Weird overcurrent condition - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: October 02, 2021, 07:00:28 pm
content: Hi,I have a number of assembled PCBs (see PDF schematic attached) which have been checked for components/assembly but don't seem to work.It is an insanely simple circuit. All it does it takes a audio signal and drives two sets of LEDs and a small coil. All three through separate current sources. Nothing more than that.To save battery, there is power switch Q1 which is turned on/off by inserting or removing the jack that supplies the audio input signal. That is done through the ENABLE signal on the switch contact in the jack (pin 10 of the jack).Whether I use the battery only (connected to J2) or the USB 5V supply or both, what happens is that when I insert the jack, VDD goes to GND and remains there after I unplug it. There seems to be some short circuit or overcurrent but cannot see anything wrong with the schematic.BTW R20 is 1K and R21 is 10K (not 0R as shown in the schematic... they were set to 0 just as placeholders for tuning later).Any idea anybody?Thank you

author: StillTrying
author_rank: Super Contributor
timestamp: October 02, 2021, 07:58:23 pm
content: Have you checked what the SIP32510DT's ENABLE does when it's stuck not working, 2M2 seems barely low enough to keep it pulled high if the ENABLE input leakage was near its max.

author: ricko_uk
author_rank: Super Contributor
timestamp: October 02, 2021, 08:05:58 pm
content: Thank you StillTryingEnable is rither 0 or 3.1V in all the various conditions.Any other ideas/suggestions?Thank you

author: StillTrying
author_rank: Super Contributor
timestamp: October 02, 2021, 08:21:40 pm
content: "Any other ideas/suggestions?"Not really,does the battery voltage stay up when VDD is down, the 3 to 6A current rating of the SIP32510DT means something should be getting hot somewhere.

author: ricko_uk
author_rank: Super Contributor
timestamp: October 02, 2021, 08:33:04 pm
content: The battery also goes to GND. But nothing gets hot.

author: StillTrying
author_rank: Super Contributor
timestamp: October 02, 2021, 08:44:09 pm
content: That's odd, with ENABLE low there should be zero load on the battery, is it a protected Li.Unless it's something to do with the STC4054GR charger's output.

author: mikerj
author_rank: Super Contributor
timestamp: October 02, 2021, 09:12:02 pm
content: No obvious short circuits on the schematic, but that doesn't rule out layout or construction errors e.g. solder whisker, an incorrect PCB footprint, an IC rotated 180 degrees etc.FWIW a completely uncompensated current source driving an inductive load is a recipe for oscillation, you'll be lucky if the coil driver works as expected.

author: Manul
author_rank: Super Contributor
timestamp: October 02, 2021, 09:17:24 pm
content: It is all to vague. Have you measured your VDD rail? Is nothing shorted? Have you tried to power VDD with external supply? Isolate problem, cut output trace of SiP32510. Does it work ok if output is cut? Power VDD externally with lab supply, what happens?

author: StillTrying
author_rank: Super Contributor
timestamp: October 10, 2021, 10:28:52 am
content: So, what was causing the missing VDD.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 10, 2021, 05:28:03 pm
content: Got a phone? Take a photo of your schematic and circuit build and put them in your post- then you may get a sensible answer.More brokern than you realise!

author: ricko_uk
author_rank: Super Contributor
timestamp: October 11, 2021, 04:03:19 pm
content: Thank you all,it turned out they assembled a completely different part in place of one of the op-amps. The laser etching on the part was not readable so found out only after replacing every IC and got to that one. After changing it on all boards that confirmed to be the case.Still no idea what IC the one they installed was.As I mentioned in the OP the schematic was insanely simple but given that I did check every track (on the CAD and the PCB under the microscope) and was working 16-hours days I thought maybe I did something silly in the schematic and could not see it, despite its insane simplicity. I need a holiday!!Thank you all again!!

author: jmelson
author_rank: Super Contributor
timestamp: October 11, 2021, 05:52:55 pm
content: Quote from: ricko_uk on October 11, 2021, 04:03:19 pmWell, this is certainly the downside of using a contract assembler for this sort of thing. Or, maybe an issue with unclear instructions to them about what parts should be used.Jon