a charm and helps to prevent a nasty shock!

04-20-2005, 12:41 PM
The Copeland Electrical Handbook states a 2 watt 15,000 to 18,000 ohm resistor. This is to be attached across the terminals of ALL start capacitors. As stated earlier this protects the points in the relays.

04-20-2005, 12:48 PM
Just hand it to your helper or homeowner,whoever's standing closer.:D I was taught to use the 20,000 ohm resistor,but the screwdriver works fine for me.

04-14-2012, 03:12 PM
20,000 ohm 5 watt resistor. discharging with a screwdriver damages the capacitor

mark beiser
04-14-2012, 03:20 PM
The problem with discharging it with a screwdriver is that the current going through it for that instant is much much higher than what the capacitor is rated for, so the capacitor can easily be damaged. Using a resistor to bleed the capacitor greatly reduces that current.

In the typical system with no trickle current CCH circuit, the run capacitors for the compressor and fan motor will discharge themselves through the compressor and fan motor windings when the contactor opens, or the power is turned off.

If you have open windings though, watch out. ;)

04-14-2012, 03:50 PM
Moved to "Tech to Tech"