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Switching power supply repair

Discussion in 'General Electronics Chat' started by lynx, Apr 10, 2014.

  1. lynx

    lynx Member

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    hi

    i'm facing a very weird issue with a DVD device...

    by mistake i've damaged the power supply from a dvd player, replacing some components i've managed
    to make work again, but the internal dvd device refuses to power up.

    there's an LM317T on the power supply board, following the rails they go to the power connector for the
    dvd but when i'm connecting it, the LM317T gets quite hot (120c) and the dvd doesn't spin, so i've tried to
    replace the LM317T in case it also got damaged, but nothing happend.

    i've measured the LM317T without load and it seems it is regulated at 11.45V also if i connect
    something else, it works just fine!! without getting hot, also noticed the regulating resistors are 220ohm
    and 1.8k so i guess the output i've measured it is quite ok.

    now if i use an external power supply for the internal dvd it starts to spin and everything works perfect!
    so then the internal dvd isn't damaged, but what happens when i'm connecting the dvd power connector
    to the internal power supply?!

    checking the label of the dvd it says that it needs 5A for the 12v rail and 1A for the 5v rail and that probably
    explains why the LM317T (TO-220) gets so hot! but how on earth it worked all this time and what should
    i do to repair it?!!

    thanks in advance if anybody is willing to help!!!
     
  2. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    I'd bet it was an LM338T which is 5A.
     
  3. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    The LM317's recommended I out max is about 2.2A.

    I suspect that it is meant to supply the regulated 5VDC rail ( 1A required) for the DVD digital circuits, not the 12VDC for the DVD motor (a normal level for a device of that sort). Could be wrong, of course.

    It would help is you could list the components you replaced. What is the input volatge level to the LM317? Understand that without a minimum load current of 12mA, the LM317's output will not be regulated properly. Thus, the 11.45VDC level you saw (with no load) is probably not accurate.

    Do you have, or can you provide some sort of power supply schematic? Or perhaps some well focused pictures showing the components and traces of the areas (or points) where you attached the external PS.
     
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  4. dave

    Dave New Member

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  5. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    A quick calc of 1.25 + 1800/220*1.25 which I did not not look up, but what I remember, and it's closer to 12V. Equation from memory.

    The 5V is probably a standard 5V regulator.
     
  6. lynx

    lynx Member

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    here's some photos.. i've replaced almost all semiconductors and some capacitors
    the 317T, BC337, TL431, BD238 the input voltage for the 317T seems to be
    around 13V it is supplied thru a huge diode.
     

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    Last edited: Apr 10, 2014
  7. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Your 5A measurement really does suggest the wrong regulator is there, You also mentioned a huge diode. The current rating of the diode would also be a clue, The part I suggested is a plug in replacement for the 317T.

    Normal design usually would make Vin about 2.5 V higher for the 317.

    Besides the LM317 is a linear regulator and not a switching regulator.

    Just replace it with a LM338T especially if the diodes have a large current rating. I highlighted the part number difference.

    This http://media.digikey.com/pdf/Data Sheets/Comchip PDFs/SB520E-G_SB5100E-G.pdf datasheet is a diode rated for 5A so you have an idea of how big one is.

    One of the diodes could be a 1N5160 http://www.americanmicrosemi.com/information/spec/?ss_pn=1N5160 which is 10A, again supporting the wrong regulator.

    You have to guess sometimes. The Japanese tend to leave off the prefixes. So a 2sd501 transistor would be a D501. They also leave off the 1S from diodes. The prefix can be a manufacture ID. LM was National Semiconductor, which is now TI. Now we have SMT "markings" to deal with and they are very cryptic.

    Get this: http://gr.rsdelivers.com/product/te...le-regulator-lm338t/0460900.aspx?query=LM338T
     
  8. ronv

    ronv Well-Known Member Most Helpful Member

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    What's on the heat sink above the regulator? Could it be a pass transistor?
     
  9. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    ronv

    To me you have a common mode filter, a big cap. near the yellow/black transformer we have an IC which could be a switching regulator. Topagraphy is consistant with a switching pre-regulator.

    The thing that you think could be a pass transistor is consistant with a "FET switch" for the unregulated supplies. Then you have a +12 regulator which isn't heat sinked much because it's already pre-regulated.
     
  10. lynx

    lynx Member

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    i haven't measured any current... and yes from your sayings and from what i can see that's what i also believe, they placed wrong regulator.. but how exactly it managed to work all this time?!


    it's a diode, the IC it's for the switching.
     
  11. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    OK, you didn't actually measure it as I thought.

    and you said you substituted a supply, but didn't say how much current it could supply.
     
  12. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    So, just to recap, in this snip of the 4 pin connector:
    5-12 pins.JPG
    have you confirmed the voltages you listed? Or are those number assumed?
     
  13. ronv

    ronv Well-Known Member Most Helpful Member

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    My guess is what you think is a huge diode is really a large transistor that works to share some of the current for the 317. If it were really just a diode feeding the 317 it wouldn't need a bigger heat sink than the 317. It could never have supplied 5 amps with the 317 alone. Can you measure the actual current when it is running from your power supply?
     
