How to identify capacitor function?

[goop]

This was from another thread:

This is monitor power supply, not critical. Is this for the startup/running cap?

How can you identify the function? What are the typical circuit stages?

[budm]

Re: How to identify capacitor function?

You have to work on lots of circuits and boards to learn how to identify each section of the board.

[goop]

Re: How to identify capacitor function?

Can you recommend something to read for an overview of common PSU or LCD PSU designs, maybe walkthroughs of some specific existing PSUs?

That capacitor you asked about was a 33uF 50V low ESR in the primary side, with 12-13V across. There's only one another electrolytic in the primary, a 0.47uF 50V standard impedance, with 1.6V.

[budm]

Re: How to identify capacitor function?

Since we have no ideas which power supply board you have, it will be impossible to tell you what caps are for what.
How about providing some more info and good pictures. Board lay out of each power supply boards are not the same but they do have same common building blocks, to narrow down to component level is another story which you will need good basic back ground of the components and how they work and how they are used, good theories in basic electronics, then you need basic understanding of the power supplies which come in more than one form.
Do the search for switching power supply, linear power supply, etc.
Doing the repair without the diagrams is a lot more difficult.
You can download some training manuals and service manuals here for studying.
WWW.ELEKTROTANYA.COM
Search example :SONY TRAINING
http://elektrotanya.com/?q=showresul...at2=schematics
You will see lots of info.

[goop]

Re: How to identify capacitor function?

I thought since you had a guess for the function of the capacitor in that quoted post, a few more details and you would be able to tell with certainty what it is for.

The PSU works now. I was just curious about what you said.

Someone uploaded shots of this specific PCB before. It's C103, the bottom one:
https://www.badcaps.net/forum/attach...5&d=1319653243
https://www.badcaps.net/forum/showthread.php?t=17123

And you referred to this cap and its sibling in a very similar board:
https://www.badcaps.net/forum/showth...026#post230887

[budm]

Re: How to identify capacitor function?

First link: C103, C105 those are small value startup/running cap, they are for filtering for the DCV feeding the VCC pin of the SMPS IC. It does not have to be super low ESR due to not very high current demanding by the circuit.

Third link: same function.
If you look up the spec sheet of the IC then you can see how they are used.
The AUX winding of the transformer provides the running Voltage ones the circuits starts running (the cap is first charged up the charge pump circuit inside the IC, when the Voltage reaches the Vstart threshold, the IC then start driving the power MOSFET which drives the power transfomer), this Voltage from AUX winding is also used to run other circuit such as the PFC IC, the SMPS IC of the main supply. That is the typical setup.

[goop]

Re: How to identify capacitor function?

So that IC runs on 12V? What's the 1.6V for? If low ESR isn't required, did they use it just because of stock or price concerns?

[stj]

Re: How to identify capacitor function?

low esr caps generally last longer and people like me dont want to stock crappy caps when we alerady have the same values in low esr stockpiled.

[budm]

Re: How to identify capacitor function?

So that IC runs on 12V? What's the 1.6V for? If low ESR isn't required, did they use it just because of stock or price concerns?

Which IC P/N? each one has its own operating range and also it sown startup Voltage, you need to look up the spec sheet to see, if the VCC is high enough to run the IC bit it is still dead then you need to find out why.
The ESR requirement of the cap depends on the circuit and for this section it does not require super low ESR like what are used in the mother board, it should be LOW ESR but not super low as required by mother board.
You can also do experiment using decade box to put resistor in series with the low ESR cap to simulate HIGH ESR and see as to how high the ESR can be until the circuit stops functioning.
Look up the spec sheet of the IC used in your board, you will see the application notes and the circuits.

[budm]

Re: How to identify capacitor function?

BTW, the 0.47uf may be for the feedback circuit around the receiving (PHOTO TRANSISTOR) side of the OPTO.

[goop]

Re: How to identify capacitor function?

low esr caps generally last longer and people like me dont want to stock crappy caps when we alerady have the same values in low esr stockpiled.

If it can work well, sure. But I guess some circuits may rely on having a certain ESR range.

budm: Thanks.

Can't check the IC model directly anymore.

Regarding adding external resistance, won't it require a more extensive analysis to tell for sure how a higher resistance affects the circuit? For example, it might seem functional superficially, but will have latent side effects. More noise, components that are driven harder, etc.

What's the function of the optocoupler?

[stj]

Re: How to identify capacitor function?

optocoupler is feedback to control the output voltage.

what do you REALLY want to know?

[budm]

Re: How to identify capacitor function?

"Regarding adding external resistance, won't it require a more extensive analysis to tell for sure how a higher resistance affects the circuit? For example, it might seem functional superficially, but will have latent side effects. More noise, components that are driven harder, etc" That is why it is the experiment you need to do to see it for yourself what affect of high ESR will do to the circuit. How else will you learn?
"What's the function of the optocoupler?" Seem to me you need to learn more about electronics component and its function. Do the search on the internet if you really need to learn, like I said, download the schematics and see how it is done.

Want to learn more?
https://www.badcaps.net/forum/showpo...&postcount=179

[goop]

Re: How to identify capacitor function?

stj: Just a better understanding of the above PSU, and similar ones.

budm: For a useful experiment I'd first need more knowledge.

I didn't mean the general function of an optocoupler, but its role in the specific circuit.

Anyway, all right. I'll proceed to unguided reading. Thanks for the PDFs.

[stj]

Re: How to identify capacitor function?

get a datasheet for the controller chip the psu uses.
it will have operational descriptions etc.

[goop]

Re: How to identify capacitor function?

I suppose when you have enough experience, you can infer or guess the function without tracing connections and reading datasheets.