I just use my tongue. It's kinda like checking a 9 volt battery except it sometimes tingles a bit more.
Naw, not really. I use an old Crastman screwdriver. Craftsmans work better than Kleins for some odd reason..
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I picked up a used service van a few years ago it was traded in by a retired hvac tech, he left about 50 caps and a few relays. The dealer asked if I wanted the caps and stuff that were left behind. After I agreed he then says that he caught his detail guy putting his tongue on the end of the caps.
Originally Posted by Cooked
me either . . . until 2 weeks ago
Originally Posted by smoke
You can use a screw driver but it really loads up the capacitor ie: high current and may damage it. a 20k 5watt resistor sounds fixed to me. I would go with the specified tool if I were doing it.
I'd go with a 0 ohm resister. AKA, screwdriver.
Unless you dealing with something big.
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An way to discharge a cap is to connect to a volt metered _
First, a meter should not be used to discharge a capacitor, unless you like buying new meters often.
Second, capacitors will hold a charge depending on how it is placed in a circuit.
The reason a run cap doesn't hold a charge is because the motor windings discharge it very quickly. It does have low resistance, but it also has greater mass to dissipate the charge via heat.
Unless it is placed in the circuit as a trickle charge capacitor. That is why there is a resistor on those aswell. To discharge the cap before a tech touches it. If you don't believe me, wire in a trickle circuit without a resistor across the terminals. Allow it to run, then pull the disconnect and check the voltage across the cap terminals.
A start cap uses the bleed resistor for two reasons. One, to protect the potential relay from welding it's contacts, and two to save a tech from getting shocked.
Yes, you need a 20kΩ 5watt minimum resistor.
Theory... higher wattage resistors can dissipate heat better. That is pretty much what you're asking it to do, in order to discharge the cap.
Typical caps are 370v when charging.
Ohms law tells us E/R = I, and IE = P.
So, when charging at 370V/20000Ω we get < 0.0185A. (charging is by reactance which is lower or < resistance of the resistor)
When discharging to line voltage at 230V
we get 230V * 0.0185A = 4.255W
Using a screwdriver to discharge a cap is a good idea. Not so much for the equipment, but for you. I would much rather eat up a screwdriver or cap than get shocked. In most cases, the cap should be discharged before you have time to touch it. If it's not wired properly or an issue with the circuit, it could hold a charge. I don't know any tech that carries a 20kΩ resistor in his/her pocket. So a screwdriver is a good sacrificial alternative.
Last edited by mgenius33; 11-30-2011 at 10:40 PM.
The charging part of that last post is wrong. Was just going off the top of my head.
Notice the arc flash when you short the cap. For your start relay it will experience that arc flash on every start up if your start cap does not have the resistor. As for discharging when wanting drain the energy then short it or bleed it (speaking of capacitors in residential or small electric motor applications),NOT VFDs.
I don't know anyone who carries around a 20k ohm resistor either. Definitely use a screwdriver. Your teachers just tell you that because they have to give you the most professional methods available.
How big the cap is,if small charge inside I just use screwdriver.