How to discharge caps?

Hi All,
I'm still working on the PSU and need to discharge the caps as I need to add some diodes into the circuit to reduce the voltage(10,000uF). How is this done in a safe way? Also - I presume that the diodes need to go in series between the rectifier and the caps?

 

Put a resistor across the terminals of the cap. The bigger the value of the resistor the longer it will take. A few hundred ohms to a kilohm is generally good. You can work out how long it will take from first principles.

 

To discharge caps .....can be lethal

don't do it if you not certain what your doing.

you need a ..... [I found this on the web tells it better than I can]

1. The items you'll need are:

- 2 foot of 18 gauge wire
- 1 150-Ohm 10 watt resistor (round not rectangular)
- 1 alligator clip
- 1 Multimeter type probe
- 1 foot of heat shrink


2. Strip the wire on one end and attach the alligator clip (solder it on if you can). Then cover that soldered connection with heat shrink.

3. At the middle of your length of wire, cut and strip each end. Solder in the 150-Ohm 10 watt resistor. Then cover the resistor and the solder connections with another piece of your shrink wrap.

4. On the remaining end, slip on a piece of heat shink. Attach your multimeter probe (solder it on if you can). Then shrink the heat shrink over the connection.

Now You're Ready To Discharge Those Nasty Caps...

5. Unplug your amp.

6. Pull the chassis out, being VERY Careful not to touch anything inside yet.

7. Clip the alligator clip end of your discharging probe to the metal of the chassis, making sure it's good and tight.

8. Now go through the chassis with the probe end and touch the + side of every cap you see.

9. That's it !!!

 

That's the safe approach....

DO NOT TRY THIS AT HOME... some of us have been known to use other things . My Leatherman tool has several weld marks on it...

 

Some people use a lightbulb to discharge their caps. I accidently found a way of using the chassis of a aluminium case, oh the sparks.

 

This is beginning to sound scary...I have about 36 volts in one cap and 27 in the other. Will the resistor approach work with these values or will I make a crater? I am also wondering if I really need to do this in the first place. I had about +/-39 volts originally and needed to reduce this to less than 36. I have added some zener diodes between the rectifier and the caps. Should the voltage at the caps end automatically right itself or do the caps need discharging. The circuit is not as yet powering anything.

 

If your not confident dont do it but the resistor will work. Isaac/zanash etc will prob know much more about this but you need to work out what size a resistor to use. Id have said you'd need something like 3 watt wirewound resistor upto 10k but it really needs working out.

 

well... 78V DC potential between + and -...

Q= CV = 0.78

Thus if you use a 1k resistor then (0.78/(1000x0.01)) = 0.078A

P = VI = 0.078 x 78 = 6.084W resistor required. You'll get away with less because it won't dissapate that much power for long at all.

 

There you go, I said a 7 watt wirewound resistor............I'll get my coat.

 

You only need a 1.5W resistor if you discharge the two 39V rails to ground separately.

 

I'll take it off again.

 

I just use the titainum plates in my leg, gives a nice warm feeling whilst discharging

 

I've always done it with an insulated croc clip, connect across both terminals which evens the charge across the plates, just keep the clip on till your ready to use it again. Thats what I was taught by some builders of big amps anyway.

 

Try soldering to a live mains connection, man that was something..

 

No

 

Health and Safety Note - Please do NOT try anything suggested in this thread. Electrickery is dangerous... mmmkay?

 

Screwdriver




(PS. Don't!)

Anyway Isaac, you don't have time to be posting here - get back to designing that DAC...