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SMT Diode id & home automation system repair

Discussion in 'Troubleshooting and Repair' started by mypcbis$crewed, Nov 29, 2009.

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  1. mypcbis$crewed

    mypcbis$crewed

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    Nov 29, 2009
    Hello all, I am in a bind with a pcb that got fried the other day.... I have identified several trouble spots that need repair: burnt out buck regulator :)eek: LM2673s adj - cant understand how that happened since its thermally protected), lifted trace off of pcb from heat and a cracked smd diode.

    The diode is what I am having a hard time i.d.' ing can you help me out on this:

    [​IMG]

    That is the best shot I could get with a magnifier, but I drew the markings from the chip:

    [​IMG]

    Thanks in advance!
     
  2. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    It could be an SMBJ15A Transient Voltage Suppressor (15V).
    But you'll have to evaluate yourself if this could be correct.
    Damage could be due to lightning or incorrect psu voltage.
    (If the 4 pins in the pic is a rectifier I'd check it for shorts.)
     
    Last edited: Nov 30, 2009
  3. mypcbis$crewed

    mypcbis$crewed

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    Nov 29, 2009
    You are amazing Resqueline :) I don't know how you knew that right of the bat, but kudos to you! The package size is correct, but I dont know how to evaluate it. I don't have the schematics for the board, nor do I think the manufacturer will be forthcoming with them. Is there a way to size this item without the schematics? And yes, the 4 large pins you see are for a bridge rectifier. I checked the pinouts:

    pin 1 = postive
    pin 4 = negative

    multimeter on diode check (sends a milliamp of current to test)

    pin 1 with positive lead from m.m and negative lead on pin 2 =

    1840mv forward voltage drop
    reversed =

    500 mv overload

    pin 1 " and negative on pin 3 = 1840 mv
    reversed = 516 mv

    pin 1 and 4 (pos and negative on bridge) = 1780mv
    reversed = 525 mv

    so it looks like the bridge is still ok. Any advice on the sizing of that diode would be greatly appreciated.

    Thanks!
     
  4. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Oh, well, a little experience coupled with a little Googling with the right keywords works wonders.. ;)
    It is possible to draw up a diagram from just looking at the board, but it takes some experience, time, & patience.
    It seems to me the anode of the diode is connected to the negative on the bridge but where does the cathode lead to? (That's where the overcurrent came from.)

    I'm missing the pin 2 or 3 to pin 4 measurement on the bridge. (Pins 2 & 3 would be shorted by the xformer.)

    What more do you want to know about the diode?
    If it is a transient diode it doesn't conduct at all in normal circumstances (up to 16V), but given the catastrophic failures it could benefit from an upgrade (as well as the PCB).
    Just use any beefier transient diode that you are able to fit on there, it doesn't have to be a SMD.
     
    Last edited: Dec 1, 2009
  5. mypcbis$crewed

    mypcbis$crewed

    58
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    Nov 29, 2009
    There's modesty for you ;P You must have some extensive background... I have very little and am honestly in over my head, but have great need to fix this device and to try and figure out what caused it!

    As for experience, time and patience.... I only have the latter two...
    I will answer the cathode question in the next post since I will post pictures to help illustrate the answer.

    More pinouts for the rectifier:

    pins 2 & 3 show 1940 mv in both directions? Can I assume that this is correct due to the rectifier's nature at the center of the bridge?

    pin 2 and 4 (-) 1850mv forward and reverse - neg lead on 2 post lead on 4 500mv

    pin 3 and 4 1865mv forward and 508mv reverse

    As for the diode, I just wasn't sure that I could substitute it without altering the function of the board, but you make a good point, the only purpose is to act as a one way valve and to prevent backflow, so I guess it would be safe to use a higher rated part.

    Thanks again :)
     
  6. mypcbis$crewed

    mypcbis$crewed

    58
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    Nov 29, 2009
    I believe pin 4 to be the ground pathway. If you look on the photo, there are some smaller 3 pin flat diode packages that have their heatsink soldered to this portion of the same trace. The trace continues on to the circle with "A" next to it. Here the trace passes into a power resistor and then to the red square which is a pair of caps, on the other side of the caps is a LM340S (other side of the board). From here it goes into a multipin ic. I hope this helps some. Let me know if you need more pics/info.

    Thanks again :)

    [​IMG]
     
  7. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    With interest, time & patience follows experience. :)

    Wery well, the bridge seems ok then. The transformer is obviously not connected.
    Measurements between the the center pins, and between the outer pins are not of vital importance since they represent two diodes in series, and they are covered separately by the other four measurements.

