Bleed down resistors for capacitors

[garagefela]

Hi all,

I am choosing to place a bleed down resistor over my capacitors, size of the power supply is 12.5A and 45VDC.

I have a 100 Ohm resistor rated at 5 watts, which will draw .45A, what I cant work out ( DUH!) is what the voltage drop will be.

Power supply will be running 3 stepper motors at 2.53A and 4.8 volts each.

If this 100 Ohm resistor is not the best one to use please suggest a better option.

Pretty simple request for all those out there that seem to have good knowledge dribbling out of every pore on thier bodies

[HomeCNC]

I use a 1.2K Ohm 5 watt on my 20 amp 36 VDC power supply and that takes about 10 seconds to bring it down to 0 VCD.

[sendkeys]

[garagefela]

Hi Guys,
Thanks for the replies, yeah 100 Ohms was a bit much I suppose, but good for warming the hands on cold days!!

Sendkeys you had indicated that there would be little or no volt drop, just wondering how this was concluded. Not that I am doubting you, Its just that I am trying to work out the theory behind it.
Its nice to know whats going on exactly when you do something to a power supply.( for me anyway)

Cheers all M

[marvinstov]

Why can't you hook it up so that the resistor is not in the circuit continuously? Can it be rigged so that when you turn off the power switch it connects the resistor to the capacitor to discharge it? Or does it have to be connected directly to the capacitor at all times?

Marv

[garagefela]

Gday Marv,
The idea you suggest is a beauty, attaching the resistor to the power switch never crossed my mind. And its not all that far to cross
All I wanted the resistor for, is to bleed the charge out of the capacitors, how that is actually done does not really bother me.
I just dont want to accidently touch the capacitors after 1 minute, 1 hour, 1 day whatever, and have to use my special naughty word!

Cheers all M

[marvinstov]

Well I was just thinking, I use a 110 ohm, 1 watt resistor to bleed mine when I need to work inside the case. I just hold it in my fingers and touch it to the capacitor terminals and it bleeds the resistor in about 3 or 4 seconds. My power supply is 30 volts and about 18 amps. The resistor barely gets warm but if it was connected all the time, it would burn up in less than a minute. I just thought you might be able to use a smaller resistor since it would only have current for a very few seconds and wouldn't drain any power when you actually had the power supply on.

Marv

[sendkeys]

[cliffhiker]

I saw an article on web about using the other half of a DPDT switch to kick in the resistors to drain caps when turned off..

I've used it several times in projects... works great!!!....
I'll see if I can find the link and post...

[cliffhiker]

ahhh found it...

PDF http://www.reed-electronics.com/ednm...es/62101di.pdf
html http://www.reed-electronics.com/ednm...bestofdi_power

btw... scan the other design ideas.... edn great source for some neat ideas...

[sbrpollock]

The voltage "drop" accross the resistor will be whatever the voltage is across the capacitor because it is parallel with it. (No matter what resistor value you choose)

There is really no reason to use such low value resistors to bleed power supply capacitors. Do you really "Need" your caps to be discharged within seconds?

You can choose a resistor to discharge your caps at whatever speed you want.

Here is the math:

T (In Seconds) = Time Constant = R * C R must be in Ohms and C must be in Farads

After five times the time constant the capacitor will be discharged.

Example #1:

1000 MicroFarad capacitor and 1000 Ohm resistor

.001 Farads X 1000 Ohms = 1 Second (Time constant is 1 Second)

5 x 1 Second = 5 Seconds (Cap will discharge in 5 Seconds)


Example #2:

5000 MicroFarad capacitor and 2000 Ohm resistor

.005 Farads X 2000 Ohms = 10 Seconds (Time constant is 10 Seconds)

5 x 10 Seconds = 50 Seconds (Cap will discharge in 50 Seconds)

Just remember to use the units of Farads and Ohms in the equations.

Unless I need my caps discharged quickly for some specific reason, I use 10,000 (10k) Ohm resistors on most of my power supplies and let the discharge take a couple minutes.



Incidentally:
When the discharge takes place, it is not constant. What I mean is that if a specific R/C combination has a discharge time of 5 seconds, 63.2% of the voltage is bleed off in the first second alone, followed by 23.3%..........etc. In the last second, only .4% of the voltage is left to discharge.

The above examples are true regardless of the voltage or current supplied by your power supply.

[marvinstov]

Also with the proper voltage regulation you could power a cooling fan (if you need one) which would bleed the capacitor for you. That way you wouldn't waist the power while running or shutting down.