Discharging capacitors

[pjpoes]

Hey Guys, I got my power supply working but now I have two 22mf caps charged at 67 volts with no drain down resistors. I was wondering if any of you knew if the 8ohm sandcast 10 watt resistor I have would be good enough? Seems like 50 ohms or something would be better, as 8 ohms is going to cause a lot of current to pass through it. I was worried about connecting it and having it burn up the resistor. Let me know the best way to safely disharge the caps.

[v-bro]

Connect a 220V lightbulb?

[Hartono]

Hi Pjpoes,

8ohm sandcast 10 watt resistor I have would be good enough?

you probably will burn/damage the resistor that way.

8 ohm and 50 ohm will take up too much power from your supply.

I'll go with 10 K ohm , 1 watt resistor. Up to 100K is not unusual, but take longer to discharge the caps.

Cheers,
Hartono

[Jan Dupont]

8 Ohm seems low at 10 Watts......
When you write mF is it uF (micro) or mF (milli) Farad

[Conrad Hoffman]

You're just an innocent victim of Ohms's law. I'm not willing to dissipate any more than about 2W in bleeder resistors, so I just figure out what that would be, and live with whatever time it takes the supply to come down. I've seen a fair number of under-rated bleeder resistors crumble away over the years. In your case that would be about 2200 ohms. I run 19000 uF (I'm over 50- do people in the modern world really use mF?) caps and 40 volts, with no bleeders at all. The bias of the amp pulls everything down to a couple volts in under 15-20 seconds. I'd need pretty power hungry resistors to do much better than that. IMO, there's some value in putting Plexi covers over big caps so nothing an be dropped on the terminals- Murphy's law says, "a dropped tool will land where it can do the most damage."

[Conrad Hoffman]

Or, if you just want to bring down the voltage of a cap on the bench, sure, touch your 8 or 10 ohms sandohm resistor across it. The total energy isn't that high that it would burn up the resistors, though they'll get slightly warm. Just don't do it with the power on!

[gootee]

Using LTspice (AKA SwitcherCad III, a free download from http://www.linear.com ), I put a 22000 uF cap in parallel with 8 Ohms and set the initial condition v(vcap)=67.

The resulting exponentially-decaying current through the 8-Ohm resistor starts at about 8.35A and gets down to 1A in about 0.375 second. The power dissipation goes from about 558 Watts to 10 Watts in about 355 ms.

Resistor datasheets sometimes specify how long certain sizes of overloads can be tolerated. Most of them are better than you might think. But if you don't have the datasheet for yours, or it's not specified, you might want to play it safer and use a larger resistor.

A 50 Ohm resistor would start at 90 Watts and get down to 10 Watts after about 1.2 sec.

A 100 Ohm resistor would start at about 45 W and get down to 10W after about 1.65 sec.

A 220 Ohm resistor would start at about 20.4W (305 mA) and get down to 10W (about 213 mA) after about 1.74 sec.

A 470 Ohm resistor would start at about 9.6W (142.6 mA) and get down to 1 Watt (about 46 mA) after about 11.7 sec.

To be safer (for the resistor), I think I'd go for at least 100 Ohms, if it's a 10W resistor. But more would be better.

You should also worry about your capacitors. Too much current through them can heat them (due to the ESR), which further increases the ESR, eventually at least. If you have the maximum ripple-current specs for them, you might want to use those, too, to decide the resistor size.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

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[pjpoes]

22mf means 22millifarads, as in 22,000 microfarads. Big caps.

I ended up using a light bulb, worked like a charm. I tried a .125K ohm resistor I pulled from a broken amp, and it blew the 10 watt resistor pretty quick, with a spart at the cap itself. I figured it would be good enough as well, but I guess not.

I ended up ordered some bleed down resistors today, 13 watt 4.7kohm resistors from Vishay. I decided to take another break from my work today as its thundering, lightning, and raining quite hard, and I'm out of ring and faston connectors.

[DonoMan]

A bit overkill on the wattage, but they'll do the trick. I'd probably have just gone for 2-3W at that resistance.

[NIC1138]

I'm with Conrad... But my doubt is: Are the resistors permanently connected in your circuit, or do you just switch them in when you switch out the circuit? Because for permanent resistors, I would not ever dare to put resistors draining more than 1/100 of the power pulled by the circuit...

I wold project this way: never go beyond the rated power for the resistor, and chosing the time with T = 3*RC...

For 1/4 W resistors, that should be 1/4 = 67**2 * Rmin, R = 17k. If you have 2W resistors, 2.2k as Conrad said...

The 220V, 100W lightbulb has 484 ohms... 127 has 161.

So discharge times should be

161 10.6s
484 31.9s
2.2k 145.2s (2.4 min)
18k 1188s (20min!!)

...What time are you willing to cope with?

For a nice time of 2 secs, you would need, let's see... a 30 Ohm resistor rated for... 150W!!...

I couldn' t find any resistors in my Jameco guide taht would be near that... There is one that can handle 125W for 5 secs... but it's lmost $10!!!

So, ar v-bro says... go for the lamp if you are in a hurry!