Voltage doubler - how to rebuild for single 400V cap

[Behemot]

I have a question about older PSU's with voltage doublers. One guy needs two 200V caps in such SFF PSU but I have only 400V one. Is there a simple way how to use it instead of two caps in series?

The PSU will be used on 230 V only so there is no need for voltage doubler at all.

[Toasty]

Re: Voltage doubler - how t orebuild for single 400V cap

Gianni did a circuit diagram of this here (Post 19):

https://www.badcaps.net/forum/showthread.php?p=80451

You are essentially removing the switch and replacing 2 caps with 1.

Here is the schematic:



PS: ask an admin or mod to fix the title plz.

[Behemot]

Re: Voltage doubler - how t orebuild for single 400V cap

Thanks I thought it's more complicated :-) I successfully removed the switch several times so now just connect single cap to the enge holes with right polarity.

Will do.

[Toasty]

Re: Voltage doubler - how t orebuild for single 400V cap

What is the capacitance of the old ones?

Generally, capacitance divides when in series and adds when in parallel.

In series, the capacitance is will be less than the lowest uF value of any one cap.

Their voltage adds in series, but you should keep the voltage and capacitance ratings as close to each other as possible. The smaller value (uF) cap will have a higher voltage across it. If you need to do this arrangement, matching value capacitors are needed, especially in a power circuit.

Voltage in parallel capacitors is limited by the lowest voltage rating of any one capacitor.

2 - 200uF 200v in series = 100uF @ 400v

2 - 200uF 200v in parallel = 400uF @ 200v

Toast

[Behemot]

Re: Voltage doubler - how to rebuild for single 400V cap

Yeah I already know this, that's why I recommended him this solution, just did not exactly know whether there are some modifications needed or not.

The old ones are like 390 uF or so and I have Samxon KM 180 uF/400 V here which is equal I think. Also there is room for this one I have 1000uF/200V Samxons here (somewhere ) but there is no way how to get them into the PSU, as I said, it's small form factor one.

[Toasty]

Re: Voltage doubler - how to rebuild for single 400V cap

Ripple current sucks on the Samxon KM. Only 704mA.

[Behemot]

Re: Voltage doubler - how to rebuild for single 400V cap

I see so this capacitor can only do 182 watts (there is also coefficient 0,8 for frequency of 50 Hz). But, there are two advantages:

1. SFF PSU's usually do not have much higher power
2. the ripple current is measured for maximal temperature si I expect it could give closer to 1 A under lets say 50-60 °C?

Anyway it's better to use it up to 150 W. I bought this caps into displays, they are fine there, LCD usually suck far less than 100 W.

[japlytic]

Re: Voltage doubler - how to rebuild for single 400V cap

Many of the "classical" power supplies with two power transistors on the primary side for the main transformer use the 200/250V capacitors to provide positive and negative rails for the main transformer, of which is a "push-pull" circuit.

[Behemot]

Re: Voltage doubler - how to rebuild for single 400V cap

I guess that means I have to discover which topology it uses?

[kaboom]

Re: Voltage doubler - how to rebuild for single 400V cap

Half bridge, usually. Replacing a pair of 330u 200v caps with a single 160u 400v will leave you without a neutral point to return the DC link cap to. It's a red or blue 1 or 2.2u 250-400v poly cap.

-Paul