Resistor to discharge capacitor

Cirkit

Oct 28, 2015
152
What is a suitable resistor to safely discharge a large main filter capacitor ~400V found in SMPSs?

Would a 220Ω 2W resistor be too small in value?

bertus

Moderator
Nov 8, 2019
3,089
Hello,

Determine the power in the resistor:

Bertus

Cirkit

Oct 28, 2015
152
Yes, I did calculate it to be around 700W but that would be continuously. For a short mS discharge, surely a lower wattage resistor would suffice?

bertus

Moderator
Nov 8, 2019
3,089
Hello,

Depends. When you want to have a (bleeder) resistor constant accross the capacitor use a 100K resistor of 5 Watts.

Bertus

Cirkit

Oct 28, 2015
152
Ah no sorry for the misunderstanding. I meant when repairing, just discharging the capacitor so that the board is safe to work on.

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,456
A 220 Ω resistor may be a bit on the low side. When discharging a 47 µF capacitor from 400 V the average power dissipated is 75 W:

I'd vote for 1 kΩ, but you can give the 220 Ω 2W resistor a try. If it blows, use a higher resistance.

Be careful when discharging the capacitor, use well insulated tools!

kenny256

Jan 4, 2020
54
a 220Ω 2W resistor would be rated for use in about a 20VDC circuit, but for short durations it can be used in higher power and energy dissipation (e.g. rule of thumb from Dale, 5x for 5 seconds). The 400V is a lethal voltage level so that is a separate important concern.

So a typical 2W power resistor can survive a 10W pulse for 5 seconds, which is 50 Joules of energy. You didn't say what size capacitor, but you can calculate it's energy and compare, e.g. 47uF at 400V is about 4 Joules.

You may see a little spark but i doubt there will be molten metal, but wear safety glasses.

Cirkit

Oct 28, 2015
152
Thanks for all the replies, the 220Ω was just a low enough figure I thought would allow a fairly fast discharge without leaving the probes on for too long. I will use something in the low kΩ range and use insulated probes of course.

WHONOES

May 20, 2017
1,217
Try a 100W incandescent lamp. It will also give you an indication of state of charge.

Bluejets

Oct 5, 2014
6,691
I use a 1k5 5w ceramic.
Hangs above my test bench in esay reach so I don't have to go hunting to find it.

Cirkit

Oct 28, 2015
152
Try a 100W incandescent lamp. It will also give you an indication of state of charge.
I had a 60W incandescent one to hand and tried that which discharged a 4.7μF capacitor with a little crackling sound!

I'd like to make a discharging tool with probes and a resistor. Could I add a neon in parallel or would it need a dropper resistor?

Bluejets

Oct 5, 2014
6,691
A neon is not going to be much use as an indicator as they require somewhere in the vicinity of 70 -90v to turn on.
The resistor I quoted works fine, simply leave it in circuit for maybe 10 seconds.
You will feel it get warm on the larger caps as it does it's job.
If in doubt, check with a voltmeter afterwards.

WHONOES

May 20, 2017
1,217
I always put a bleed resistor across any large power supply Cap's so that they discharge after 20 seconds or so. Discharge rate depends on the size of the cap and value of resistor.

Cirkit

Oct 28, 2015
152
I always put a bleed resistor across any large power supply Cap's so that they discharge after 20 seconds or so. Discharge rate depends on the size of the cap and value of resistor.
I agree that is good practice, but not all SMPSs have them and if there is a fault rather than the capacitor being discharged on power down, the terminals remain at high voltage.

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