# Capacitor Safety Threshold Voltage

#### mcgyvr

Joined Oct 15, 2009
5,394
Just for my reference...
What is typical or how to determine the "Safety Threshold Voltage" for a larger aluminum electrolytic capacitor for proper bleeder resistor sizing?

I'm assuming max. 50V at this point.. but wasn't sure if there is a mathematical formula that I should know.

For reference i have 2 x 23,000 uF 75V aluminum electrolytic caps in parallel with a 54V charge voltage. I'm planning on using a 700 Ohm 10W resistor as the bleeder. (Should discharge in 1.25 seconds or so I believe)

#### MrChips

Joined Oct 2, 2009
20,846
I'm not quite sure what the value of the bleeder resistor has to do with the capacitor voltage.

You need to make sure the voltage rating on the capacitor exceeds the maximum charge voltage plus a safety margin. A rating of 75v for a 54V circuit is reasonable.

#### mcgyvr

Joined Oct 15, 2009
5,394
That welwyn calculator has a field for "safety threshold voltage" which is the voltage at which the capacitance is considered safely discharged.
So my question is still how to determine when a capacitor is considered safely discharged..

#### MrChips

Joined Oct 2, 2009
20,846
To safely discharge capacitors with those ratings, you have to leave the bleeder resistor connected for a very long time (indefinitely), much longer than the RC time constant. Capacitors with no discharge path will regain a voltage potential from residual dielectric effects.

#### mcgyvr

Joined Oct 15, 2009
5,394
The bleeder resistor WILL be always connected across the capacitors.
My question still stands... at what voltage threshold is a capacitor of that value considered "safely" discharged.

According to the calculators with the resistor I specified above the caps will be below 50V after only 1.25 seconds.. But I want to know how long it will take (using that same resistor) to bleed that cap to a "safe" level. The calculator has a note that says 50V is the "typical" value entered for that "safety threshold voltage"... However I don't know what the real voltage needs to be for that cap to not be able to cause a shocking experience. I don't want to just enter the typical 50V if its not safe with 50V on it..

#### MrChips

Joined Oct 2, 2009
20,846
Off the top of my head, I would say six times the RC time constant = 3 minutes.

#### mcgyvr

Joined Oct 15, 2009
5,394
Off the top of my head, I would say six times the RC time constant = 3 minutes.
huh? so you are saying that those caps would be considered safe when they are discharged to below .005V ???

#### MrChips

Joined Oct 2, 2009
20,846
We all know that it is not the voltage but the current that presents a shock hazard.
One RC is 32 secs, 4RC is two minutes. So lets wait for 3 minutes to be on the safe side.

#### thatoneguy

Joined Feb 19, 2009
6,359
The reason for 3 minutes is when you get to large capacitors that have held a voltage for a very long time, when the load is removed and the cap is discharged, the cap will still "bounce back" to some voltage over time, usually minutes. So it is safer to hold the terminals at an equal value.

Disconnect the caps after they've been powered up for a year, discharge them, wait 5 minutes, and measure their voltage again. It's surprising.

Same thing with CRTs. You could short out the flyback to ground, but it'd have 1,000V charge back on it in under an hour, that's why working with CRT you need to keep the Anode grounded while working on it (when the power is off).

If a capacitor is passing AC/blocking DC, then it never gets a charge, all AC is passed, so capacitors in this usage do not self-recharge.