low voltagw on 150uf 450v cap?

[capted]

Hey guys, I have a question. I keep reading about checking the voltage on the big cap for an lcd power board. I am reading 60 volts instead of 160v on some test boards I have. Where would the problem be if I am not getting the 160v? I have 120v input also. It is a dell sp2008 monitor I am working on. Thanks for any info.

[mariushm]

Re: low voltagw on 150uf 450v cap?

Make sure you test DC and not AC on the big capacitor.

See if the bridge rectifier is bad.

If the power supply has a PFC circuit, the PFC controller may be bad - and you should have about 300-400v on that 450v capacitor.

If there's no PFC you would have 110v x 1.41 = ~ 160v or 230v x 1.41 = ~ 330v

[capted]

Re: low voltagw on 150uf 450v cap?

am I correct, the bridge is the 4 pin big diode on the heat sink. I ask because I get 120v to one pin and 60v on two other pins.

[ben7]

Re: low voltagw on 150uf 450v cap?

am I correct, the bridge is the 4 pin big diode on the heat sink. I ask because I get 120v to one pin and 60v on two other pins.

You should get about 120v AC across the pins marked with the '~'

You should get about 160v DC across the pins marked '+' and '-'

If this has PFC, there usually is no filter capacitor directly after the bridge rectifier, so there might be a fairly large amount of ripple (which you might be measuring)

[budm]

Re: low voltagw on 150uf 450v cap?

"I am reading 60 volts instead of 160v on some test boards " I think you are using the chassis ground as your ground reference point, that is why you are getting the reading around 60V. You need to use the negative leg of the main DC filter cap as a ground ref point when you are working in the primary side of the power supply.
And sure you DO NOT TOUCH any part of the primary section with your hands or body.
To be safe when you work on the anything that is plug into AC, you should plug it into an GFI protected outlet.
You should make something like this if you are working with AC.
http://s807.photobucket.com/albums/y...t=DSC00073.jpg

[capted]

Re: low voltagw on 150uf 450v cap?

I did check the voltage on the legs of the cap. I have some time behind the wheel with testing and not getting zapped!!! I checked the bridge and I get 120 on the two legs and 60v on the other two(fluke 88 meter). I'm thinking the bridge is bad but you guys have way more know how than me.. Oh. all the caps but the big one I changed with nichicons.

[ben7]

Re: low voltagw on 150uf 450v cap?

I did check the voltage on the legs of the cap. I have some time behind the wheel with testing and not getting zapped!!! I checked the bridge and I get 120 on the two legs and 60v on the other two(fluke 88 meter). I'm thinking the bridge is bad but you guys have way more know how than me.. Oh. all the caps but the big one I changed with nichicons.

Hm, that bridge does sound fishy.

Do you have extra caps of around the same uF rating and voltage rating (no less than 450v!)? If so try swapping it out. It may be dry. What brand is it?

[budm]

Re: low voltagw on 150uf 450v cap?

"get 120 on the two legs and 60v on the other two(fluke 88 meter)" OK, you are getting 60V at the output of the bridge, not at the filter cap itself, since this unit more likely to have PFC circuit, the + side output of the bridge should be connected to Anode side of the PFC isolation diode, the Cathode side of that Diode will be connected to the + leg of the cap.
So if you measure at the + side of the bridge, you will not see the 160V due to the isolation Diode. but if the cap is open then I can see why you will read 60VDC on the cap itself, but the cap has to be completely open and there is no other cap in the circuit to let the voltage charge up to peak.
So can you verify if the setup is like what I indicate above?

[capted]

Re: low voltagw on 150uf 450v cap?

I have some spare power boards that have that size cap from a few screens that were cracked. I will take the cap out and try that...

[capted]

Re: low voltagw on 150uf 450v cap?

"get 120 on the two legs and 60v on the other two(fluke 88 meter)" OK, you are getting 60V at the output of the bridge, not at the filter cap itself, since this unit more likely to have PFC circuit, the + side output of the bridge should be connected to Anode side of the PFC isolation diode, the Cathode side of that Diode will be connected to the + leg of the cap.
So if you measure at the + side of the bridge, you will not see the 160V due to the isolation Diode. but if the cap is open then I can see why you will read 60VDC on the cap itself, but the cap has to be completely open and there is no other cap in the circuit to let the voltage charge up to peak.
So can you verify if the setup is like what I indicate above?

I do think it is that set-up...

[budm]

Re: low voltagw on 150uf 450v cap?

Then start tracing all that path from the + of the bridge to the + leg of the cap to make sure they are all connected.

[capted]

Re: low voltagw on 150uf 450v cap?

Then start tracing all that path from the + of the bridge to the + leg of the cap to make sure they are all connected.

I think the path is good to the cap from the bridge.. What damage can I do if I take the cap out and check the bridge and the voltage to the cap socket?
I also should mention this power board is out of the chassis and no other boards are connected( I did get the same reading with all connected)

[budm]

Re: low voltagw on 150uf 450v cap?

Right now the bridge is not seeing the cap through the isolation diode to charge up the cap to the peak of the sine wave, the isolation diode is good, some time something so simple can cause the problem that is why it is important to check everything. You can run board by itself without another board attached but I will not do without the cap attached since you do not want the AC to be fed to the circuit. You can just jump another cap in parallel with the old cap, it does not have to be big value, 47uF 450VDC will be fine. By the way, are you seeing 60VDC on that large cap right now?
Can I see the pictures of the board, both sides.

[capted]

Re: low voltagw on 150uf 450v cap?

Right now the bridge is not seeing the cap through the isolation diode to charge up the cap to the peak of the sine wave, the isolation diode is good, some time something so simple can cause the problem that is why it is important to check everything. You can run board by itself without another board attached but I will not do without the cap attached since you do not want the AC to be fed to the circuit. You can just jump another cap in parallel with the old cap, it does not have to be big value, 47uF 450VDC will be fine. By the way, are you seeing 60VDC on that large cap right now?
Can I see the pictures of the board, both sides.

If the cap was open would it be the same as the cap not being there? I will get some pics up. I did not know you can jump another cap in parallel!!!! What if I use the same size cap uf and volts??

[capted]

Re: low voltagw on 150uf 450v cap?

Hm, that bridge does sound fishy.

Do you have extra caps of around the same uF rating and voltage rating (no less than 450v!)? If so try swapping it out. It may be dry. What brand is it?

The old brown kxg type one.. I have known good ones I could remove from other spare boards I have.

[budm]

Re: low voltagw on 150uf 450v cap?

Yes you can jump the caps in parallel. I cannot wait to see what you find out!

[capted]

Re: low voltagw on 150uf 450v cap?

Have not tried the cap yet. I did see a schematic somewhere on the forum for a pfc voltage booster circuit. Does anybody know where it is? I can't seem to find it again.

[capted]

Re: low voltagw on 150uf 450v cap?

I found the schematic. https://www.badcaps.net/forum/attach...4&d=1344964111

In case anybody needs it!!!

[budm]

Re: low voltagw on 150uf 450v cap?

I am the one that posted that sch with the notes explaining how the PS_ON works and turn on the PFC circuits.

[capted]

Re: low voltagw on 150uf 450v cap?

I am the one that posted that sch with the notes explaining how the PS_ON works and turn on the PFC circuits.

I should have known that!!!! I have not looked at the board yet but I will keep you posted.