Different primary capacitors in ATX Power Supply

[Hitto]

I am reparing a quite cheap ATX Power Supply which will be used just for testing motherboards.

I am planning to use different capacitors for the primary (470uF@200V + 560uF@200V).

The capacitors measure perfectly and are of a good brand.
Howewer, they are different in capacitance.

Is this to be avoided?

I was thinking about reducing the size of the bleeder resistors (and use 1W parts) as a countermeasure to improve stabilization.

[stj]

Re: Different primary capacitors in ATX Power Supply

yes, never do that.
when you have 2 capacitors on the primary and your using 230v they are operated in series and must be in balance.
you will notice the 230/120 switch has one wire that leads to the center connection between the 2 caps.

[Hitto]

Re: Different primary capacitors in ATX Power Supply

Thank you.

It is to be avoided as well if the rated capacitance is the same but brands are different?

I've seen oftenly identical capacitors (same voltage, same capacitance, same producer, same line) that are never equal in capacitance because of the 20% tolerance.

[stj]

Re: Different primary capacitors in ATX Power Supply

well they wont be matched in esr or ripple/current capacity, so fully matching only.

even if you had 2 that were the same brand, they should also have the same use level and age.
you get the idea

[Hitto]

Re: Different primary capacitors in ATX Power Supply

Clear. Thank you for the fast reply.

[momaka]

Re: Different primary capacitors in ATX Power Supply

It is to be avoided as well if the rated capacitance is the same but brands are different?

No, you can mix and match there. I've seen it done and generally it doesn't cause problems. But you'd have to be careful that the capacitance of the two caps is within 20% of their (identical) rated capacity and that the caps have somewhat similar ESR (for a good measure, I'd say they'd need to have ESR within 50% of each other or preferably less.)

And yes, even identical capacitors of the same brand and specs will not have same capacitance due to tolerance. But as long as you match the caps close enough (same voltage, capacitance within 20% of rated value and both caps with same rated capacitance, and more or less similar age and size), it will probably be OK.

[Hitto]

Re: Different primary capacitors in ATX Power Supply

Will reducing the size of the two bleeder resistors help balancing them?

[momaka]

Re: Different primary capacitors in ATX Power Supply

Will reducing the size of the two bleeder resistors help balancing them?

By reducing the size... you mean resistance?

If you reduce the resistance, then the voltage across the caps may indeed balance a little better. But you can't solely rely on that to balance the voltage correctly. And also, reducing the resistance will mean more heat dissipation by the resistors, so you will actually need physically *bigger* resistors. Of course, that's easy enough to calculate. Each cap will have approximately 160-170V DC across it. So with that, for example, a 330 KOhm resistor will dissipate
Pd = V^2 / R = (170x170)/330 000 = 0.08758 Watts. So in that case, 1/4 Watt -rated resistors will be adequate. But if you use a 100 KOhm resistor, the dissipation for each resistor will be Pd = 0.289 Watts. So here, you'd want to use 1/2 Watt -rated resistors. (One rule of thumb with resistors is to have your resistors' power rating at least 2x the expected dissipation. And also try to keep the heat dissipation of "naked" resistors under 1/2W in enclosed electronics, where there is little space for ventilation.)