Accidently connected polarity wrong to a metering panel PM438 - Page 1
Forum section: electronics / beginners

author: ZeTeX
author_rank: Frequent Contributor
timestamp: July 13, 2016, 10:10:41 pm
content: Hi,I've accidentally connected a metering panel power supply backwards, so the + and - of the supply got connected to the - and +, unfortunately the meter didn't survive that and just stopped working, nothing smoked but I don't see anything on the display.Is there a way to fix it maybe? I don't have a schematic and I'm not sure what IC they are using as it is a chip on board.

author: Signal32
author_rank: Frequent Contributor
timestamp: July 13, 2016, 10:13:21 pm
content: Post pictures of the front/back of the meter PCB for better suggestions.One thing to easily check is to see if the voltage regulator on the meter is blown, check to see if it actually does have an external voltage regulator and check input / output voltages.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: July 13, 2016, 10:36:48 pm
content: Quote from: Signal32 on July 13, 2016, 10:13:21 pmthe other side is just the LCD display itself.I dont see any voltage regulator, only resistors and capacitors and the IC.

author: Gyro
author_rank: Super Contributor
timestamp: July 14, 2016, 08:23:45 am
content: That's an ICL7106 (or equivalent) based meter. They don't normally have a voltage regulator as their input voltage is fairly flexible (9v battery range). I don't see a reverse protection diode either. If you really want to keep it, you could remove the glob-top IC in the centre and solder a new one to the pad footprint, but whether it's worth it depends on how cheaply / easily you can get a replacement.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: July 14, 2016, 11:16:48 am
content: Quote from: Gyro on July 14, 2016, 08:23:45 amHow do you desolder \ remove the glob? do you need something special?

author: Gyro
author_rank: Super Contributor
timestamp: July 14, 2016, 11:49:41 am
content: Quote from: ZeTeX on July 14, 2016, 11:16:48 amThe glob is epoxy so I would think a smelly poke with a hot soldering iron might do the trick. It might be a bit time consuming though as you need to get it flat to be able to solder on a new chip. The die would probably come off the same way, make sure you clear all the bond wires to avoid shorted traces.I really wouldn't consider it until you've priced a replacement 7106 and made sure it's less than a replacement meter. Nothing to lose at this point however.Edit: just a quick check on ebay and the one listing for the PM438 is quite pricy whereas chinese ICL7106s are quite cheap. I don't know where you bought it though.

author: Ian.M
author_rank: Super Contributor
timestamp: July 14, 2016, 12:30:55 pm
content: +1 for digging with a very hot OLD soldering iron bit. Don't use a good one!I'd mask off everything except the blob with aluminium foil over cartridge paper as a thermal barrier, taping down in a square round the blob with aluminium foil tape, (or Kapton tape if you stock it) so you don't mess up anything else on the board.Use some small fragments of lump Rosin to improve heat transfer to the Epoxy. It also attacks and softens epoxy when boiling hot, so keep on scraping off what's loose and adding a little more Rosin.Get the board cleaned up (or fail) before ordering any parts!

author: Gyro
author_rank: Super Contributor
timestamp: July 14, 2016, 12:43:57 pm
content: Just one other thought - don't cook the LCD on the other side of the board. Hopefully it's spaced off sufficiently.

author: Brumby
author_rank: Unknown
timestamp: July 14, 2016, 12:48:45 pm
content: This is something I've never thought about - but I have to ask: How practical is it to try to replace a COB IC?

author: Gyro
author_rank: Super Contributor
timestamp: July 14, 2016, 01:00:14 pm
content: I think we're hopefully about to find out, I'll be interested to see the result too. Please document and show us pictures ZeTeX.

author: Ian.M
author_rank: Super Contributor
timestamp: July 14, 2016, 01:02:26 pm
content: Its impossible to replace a COB without a wire-bonding machine. but the O.P's board also has a footprint for a conventional SMD package, so all that has to be done is get the COB 'blob' and die off without b----ring up any tracks that go to vias under the blob (if any) then make sure its been cleaned up flat with no residual bond wires.

author: Brumby
author_rank: Unknown
timestamp: July 14, 2016, 02:04:53 pm
content: ... and a bit of detective work to figure out Pin 1?

author: Audioguru
author_rank: Super Contributor
timestamp: July 14, 2016, 02:24:43 pm
content: It is a cheap Chinese meter. Why not simply replace it??