    Now, I suspect there is a battery involved here, connected to the black & red wires at the top of the picture. I guess it could have been connected backwards, and this has made current pass through the power transistor above point A - via the burnt track - through the diode and into ground via bridge pin 4.
    It is the components connected to this track that might have been damaged.

    I guess bridge pin 1 supplies a charging circuit for the battery.
    What are the labels on the power transistors, and where is the LM2673?
    The LM340 delivers regulated 5V to the digital circuits and would be protected against reverse polarity damage by the power resistor.

    To get better pictures you could try to get the illumination at an angle and the camera square to the pcb. Try to find the closeup mode (flower symbol) and use no flash. If no closeup mode is available then it's better to use a long enough distance so that the focus will be in range. Both sides of the power area would be of interest.
     
  8. mypcbis$crewed

    mypcbis$crewed

    58
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    Nov 29, 2009
    Very well said :)

    Thank you for the explanation of the bridge pin out, it seemed logical to me that that should be the case, but I wasn't sure on how to interpret some of the readings.

    Yes, there is a battery backup involved, however, it has been connected to the panel for some time while the panel has been in operation without incident. I will post the background of this device and how it came to be damaged in the next post, as this post will be lengthy with photos and I think the background info will only serve as humor for you!

    Back side of board with description of chips
    [​IMG]

    closeup of backside
    [​IMG]

    Front side of board around power regulator ic LM2673, LM340 is above coil.
    [​IMG]
     
  9. mypcbis$crewed

    mypcbis$crewed

    58
    0
    Nov 29, 2009
    The pcb is off of a home automation system. The circumstances are really messed up..... I got the system a couple of years ago and managed to wire up a lot of it. Life and money prohibited me from finishing the majority of it, but I picked it up again a few days ago. I was installing a piezo sounder using cat5 for the external alarm and thats where things got messed up. The total load for all devices on this board can not exceed 1 amp. I looked up the specs on all the powered devices and came up with about 570mA, which is way under the 1A max. So it got me thinking about the wiring....42 feet of 24 gauge cat 5....everything worked fine until the sounder was added. My thinking on this is that the resistance in the wire used to hookup the sounder somehow pushed the limit of the transistor. Although everything worked fine on cat5 wiring, but the sounder, the system maybe having too much voltage drop over the length of the wiring or just too much amperage is being conducted due to the low resistance of the wiring? I have no way of measuring this now, but that is what I am leaning towards. So hopefully if I can salvage this board, which is no longer made, I can rewire the sounder with larger gauge wire, take it off the system and measure the amperage at the connection to the system of all the devices, in steady state and in active state and do the math.
     
  10. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    I had to both chew & sleep on this one..
    I doubt that a small sounder would overload the regulator, and that thin wiring could be blamed also.
    Thin wiring & its resistance would only serve to decrease the current draw.
    The charging of the battery places a much more variable load on the LM2673.
    Unless you dropped some wire clippings on the LM2673 shorting it out, I lean more towards static discharge damage or just coincidence. It's supposed to be very overload & short circuit proof. It's good for 3A dc out, and the datasheet suggest a 4.5A peak limit whereas this circuit sets it to 3.7A peak.
    Nevertheless, whatever made the LM2673 fail, it shorted out and delivered the full raw voltage to the battery which absorbed most of the current - even if protected by F1. Then the track failed and the poor little transient diode got the full voltage, releasing its magic smoke quickly.
    Did you check out D51 btw.?
     
  11. mypcbis$crewed

    mypcbis$crewed

    58
    0
    Nov 29, 2009
    No problem, I am patient and quite grateful that you have been able to help me thus far :) The manufacturer was to call me back yesterday to let me know if they would give me the schematics as this is an out of date board, but I wont hold my breath....

    How did you determine that this circuit was limiting the lm2673 to 3.7A? Is it by one of the other components value?

    What part is F1 referring to? I see it on the pcb near the wires that charge the battery, but I am not sure what it refers to.

    D51 is shot - 4k ohm reading on tab (cathode) and each leg, forward and reverse (its a 50WQ06FN)

    The back of the board has another 50WQ06FN that is shot as well, its the one with the smaller red arrow pointing to it. 4k ohm readings forward and reverse.

    The good 50WQ06FN on the back side of the board with the large red arrow shows 125k ohm forward on both legs and 5k reverse

    Also between the lm2673 and d51 is a part marked "13" any idea what this is? A continuity test of both sides goes to 0 on the meter, and I get a reading of 0.3 ohm forward and reverse.

    Seems like the plot deepens a little, more parts that I had originally anticipated have failed. Tally thus far lm2673, 1 trace, smbj15a and two 50WQ06FN? More of your thoughts, when you get a chance please! :)

    thanks in advance.