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  14. lynx

    lynx Member

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    hi, yes, the 5v rail it's 5.25V and the 12v rail it's 11.45V without load.
    the 12v rail tasted with an external load and it worked..the consumption was around 200mA and the LM317
    didn't went hot.

    it's not a large transistor it's a power diode on a heatsink

    http://www.farnell.com/datasheets/1761342.pdf

    the diode that supplies the lm317 it's obvious on the second photo behind the heatsink of the regulator
     
  15. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello there,

    Another possibility is that the rating for the 12v line (5 amps) is NOT the current for JUST the LM317. That is, the 12v supplies the LM317 but also supplies other things. That would mean the LM317 would work comfortably with whatever current it needed to supply the regulated output on the output side.

    The LM317 could never work at 5 amps because there are two internal mechanisms that would cause it to shut down after a very short time, if it ever started at all. So the questions that come up are two:
    1. Is it really an LM317, how do you know it is, are there clear markings on it?
    2. If it is an LM317, what circuit is the output powering, could the 12v input be supplying other circuits without using the regulator?

    Remember the LM317 could never handle 5 amps, ever, possibly not even if it was cooled with liquid nitrogen.
     
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  16. lynx

    lynx Member

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    the 5A/12V and 1A/5V markings are on the label on top of the internal dvd device it's the power supply it needs so the motor can spin
    and the rest of it's electronics can power up.

    the dissoldered lm317 it's somewhere around here, :) i'll try to find it and see if i was mistaken, but i don't think so.

    also!... notice the power line in the first picture that goes in the left side of the pcb to a pin
    these pins are various output voltages 3.3v, 5v, etc seems it also supplies 12v to the processing board, so then the current consumption is high enough and obviously a single lm317 can't make
    it.

    so i'm puzzled with all this thing.....
     
    Last edited: Apr 11, 2014
  17. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,

    Oh so you are reading the product label then and assuming that all the current of the 12v line goes to the LM317 for some reason. I dont see why you would assume that it has to go to the LM317. If the device has a motor in it then it likely also has a motor speed controller that powers the motor through a series transistor or bridge. It is unlikely that it would power the motor using the LM317 as a regulator because that would never be good enough to get the motor speed to be constant and of the right absolute speed, as it would need to actually actively regulator the speed itself not the terminal voltage of the motor.

    So it looks like all that is happening here is that the 12v line goes to other things as well as the LM317 and it's as simple as that. If the LM317 is overheating then it is due to the load at the output, where something must be wrong. Perhaps another part shorted out or went to a much lower resistance than normal. You'd have to check out the parts in the output circuit of the LM317 to find out. If there are caps on the output then it could be one of them for example.

    If the LM317 is powering the motor circuit too (with speed regulator) then it could be that the particular LM317 can not provide enough start up current to the motor. Another LM317 might work, or else another IC that can handle higher current might work better. Note that the LM317 current limits spec is very wide ranged. To test this you'd have to start up with a power supply and then switch to the LM317 while the motor is still turning, or try giving the motor a quick turn with your fingers (being careful of course).
    One way to do this would be to use two diodes to get the motor to run with the external supply through one diode and then when the external supply is shut down the LM317 takes over though the other diode.
     
  18. lynx

    lynx Member

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    Hi the 12v power supply comes from the regulator...the regulator is supplied by the axial diode next to the btown capacitors
     
  19. ronv

    ronv Well-Known Member Most Helpful Member

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    There are a lot of things that don't make sense.
    1- If the diode is on the big black heat sink when it only dissipates 2.5 watts and the 317 is on a little heat sink when it would need to dissipate over 10 watts. Besides the diode looks like it has 3 leads when viewed from the bottom side.
    To me it looks like the 317 is fed from the little diode next to the text isolated.
    2- If the input to the 317 is 13 volts it can't be a regulator for 12 volts as it needs ~ 2 volts (14v) higher than the output. But since the divider is set up for 11.5 volts it would seem the input is to low.
    Anyway we are missing something.
     
  20. lynx

    lynx Member

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    indeed it looks like that...but the middle pin is not connected anywhere...
    also the lm317 INDEED its feed from the smaller diode the axial one! i said
    that before also there's a red line on the second photo that shows it.

    here's an update..

    i tried an lm338t costed but nothing seemed to power up...then i got back to the lm317T and although it powered up.. i've lost the output from the lm317t!

    when i tried to measure the voltage on the supplying diode it was almost zero (few mV)
    that voltage looks like it is coming from one of the windings of the transformer so i probed there too but nothing! so i thought that the lm338t was a bad idea and the tranformer winding burned istead of the regulator.

    while i was inspecting the board i thought i should check if there might be any component
    that acts as a load (short circuits the output of the 317) and i dissoldered a 1000uF/25v
    capacitor so i can measure it...the cap was ok and i soldered it back i tried again the power supplyand everything worked perfectly :wideyed: no heat on the lm317r nothing! the dvd was working just fine!!!! then i decided to put the power supply board back to it's place and close it...but when i powered the dvd nothing worked no leds no display nothing! and after few seconds a heard a noise and the SMPS ic litarary exploded!!!

    what a mess........

    i don't even remember the part number of that ic...i only remember it was started with "ICE" which is part code of the infineon........

    help!
     
    Last edited: Apr 12, 2014
  21. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Junk it, buy a new one :)

    I rarely tell anyone this, but this is one of those exceptions. If you could not figure out what was happening before, you'll never figure it out now without getting extremely lucky.

    If you seriously still want to try to fix this you'll probably have to get a schematic. Maybe tracing out the PC board would help, but without knowing the IC part number it's very difficult.
     
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