Replacing two 200V capacitors with a 400V

[Rulycat]

Lads,

Got this crappy psu here. As an experiment I want to replace the two 200V 680uf caps with a 400V 470uf cap. Now, as I understand it, these two caps will be in series, so they act as a single 400V 340uf cap. The main concern for me will be the fact that the two caps will have a higher impedance (?) esr (?). There seem to be two resistors presumably to discharge the caps, I imagine leaving these in place will be fine?

The original caps are Fuhjjyu. The new cap is a Samwha cap. Its dimensions are larger but I can work around that. My main concern is if the new cap will work in place of the old ones. This is just an experiment so I won't waste my money with japanese parts. I have attached pictures for your perusal. On the underside I have marked in red where the two primary caps are.

[budm]

Re: Replacing two 200V capacitors with a 400V

The orignal two may be used for the voltage doubler circuits function, need to verify that. From what I can see from the trace layout, the junction between the two caps is connected to the circuits also.

[tom66]

Re: Replacing two 200V capacitors with a 400V

If it is a two switch forward design, you probably need the centre tap between the caps for the PSU to work.

[Th3_uN1Qu3]

Re: Replacing two 200V capacitors with a 400V

If it is a two switch forward design, you probably need the centre tap between the caps for the PSU to work.

Wrong - if it isn't a two switch forward, then you need the center tap. A half bridge won't work with a single primary lytic (unless you add a capacitive divider with plastic caps, which are expensive), but a 1 switch or 2 switch forward will, because it switches the full input voltage into the transformer on and off.

A half bridge uses the capacitive divider to make a "midpoint", because the transformer's "zero" voltage is 1/2 the supply voltage, and it is switched between that and either the positive or the negative of the primary caps.

[tom66]

Re: Replacing two 200V capacitors with a 400V

Wrong - if it isn't a two switch forward, then you need the center tap. A half bridge won't work with a single primary lytic (unless you add a capacitive divider with plastic caps, which are expensive), but a 1 switch or 2 switch forward will, because it switches the full input voltage into the transformer on and off.

A half bridge uses the capacitive divider to make a "midpoint", because the transformer's "zero" voltage is 1/2 the supply voltage, and it is switched between that and either the positive or the negative of the primary caps.

You know what I mean. :P I'm a noob with regard to SMPS design. But the midpoint may be important and unless the board is closely studied, I can't be sure.

[budm]

Re: Replacing two 200V capacitors with a 400V

Bottom line that he need to trace out that junction of the two caps to see how it is being used. Alot of PC PS in the US has Voltage selector swith, when it is for 120V, the rectifiers are setup as voltage doubler circuits, when it is in 240v MODE, it is setup as regular bridge setup.

[Th3_uN1Qu3]

Re: Replacing two 200V capacitors with a 400V

Bottom line that he need to trace out that junction of the two caps to see how it is being used. Alot of PC PS in the US has Voltage selector swith, when it is for 120V, the rectifiers are setup as voltage doubler circuits, when it is in 240v MODE, it is setup as regular bridge setup.

Yes, but even set up for 240v, it still needs that capacitive divider to work if it's a half bridge design, for the reason explained above. Edit: Just took a look at the pics. That right there is the classic TL494/LM339 affair with group regulation, which is a dead giveaway that it's a half bridge. Therefore you cannot use the single 400v capacitor.

[tom66]

Re: Replacing two 200V capacitors with a 400V

You can also see one trace going to AC filter area (for line voltage switch) and one trace going to the transformer/transistor area via a poly cap.

[Rulycat]

Re: Replacing two 200V capacitors with a 400V

Alright. Thanks guys. I'll find some other use for this capacitor.

[budm]

Re: Replacing two 200V capacitors with a 400V

The voltage doubler is the for supplying the B+ to power what ever you are for the SMPS. It is just for generating main B+. I am not talking about the switching circuits. This is the most simplest form for generating B+ for the load and if you add input voltage switch/more diodes you will maintain the same B+ output to feed you switching circuit when you are feeding it with either 120 or 240VAC. They use this setup a lot in old PC power supply that uses input voltage selector to manually select the input voltage.
Since I cannot see the input voltage selector switch any where in the pictures, then it looks like it is supplying +/- with ref at center junction of the two caps which is connected to the Brown cap for the 1/2 bridge return current path, not for Voltage doubler circuits. This power supply probably can take either 120/230V input.

http://www.coolcircuit.com/circuit/voltage/

https://cdn.badcaps-static.com/pdfs/...4fc807c47b.pdf

[tom66]

Re: Replacing two 200V capacitors with a 400V

Yes, it is a quite clever configuration - in the 115V mode, the bridge rectifier is used as part of the voltage doubler. IIRC, only two of the four diodes are used in the bridge when in the 115V mode, so they have to handle a lot more current (two times at 230V, plus twice as much per diode.)

[budm]

Re: Replacing two 200V capacitors with a 400V

More with half bridge AC in selector switch. Very cleaver indeed, have seen them used in the early day of computer power supply and industrial equipment (in the 80's) that can be sold all over the world due to 120/230V requirement. Fixed a lot of them also.
There are many topologies of Half bridge drives:


https://cdn.badcaps-static.com/pdfs/...d96495f350.pdf