    - John
     
  12. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    The resistor under the LM2673 marked 103, connected to pin 5, determines that. The datasheet suggests using 8.2k instead of the 10k used here.
    If you tell me the numbers on the last resistor in that row I can tell you the output voltage.

    F1 is the yellow device, a PTC self-resetting thermal fuse.The markings on it will reveal the rating.

    If D51 were the first thing to go, then the pulse current could have risen too fast for the protection circuits to shut it down in time. The output transistor would short out and dump the primary energy, overpowering D51 & the transient diode. Then battery current could flow back through the now shorted transient diode, finishing off the track.

    Beware that some suspect components might still be ok. The measurements can be a result of other components in connection to the one in question, so you'll need to do some desoldering to tell for sure. Sometimes I use a lab-supply set for low voltage & high current in parallell to the suspect component to see if it heats up or not.
    The LM2673 is going to need a big, powerful, & temperature controlled soldering iron to de- & resolder safely. Snip off its legs first before desoldering the body.

    The part "13" is just a ferrite core. It's probably just 1 microHenry and should measure 0 ohms. The task of it is presumably to reduce ringing & risetime of the switching current.
     
  13. mypcbis$crewed

    mypcbis$crewed

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    0
    Nov 29, 2009
    The last resistor under the lm2673 was marked 1052. The yellow thermistor? is marked:
    Line 1: B
    Line 2: R400
    Line 3: 5021S

    When I get some more time, I will desolder the suspect diodes and retest and post the results. My car crapped out on me recently and I am stretched to get that resolved first (struts went out) it pours sometimes ;)
    Can you make a suggestion as to what type of iron I should look
    for? Thanks again! :)
     
  14. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    The raw charging voltage from the regulator is ((1.25V/1020)*10500)+1.25V=14.12V
    Subtract then a little drop through the 4.7 Ohm resistor and the 4539 dual N/P MosFet (8 pin "IC") and you get about 13.8V at the battery.

    The B on The PTC is the Bourns trademark.
    The R400 is the part number and also indicates that it's a 4A unit.
    The 5 and 021 indicates it was made in 2005, day 21, and the S means it was made in China.
    It'll stand 8A for quite some time, and it can take up to 12 seconds (usually 5) at 20A before it trips, so it's no wonder the track could burn before the fuse tripped.

    Iron, well, a 50W (or more) temperature controlled one with a(n exchangable) flat-blade tip at least half as wide as the transistor tab would be ok. While it'll be somewhat costly it'll enable you to do the job with a better chance of success than a big uncontrolled iron. Some of those can soar to very high temps and quickly damage the IC & the PCB. A lesser powered iron will not be able to melt the solder quickly enough (or at all), and can damage the parts due to heat*time.
    You'll also need a smaller tip to solder the legs of the LM2673.
    I'm sorry I'm unable to suggest any brands or places, you'll just have to shop around.

    But first things first, you'll want to get you strut issue straightened out before anything else!
     
  15. mypcbis$crewed

    mypcbis$crewed

    58
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    Nov 29, 2009
    Again, many thanks. I do have a smaller iron so I will have to look around for something else. So far I have seen these two one is 50w and the other is 45w. I haven't seen larger than that. Will either of these be suitable for the task?

    Any comments on either unit?
    http://www.amazon.com/gp/product/B000I30QBW/ref=cm_cr_asin_lnk

    http://www.amazon.com/Weller-WES51-...ref=sr_1_2?ie=UTF8&s=hi&qid=1260150561&sr=8-2

    The strut project commences tomorrow....I hope to win that one ;)

    Do you think that the PTC is too highly rated? Should it have been of a lower rating or was it meant for transient spikes?

    Should I desolder using a braid or bulb for the back of the lm2673?

    Thank you
    - John
     
  16. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    The Weller is a classic that I use myself and I know there is a wide selection of tips available for it.
    The other one also looks ok but I have no idea about the tip selection for it. The tip in the picture looks too slim for working the body of the 2673.

    The PTC is probably about right, so it's the transient diode & the track that needs upgrading.

    Removing the 2673 I'd first clip the legs off it with a slim, pointed pair of cutters. Then heat the tab while trying to slide it off sideways & up using a pair of pliers. Then remove the remaining legs with the iron and clean up the pads with a braid.
    I never tried a bulb, but I use a spring-loaded plunger sucker for hole-mounted parts.
    When resoldering I imagine a dab of paste-solder under the tab wil make positioning easier, but lacking that I'd squeeze a piece of solder real flat and put it under the tab. That way you have one hand free for positioning it with a pair of pliers while the solder melts where it counts. Do the legs last, with a pointed tip.

