Help finding an smd capacitor rating

[sam67]

I have come a cross a smd cap that i would like to replace but it does not appear to have any markings .. it looks similar to this -

http://cgi.ebay.co.uk/ws/eBayISAPI.d...m=220553472913

How can i determine what the value is as there is no other markings on the cap..it's c61 from a bestec 300w 12z psu.

thanks

[sam67]

Re: Help finding an smd capacitor rating

No one have any ideas to how i can find out the correct cap ??

[Radio Fox]

Re: Help finding an smd capacitor rating

If there are no markings on either side of the component, then your options are rather limited.

All I can think of is trying to find the schematic, or finding another PSU of the same model & removing the capacitor to measure it.

Is the capacitor phsically damaged?

[sam67]

Re: Help finding an smd capacitor rating

Is the capacitor physically damaged? well i have tested every smd cap on the back of the pcb and this is the only one cap that has a shorted reading ,so without a schematic i cannot get any further as i need to know what rating and voltage it is.

[sam67]

Re: Help finding an smd capacitor rating

Here is a pic of the bottom of the board with the cap highlighted ..Any help appreciated.

ps can anyone give me an idea as how to work out what voltage line is what as i am trying to work out how to do this without a schematic ,maybe some one could edit the pic and give me a clue ( i am very eager to learn these things and feel robbed that i was never taught anything to do with electronics and physics when i was at school

Thanks pps the board still has flux residue that needs to be cleaned .

[Radio Fox]

Re: Help finding an smd capacitor rating

...i have tested every smd cap on the back of the pcb and this is the only one cap that has a shorted reading...

While the cap is still on the board, you cannot know for certain that it is the cap that is s/c.

It may be soldered across a component that will read s/c on a meter, such as a transformer winding or an inductor.

The only way to know for sure, is to desolder it from the board.

[sam67]

Re: Help finding an smd capacitor rating

While the cap is still on the board, you cannot know for certain that it is the cap that is s/c.
quote
It may be soldered across a component that will read s/c on a meter, such as a transformer winding or an inductor.

The only way to know for sure, is to desolder it from the board.

It looks like my only option is to try it off the board ..which is better ,analogue meter or dvm ..also roughly what reading should i expect with this cap..

Thanks

[Radio Fox]

Re: Help finding an smd capacitor rating

It looks like my only option is to try it off the board ..which is better ,analogue meter or dvm ..also roughly what reading should i expect with this cap..

Thanks

If you mean what to expect using the Ohms range, then the capacitor should read open circuit on either meter once it is off the board.

If you have capacitance ranges on your DVM, then it could read anything from a few pF to a few nF, as there is no way of knowing the exact value.

I suspect that the capacitor is OK, & that the s/c is somewhere else on the board. Can you follow the tracks from the capacitor to see if they go to a transformer or inductor?

[momaka]

Re: Help finding an smd capacitor rating

Is this the intermittent Bestec PSU you're troubleshooting? If so, I wouldn't count on this being the problem. Not only do ceramic caps almost never short out (or fail in general), but if this cap was indeed shorted, the PSU would've probably exploded by now. (Just word of warning - don't mess with components on the primary side too much, unless you're positively sure something is wrong there - i.e. PSU has a blown fuse or appears completely dead. Otherwise you may end up with lots of sparks and smoke.)
Nonetheless, if you still want to check this cap, first trace out what the rest of the circuit around it does. Like Radio Fox said, there can be a low-ohm resistor or an inductor in parallel with this cap, giving you false readings.

Also, when you asked "how to work out what voltage line is what", did you mean the 3.3/5/12v output rails?

[sam67]

Re: Help finding an smd capacitor rating

quote-
Also, when you asked "how to work out what voltage line is what", did you mean the 3.3/5/12v output rails?

Hi momaka,

I have wondered how some of you guys know which trace on the solder side pcb relates to the specific voltage rails ? i can just about follow a schematic but seem lost when trying to trace which rail is which without a schematic .. Can you give any tips on how to go about this ? you see i am still learning but it can be hard when there is no one to guide you especially now i am hooked on electronics thanks to this forum and all i have is the internet to find the answers i need .

I have looked and searched high and low to see if there is anywhere near me that might be running a course so i can get a hands on experience but up till now i have had no luck so i have just been practicing on broken monitors to try and learn more.

Thanks

sam67 p.s i have put the psu to one side for now as i cannot find the fault ..i soldered out the smd cap and it was good so you guys were correct about other circuits interfering with the cap.

[momaka]

Re: Help finding an smd capacitor rating

The first step would be to learn the basic components (capacitors, inductors, resistors, diodes, and transistors) and just get a very general idea of what they do. Wikipedia contains some good info on these. With this alone and a general idea of how SMPSs work (see this article), you can figure out just about any PSU. If you already know these things, the best approach is to start at the output of the PSU where the wires go out to the computer/LCD screen and work your way backwards into the PSU. Then with the PSU board in hand, pick one rail at a time (in computer PSUs, you know which rail is which by the colors of the wires) and use your finger to follow the traces on the board until you get to the main transformer. This will also allow you to see how capacitors are distributed on the PSU, and on what rail each is connected to.
I hope this helps a bit. Let me know if something is still unclear, though.

[sam67]

Re: Help finding an smd capacitor rating

Hi momaka,

Thanks for the info it is much appreciated ..I do know what most components do and how to test them off board ,it was tracing along the pcb without a schematic that i was not to sure about but i will look closely to some boards i have and see if i find the right paths for the right rails..I did try a search for some tips but could not find any that i was looking for .

Cheers

sam

[momaka]

Re: Help finding an smd capacitor rating

If you're tracing out a power supply, just try and only follow the major traces for right now and ignore the small traces that lead to the logic ICs (at least on the first pass). Start by identifying the ground trace as that's the easiest one to find - it's usually connected to the case of the power supply. Then either start from the output wires and trace backwards or start from the schottky diodes (the 3-terminal devices mounted on the secondary heatsink) and go forward until you get to the output wires. Usually, you'll only see a coil or two between the output wires and the diodes with a capacitor or two after each coil.
Now your Bestec may not be the most intuitive PSU to start with because its layout is like the one in diagram B on page 9 of the hardwaresecrets article I posted earlier. The layout for diagram A is more common and probably also much easier to understand. Nonetheless, try to trace it. Like I said, there would only be a coil or two separating the middle terminals of the schottky diodes and the output wires (middle terminals are the outputs for each rail).