Resistor size for discharging caps-is there any agreement

[Normando]

I have been following some posts and reading the archives about this topic and there appear to be ohm and watt numbers all over the place as to what is a good resistor value to discharge a cap. Is there any agreement on what consistutes a good resistive load to discharge a cap safely? Also occasionally I see the word ohm, where the "O" is capitalized. Is this a shorthand for a higher strength measurement or meaningless. THanks . N

 

[Zilch]

Why is this even an issue for loudspeakers?

"Ohm" is a person's name. I suppose there is some standard, but it has nothing to do with the magnitude.

 

[Normando]

I am guessing from what you say that this is not an issue for loudspeakers just for larger caps in power supplies, tube and solid state gear. My question was obviously an ignorant one. Sorry for lowering the level of discourse. N

 

[Zilch]

No, I just wanted to make the point that, generally speaking, the voltages involved in loudspeaker crossovers, for example, are low enough, under 10V, typically, for it not to be an issue. Also, audio is AC, not DC. The primary purpose of discharging is to lower the stored charge below a dangerous voltage, 60V DC or below.

The numbers are all over the place because the appropriate resistance to use for discharging a cap depends upon the size (capacitance) of the capacitor and the charge voltage. A higher resistance will ultimately discharge any capacitor; it just occurs at a slower rate according to the time constant of the R/C combination.

A lower resistance will discharge it faster, but then the power becomes a consideration, as that must be dissipated in the resistor. Too low a resistance, and that becomes dangerous, too. Very low resistance like a screwdriver blade, for example, and we've all likely experienced what happens.... :

 

[toxcrusadr]

I confess I've used a screwdriver blade on an electrolytic just to make sure it's discharged after turning off the power. I've never even seen a spark doing that, so either they're already discharged through the circuitry or it's not enough to make a spark. What's the dowside of doing that?

 

[EchoWars]

Direct shorting can cause large surges of current that can damage the cap. Discharge with a resistor. I have a big 8 ohm 50W resistor that I use to discharge caps.

 

[clydeselsor]

I confess I've used a screwdriver blade on an electrolytic just to make sure it's discharged after turning off the power.

Me too!!

 

[markthefixer]

I confess I've used a screwdriver blade on an electrolytic just to make sure it's discharged after turning off the power. I've never even seen a spark doing that, so either they're already discharged through the circuitry or it's not enough to make a spark. What's the dowside of doing that?

When it IS charged, it'll take a chunk out of your screwdriver. I got tired, as a kid, of destroying my screwdrivers.

Like EW said, it hurts the cap as well.

As to what resistance to use, it's ohm's law, and it somewhat depends upon the wattage of the resistor and the voltage the cap is charged to, unless you do like EchoWars and use a HUGE wattage resistor that can handle the surge without being hurt.

BUT that surge through 8 ohms is FAR less than the surge through the screwdriver.

There are special precautions taken in manufacturing caps for very high discharge currents. That's why they're much more expensive.