    So how did the struts do? Did they pose any challenge? Who's the winner? Will there be a rematch? ;)
     
  17. mypcbis$crewed

    mypcbis$crewed

    58
    0
    Nov 29, 2009
    Oh my God....the struts took their toll on me! It was a 10 hour grudge match with me refusing to throw in the towel. I ended that day with one side done and the other side stubbornly not going in..... so we called it a day with a tie. I on the other hand in retrospect probably lost as I limped away with a compressed nerve, numbness in half my big toe for 3 days, about 95% better now. I also could barely walk for a couple of days and all because I didnt want to undo the lower ball joint..... the torx head bolt was really rusted up and I feared that the head would mushroom out and I would be forever there drilling out this thick bolt! I resumed a few days later and took apart that lower ball joint bolt without any complications and was finished on that one side in under an hour :O just more insult to injury...

    Enough of that horror story, its finished and it rides really well again....I now have to do the rear struts and rear brakes....oh.... ;P

    Back to the task at hand. Please verify with me then the proper order of what I need to do since the manufacturer has decided that they do not provide schematics....

    Should I first obtain the Weller iron and necessary tips/braid/plunger/etc. and desolder the broken components, followed by desoldering at least one leg of some of the suspect components in order to verify their function? Can you recommend the weller tips that I may need? I was looking at the ETD and ETS sizes (large flat screwdriver for the LM's back and the ets for the pins)

    Will desoldering one leg be sufficient?

    Next, if suspect components are okay, order replacement parts for the burnt out items, replace, jumper the burnt trace with some wire and retest?

    What size wire would you suggest?

    After fixing all these items, I would reconnect with everything minus the sounder and check function and settings.

    At that point should I rewire the sounder and have another go or would I be better playing it safe and getting an external power supply to drive the sounder? (still not knowing for sure what caused the failure)

    Also, so that I understand this correctly, the diode next to the burnt trace, which direction was electricity flowing? Was it flowing from the negative side to the cathode and then through to the burnt trace? I was trying to reason out the flow and the side of the damage, but was unsure.

    thanks in advance! :)
     
    Last edited: Dec 14, 2009
  18. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Omg! My sympathies, I know what a fight a car can put up..Hehe..

    No schematic; no surprise..

    I find that I hardly ever use the long tips, I find they quickly loose heat out that long narrow tip, and using a big tip on small pads is actually no problem (but in your case the reach may be). Anyway, I'd consider the ETK instead of the ETS, but these tips are so cheap so why not throw in a couple extra sizes. The ETD seems the biggest so you'll want that.
    Rememer to put on fresh tin first thing when you first turn on the power. The worst tip-killer is leaving them hot & dry. And so you don't wipe them after soldering & before putting the handle in the cradle like many "informed" people tend to do, if you need to wipe at all you do it immediately before soldering!
    Also beware that the new lead-free tin requires higher temps than the good old 60/40 tin. I hold on to my old rolls like they were gold, it's hard to find such quality these days..

    The diodes have both legs in parallell so most likely you'll have to lift both, but you can try one at a time. I often slide a thin knife (scalpel) between the leg & the pad while heating with the iron to separate them in a quick & controlled manner.

    Suggested sequence of repair seems ok to me.

    I'd patch the track with one 20 gage or two 24 gage wires. One 24 gage is a bit thin, & 18 gage is overkill.

    After initial test w/o sounder I'd suggest connecting a small signal lamp in series with the wiring from the board. That way it'll light & protect the board if there's a short somewhere.

    The exact sequence of events had me pondering for a long while, and I can't say for sure what took place when, but I'm quite sure battery current flowed back & through the diode once it had overheated & broken down due to overvoltage from the psu after the IC had shorted out, and lastly the trace burned out.
     
  19. mypcbis$crewed

    mypcbis$crewed

    58
    0
    Nov 29, 2009
    Ok, I will be ordering the necessary equipment in a day or so and as soon as it comes in I will post back. Thank you ever so much for your help, you have been a life saver! :)
     
  20. mypcbis$crewed

    mypcbis$crewed

    58
    0
    Nov 29, 2009
    Well, my luck has changed some! Resqueline, the board manufacturer had gotten back to me over the three week period and initially told me no schematics, etc. but apparently they took pity on me b/c the board was out of date and said that if I shipped it to them they would upgrade it to the current model! Wow.... I am grateful for your help and diagnostic skills and with any luck when I receive the new unit, it won't blow out on me as well :eek::eek: Only downside was that I ordered all the parts....at least the soldering station I was able to cancel.. Thanks again :) I will post again when the unit arrives and with some testing to let you know the values of the devices as I add them onboard. Maybe we can further clarify the mystery of the blown LM2673
     
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