Jump to content

Power Supply Fixing


napierzaza

Recommended Posts

Does anyone know much about switching power supplies (I believe it's flyback style)? I think I might have broken mine somehow and am unsure what I should replace. Basically I had a switching PSU with 5v (switching) and linear 12+ and 12-

I found the datasheet for the reference voltage IC and figured out that I should change the value of a resistor so that it will generate aprox. 9v DC (so then I can put it through a linear reg and everything would be great). However something went wrong in that area and now the unit produces no (.11v) voltage whatsoever, even when it's changed back to the original resistor value.

Does anyone know what the most sensitive IC in this type of power supply is, or what I might try replacing? I have replacement parts so I can swap whatever transistor IC etc that I need. Or if anyone knows a good PSU forum that would be appreciated, I can't find any.

Link to comment
Share on other sites

Is this a mains powered switcher? If so be very careful - they have lethal voltages on the board, and can stay charged long after switch off.

Major word of advice: make up a dummy test load before any other work - say a BIG 10 Ohm resistor, that way you won't risk frying any of your PIC's or SID's if it all goes wrong. Some switchmodes won't work without a load, by the way. Some have an over voltage crowbar circuit, design to short the output should the voltage go too high, just check you haven't got one set below 9 volts.

I have a lot of experience working with things like this, starting with television repairs as a student. Even so, I think twice about working on one, and will only do so if there is no other option.

There are some very good guidlines here http://www.eio.com/repairfaq/sam/smpsfaq.htm

Once again: be careful.

Mike

Link to comment
Share on other sites

>Is this a mains powered switcher? If so be very careful - they have lethal voltages on the board, and can >stay charged long after switch off.

Gotcha.

>Major word of advice: make up a dummy test load before any other work - say a BIG 10 Ohm resistor, that >way you won't risk frying any of your PIC's or SID's if it all goes wrong. Some switchmodes won't work >without a load, by the way. Some have an over voltage crowbar circuit, design to short the output should >the voltage go too high, just check you haven't got one set below 9 volts.

I of course do not have anything connected when I am doing this mods. But it has been sending out a steady voltage when nothing is connected in the past. I might scrape around for a large resistor though to make sure.

Hmm, crowbar circuit? I'll look into it.

>I have a lot of experience working with things like this, starting with television repairs as a student. Even >so, I think twice about working on one, and will only do so if there is no other option.

I don't have any other PSU unfortunantly. I had the alternative of throwing a 9v wall wart into the mix but now that this thing isn't working, at all, I have no 12+- to speak of.

>There are some very good guidlines here http://www.eio.com/repairfaq/sam/smpsfaq.htm

Thanks a lot for the help, I'm going to get reading!

Link to comment
Share on other sites

Some thoughts, so what you're saying is that, possibly, the 5v line has a crowbar circuit on it? Any that it might in fact be configured to spring if over 5v by a certain amount? Any that therefore hacking the output to 7-9v will make it not work at all? This would explain a few things, why I blew the fuse on one of the PSUs when only increasing a bit.

Sketchy, so I suppose this is bad news as crowbars are dangerous to a psu's components if you happen to have bypassed the fuse ... ;-P

Link to comment
Share on other sites

Some thoughts, so what you're saying is that, possibly, the 5v line has a crowbar circuit on it? Any that it might in fact be configured to spring if over 5v by a certain amount? Any that therefore hacking the output to 7-9v will make it not work at all? This would explain a few things, why I blew the fuse on one of the PSUs when only increasing a bit.

Sketchy, so I suppose this is bad news as crowbars are dangerous to a psu's components if you happen to have bypassed the fuse ... ;-P

Yes, well crowbars are designed to save the expensive electronics, at the cost of a power supply that has gone wrong anyway. A typical 5V rail crowbar will fire at 5.6V. If you look around the 5V output, you may find an SCR, (usually in the same style of package as a voltage reg), wired across from 5V to gound. That would be the crowbar SCR. If you increase the fuse or link it out, then you will likely lose the switching transistor, or the input rectifier diodes. Some switchmodes have a thermistor, (looks like a big disc ceramic cap usually), in series with the live, this is to limit the startup inrush current. A short may blow this, and they are hard to get as spares.

If you've replaced parts, and are wary about starting it up again, the series mains bulb trick, (mentioned in the link I gave), is an old fashioned technique that can save you money in power transistors.

If you hvae trouble circuit tracing, try putting the PCB, copper side down, on a scanner and doing a big print, you can then 'ghost' in the parts and you are part way there.

Best of luck, and again: careful!

Mike

Link to comment
Share on other sites

Like a big 220 package? How many leads will it have? Three? Oh wait, I just found, marked on the board, a small IC with a PCB label of SCR. I hope I haven't destroyed anything. I'll have to now swp different replacement ICs around I guess. Thanks man.

It really sucks because I was trying to get all the datasheets for the parts and that was the one I wasn't able to get a datasheet for. Of course who knows if I would have known what it meant had I found one anyway.

Link to comment
Share on other sites

Okay, I need your help again. I, somewhat, got it working again!

Basically the info you gave me allowed me to check if the different ICs were broken or not. Some were, but it still didn't work. I then realized there was a small NPN transistor in the circuit I didn't check yet. It was a 2SC3203 NPN Transistor. So I realized I had a lot of MPSA13s lying around and they had the same pinout and the same (NPN) config. So I swapped them both and bang (not literally) the PSU came on giving its old voltage that it used to. This was good except that now it gives off a high pitched whine (very audible) all the time and it certainly didn't do that before. Is this because of the transistor?

Link to comment
Share on other sites

Crap, I have one, but I think it's reserved for one of my future midibox projects.

What if I had a BC549?

EDIT:

Okay, now it's working with no noise. It's now functioning at 3v which is odd. I recall it going around 4-5 previously (even the bad transistor replacement). I think it might be a reference voltage issue, but I swapped that out and no such luck. Do you think this is this still a transistor issue? I believe that is at least near the base that the 431 can output (2.5 I think).

Link to comment
Share on other sites

Okay,

It seems to be working at least as well as the BC549 did, but it's still giving 3volts whether I have the original resistor in the the 431 circuit or the newer one I picked out. However it appears to change voltage from 3v volts to 3.08 volts if I change the pot setting. This is somewhat perplexing!

Link to comment
Share on other sites

We're getting very close to the point where you have to say: "OK, I've fought this one down to the wire, but that's it".

A few thoughts: do you hvae details of the make and model number of the supply itself?

Can you give me some of the main chip numbers.

I think you've done pretty good so far, but I need a little more info.

Mike

Link to comment
Share on other sites

I've read that paper and it really helped me understand the circuit. Looks like a really standard flyback circuit using mostly the standard parts.

tl431 as a reference voltage (hooked into a 4n35).

c2553 as a switch

rectifier diode as a supply side rectifier.

BC337 on the high voltage side, being used for... ?

Link to comment
Share on other sites

here is a schematic of the ref voltage circuit. I figure that if it's constantly holding at about 3 - 3.1 volts that there is something wrong, but am unsure about what, in this circuit, that may be. That is to say what specifically would have to break in order for it to not work.

I'm really not certain what the issue is but it appears as if it might be in the feeback section (431 & 4n35).

I've tried 3 different 431s and they only offer slightly different results.

I've also tried 3 different octocouplers with slightly different results.

tl431_thumb.gif

1237_tl431_gif201c5a6c14f866c26bc8460eb4

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...