author: ZeTeX
author_rank: Frequent Contributor
timestamp: July 14, 2016, 04:09:09 pm
content: Quote from: Gyro on July 14, 2016, 11:49:41 amAlright, I'm going to use an crappy tip on my soldering iron and setting it to the max tempeture, I'm also going to put an tape over the board so nothing will be damaged.The metering panel is dead now, so I cant ruin it more, might not work out but who cares, 7106 cost nothing compare to this panel which the only seller that ship to my country is this one:http://www.aliexpress.com/item/Digital-panel-meter-DC-voltage-measurement-Digital-LCD-header-PM438/32517120449.html?spm=2114.13010608.0.81.cwalhuand I also need to wait about 2 weeks to receive it.. if I can fix it for a couple of cents with a chip that my local electronic store probably sells then why not.Quote from: Audioguru on July 14, 2016, 02:24:43 pmits not that cheap. its 16$ for 5 of those - ~3.2$ for one - but there is only one seller that ships to my country and he is from hong kong so a lot of waiting time. also minimum order is 5..Quote from: Ian.M on July 14, 2016, 12:30:55 pmI will try aluminum foil + cartridge paper, hopefully it will work.I don't have rosinQuote from: Gyro on July 14, 2016, 12:43:57 pmhopefully it wont happen. there is not much I can do about it but I will try my best to not get too much heat.Quote from: Gyro on July 14, 2016, 01:00:14 pmI will!Thanks for helping everybody!

author: ZeTeX
author_rank: Frequent Contributor
timestamp: July 14, 2016, 06:42:40 pm
content: Well I managed to remove the COB, no vias where damaged.I think its ready for a new IC.it was ugly but at the end I managed to remove it pretty cleanly, I think with some practice and right tools I will be able to remove those without a lot of mess.and the smell..http://imgur.com/a/ipsCs

author: Gyro
author_rank: Super Contributor
timestamp: July 14, 2016, 09:07:36 pm
content: Well done ZeTeX it looks like you had fun. Thanks for posting the images.Well I did warn you about the smell... though in your case I reckon it was mainly the plastic surround!It looks as if you're in good shape to buy a new 7106 now.

author: batteksystem
author_rank: Regular Contributor
timestamp: July 15, 2016, 05:57:17 am
content: Quote from: Gyro on July 14, 2016, 08:23:45 amThat is not an IC, it is a wire-bonded die inside it. I would say buying a new one would be a far easier job.

author: Brumby
author_rank: Unknown
timestamp: July 15, 2016, 06:58:55 am
content: Quote from: batteksystem on July 15, 2016, 05:57:17 amAhhhh.... the dieisthe IC.It's just notencapsulatedin a standard package like SO or DIP - but rather by a blob of black stuff on one side and the PCB on the other.

author: batteksystem
author_rank: Regular Contributor
timestamp: July 24, 2016, 05:06:58 am
content: Quote from: Brumby on July 15, 2016, 06:58:55 amYou're right, just I always treat the wire-bond die and DIP/SMD IC as two complete different things, as the manufacturing process are very different.

author: mikerj
author_rank: Super Contributor
timestamp: July 24, 2016, 01:36:03 pm
content: Quote from: batteksystem on July 24, 2016, 05:06:58 amThe manufacturing process of the die is exactly the same, because it's the same die. Instead of wire bonding the die to a lead frame and encapsulating it, you wire it to a PCB and encapsulate it.

author: batteksystem
author_rank: Regular Contributor
timestamp: July 25, 2016, 01:15:05 am
content: Quote from: mikerj on July 24, 2016, 01:36:03 pmOn the other hand, there are no need to wire bond the die and wait for the epoxy to dry if you use the normal IC. These days except very low cost, very high volume items (watches, LCD, etc) most already use pre-capsulated IC.

author: Ian.M
author_rank: Super Contributor
timestamp: July 25, 2016, 01:29:39 am
content: Lets further increase your confusion!What about Flip-Chips?Take a bare die, passivate it, add solder bumps, and flip it over and solder it direct to the PCB

author: Brumby
author_rank: Unknown
timestamp: July 25, 2016, 05:14:57 am
content: Quote from: Ian.M on July 25, 2016, 01:29:39 amI was waiting for someone to throw that one into the mix....

author: batteksystem
author_rank: Regular Contributor
timestamp: July 25, 2016, 06:49:21 am
content: Quote from: blueskull on July 25, 2016, 03:08:57 amThis would depends on how thick and how good (gold!) the wires you use for wire bonding.

author: mikerj
author_rank: Super Contributor
timestamp: July 25, 2016, 06:52:01 am
content: Quote from: batteksystem on July 25, 2016, 01:15:05 amThis is a low cost item, so I don't really see your point? The cost of a bare die for a reasonably simple IC is very small so packaging costs can be a very significant overhead.