SMPS question, how to discharge the power capacitors

Thread Starter

LA1

Joined Dec 1, 2013
6
on a SMPS that has MOSFETs and other transistors, with the board remover from the power supply to discharge the power capacitors or any capacitor on the board by grounding the two leads together. if not what is the preferred way?
 

Papabravo

Joined Feb 24, 2006
21,225
The preferred method in power supply design is the use of bleeder resistors. The bleeder resistor is chosen based on the capacitance to be discharged so that lethal voltage levels are reduced quickly after the power is removed. 63% of the voltage will be removed in one time constant. The presence of such resistors also represents a minimum load on the power supply which can be helpful in some circumstances, but it will affect the overall efficiency.

IMHO it is a really bad idea to short a capacitor to GROUND. It should always be done through a resistance.
 

MisterBill2

Joined Jan 23, 2018
18,463
What voltage is the capacitor charged to, and what is the value of capacitance? And what is the reason for discharging them? Is there is a danger of damaging other devices with the board removed? Or is this just a concern of somebody in management??
 

MaxHeadRoom

Joined Jul 18, 2013
28,681
on a SMPS that has MOSFETs and other transistors, with the board remover from the power supply to discharge the power capacitors or any capacitor on the board by grounding the two leads together. if not what is the preferred way?
What are you refering to the PS itself? or some Mosfet circuit connected to it?
Normally a SMPS style PS does not store power when off, not for very long at all. Not enough to worry about.
 

Dodgydave

Joined Jun 22, 2012
11,301
thanks I just ordered some 5 watt 20k resistors, if 20k is too big can parallel 2. going to put them on the end of a short plexiglass rod
Yes it should work with 20K resistors, and if you want to put two in parallel to make 10K it will work just fine.
 

MisterBill2

Joined Jan 23, 2018
18,463
Are you pulling the boards out of a SWITCHMODE POWER SUPPLY WHILE IT IS OPERATING???
or after it is switched off?? And what sort of voltage are the caps charged up to?? and what value of capacitance??
I have never come across such a situation
 

MisterBill2

Joined Jan 23, 2018
18,463
If that is the case, then the simple solution is to connect a load, such as a suitably high wattage resistor of a value to draw some small portion ( 10% )of the rated output current. That should discharge the capacitors in a hurry and not require any connections into the board.

And why is the TS working with a switcher with a separated circuit board? The resistance and inductance of almost all connections on a switcher affects the operation. An additional explanation seems to be in order.
 
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