Hi, I was in the process of reforming some capacitors - wanted to check what is the safe level to discharge them down to without causing permanent damage, so as to measure capacitance using a multimeter?
Im using a 100 ohm 100 watt resistor to discharge them ( 10,000 uf, 100 V caps) down to almost 0 volts.
Is it ok to discharge them all the way to zero volts? I think I might have damaged them by discharging them to zero or close to zero by leaving them shorted using the resistor for a few minutes - as they are reading lower ratings than where I started 🙂
Im using a 100 ohm 100 watt resistor to discharge them ( 10,000 uf, 100 V caps) down to almost 0 volts.
Is it ok to discharge them all the way to zero volts? I think I might have damaged them by discharging them to zero or close to zero by leaving them shorted using the resistor for a few minutes - as they are reading lower ratings than where I started 🙂
I used the 100 ohm resistor to drain it down to about 0.3V, and towards the very end, just for a second or two I shorted the leads ( at 0.3 V) to drain it all the way close to zero. ( Another question is is it even necessary to have them. that close to zero to measure capacitance accurately). They have definitely seen a drop in both capacitance and current capability - when I did it the first time, the resistor would have a blue spark when I connected the leads, and a small pop, but now that's mostly gone.
To measure the cap, they must be fully discharged. The measurement is usually made by the meter applying a constant current to the cap and charging it up to some specified voltage. Cap is then calculated by the time it took. Large caps such as yours can't measure that large of a cap. My handheld tops out at 20,000uF. I've got another that tops out at 1000uF.
Capacitors are not like batteries, you can fully discharge them to zero.
Before measuring, keep them shorted for a few minutes after they reach zero, as they may recover somewhat when open circuited.
Before measuring, keep them shorted for a few minutes after they reach zero, as they may recover somewhat when open circuited.
One thing to maybe take note of is that if you have HV capacitors, sometimes they can recharge themselves after being shorted out and seemingly fully discharged. In some cases they can recharge to lethal voltages. For such capacitors it is common to leave a shorting wire attached across the terminals when they are not installed in a circuit.
First time years ago?
You need them down to zero, to time how long it takes from zero to full. Bigger the cap, the longer the charge time. Sometimes timing out, if beyond the multimeters range. Least that is how both mine appear to work.
You need them down to zero, to time how long it takes from zero to full. Bigger the cap, the longer the charge time. Sometimes timing out, if beyond the multimeters range. Least that is how both mine appear to work.
It does not do any harm, but I don't see why you want them discharge to zero. It definitely does not give you a more accurate measurement. Discharge to 37% of the original voltage. Then you have the RC time constant.
If you use an analog meter, don't forget to calculate in the meter resistance.
If you use an analog meter, don't forget to calculate in the meter resistance.
He wants to use a multimeter. I just noticed the size of these things though. 10,000uf is a lot for a multimeter. One of mine has a high range of just 200uf. The OP may have to measure discharge rates, as you say