Help in Capacitor Discharge

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keiththevp

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I have a 9800uf 200v capacitor in a power supply that I would like to discharge in 1 second or less. I am connecting a bleeder resistor or a series of them to the cap to discharge but I can't figure out which one to use. I know that I can only have a max resistance of 45 ohms to get the cap down to 20v in about 1 sec. Also I have 5 10 ohm 10 w ceramic resistors that I would like to use if possible. What would your recommendation be for the proper resistor or is there a way to wire the ones I have?
 
hi,
To what voltage level do you want to discharge down to in 1 second.???
 
keiththevp said:
Down to 20v
Click to expand...

Hi
Look at this family of discharge curves, 10R to 100R in 10R steps.

EDIT:
Attached the Power curves
Connect 4, 10R 10W in series
 

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That graph just shows exactly what I posted that at about 40R the cap would be down to 20v in 1 second. This info I already know. What I am asking is how I would wire my 5 10 ohm resistors to accomplish this. I know I can put 4 in series to get 40R however I don't know if the resistors can handle the 5 amp surge at the start of the cap discharge.

So will the resistors be able to handle the 5 amp surge?

If not can you recommend the part number or specs of a resistor that will be able to safely discharge the cap without failing?
 
Are you attempting to short out the cap to work on the circuit or as a permanent safety discharge feature? If the latter, one normally expects to see a value much higher value of resistance that will drain the supply at a much slower rate (like about 5% to 10% of the power supply’s rated current) taken directly to ground. That is why one will normally give a power supply 30 to 60 seconds after removing power before touching it.

If the former, then the size is relatively unimportant because heat is generated for such a short time (about .7 second in your case). While a 5 watt rated resister is required if it was permanently connected, you can probably get away using a half watt resister since you will probably want to use some type of handle that will also dissipate some of the heat.
 
I made a dual output power supply that puts out 150v dc or 12v dc. Problem is when you switch from 150v to 12v those caps are charged at way to high of a voltage so I am making a circuit to completely dump the caps when ever you switch sources or turn the unit off. Since it only takes 2-3 sec to switch from HV to LV it needs to dump fast.
 
hi Keith,
If self heating of the 10R's becomes a problem, if you switch frequently, fasten the 10R's to a small aluminium plate.
I am assuming that they are the rectangular, white bodied type.

EDIT:
Look at this current discharge plot, thru 40R, choose a dump switch contact to suit.
 

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Since the resisters are only connected during the dump phase (I’m assuming that you intend to use something like a 3-position rotary switch to ensure proper timing), then power dissipation should be only a minor issue. ½ what should work. If I was building it, I would use 1 watt since I tend to over engineer everything.
 
Thank you everyone for all your help!!!

ericgibbs: I use a 40amp 300v dc SSR to switch dump load on and off, so I should be good. Also they will all be mounted to a large heat sink since I made the case for the PSU out of heat sinks.

EternityForest: yes the two switches that control the HV and LV modes can not be activated unless the opposite side is off. So if I have the LV side switch in the on position it will only actually turn on if the HV switch is in the off position.

Gary B: I am using two DPDT switches and some relays and an SSR to control instead of a 3-way rotary, sounds more difficult then it had to be but that is what I had. And as far as timing I am not flash gordon so I figure it would take far more then 1 second to turn off one switch, switch output cables(since no 1 load I use will need HV and LV), and then turn the other on.

Keith

P.S. I am really glad I gave this forum another try. The first time I came here for help was about this power supply and I was told it is not possible and will blow up or worse!! Well, guess what, it works flawlessly. So again thank you everyone for all the help.
 
Sorry I was thinking of the 380v ones I have that are AC, the DC ones are only 220v still way more then I need. Here is a link

**broken link removed**
 
I calculated that resistance for getting 20V after 1 second would be 0,093 ohms using this formula:
 

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