Finding a short on Common Anode RGB LED Array - Page 1
Forum section: electronics / beginners

author: chrono68
author_rank: Contributor
timestamp: January 24, 2024, 03:43:24 pm
content: Hey all,Attached is a very basic, fundamental schematic of a common anode, LED array I am helping with. Each cluster of 3 Diodes is Red, Green, Blue in a 3-in-1 Common Anode package.The circuit is sourced by a muxing circuit which controls the rows individually, and a different constant current driver for each color, sinking each column individually (the example is a simple 3 row, 1 column array).How would one find where a cathode side short between two colors of one LED package (Red line in pic) is located? Here's the catch:he only access to measure on the circuit is at the muxing chip and CCD leads. There is no access to the traces as it is mostly vias or the traces run under the LEDs.I have tried a few ideas like miliohm-meters and other suggestions like here:https://www.eevblog.com/forum/testgear/advice-for-a-short-circuit-detector/No luck. I have also tried to find the short using an IR camera but the way they are muxed there's no detectable difference.It seems like a lost cause since the only thing I can measure is effectively from the anode to cathode of an LED which won't include the short.

author: RAPo
author_rank: Super Contributor
timestamp: January 24, 2024, 03:49:22 pm
content: try an infrared camera I use infiray with a macro lens adapter).

author: chrono68
author_rank: Contributor
timestamp: January 24, 2024, 04:05:42 pm
content: Thanks for the reply!Unfortunately, it does not show up. In white hot mode, you can see the column radiates differently cause of the color, but I'm unable to detect a difference in temperature of any LED in the column.

author: RAPo
author_rank: Super Contributor
timestamp: January 24, 2024, 05:04:44 pm
content: hmm, thats to bad. I´m out of options now.

author: chrono68
author_rank: Contributor
timestamp: January 24, 2024, 06:39:57 pm
content: At least it means I'm not totally a dummy! (Only partway for getting roped into this)

author: Ian.M
author_rank: Super Contributor
timestamp: January 24, 2024, 06:48:55 pm
content: Isolate the two cathode connections to the faulty column, inject a reasonably high current through the fault cathode to cathode, e.g. 100mA, and see if you can locate the fault with an analog output hall effect sensor. It may help to chop the current at a frequency not harmonically related to your mains frequency so you can use AC coupled amplification of the sensor output for more sensitivity without saturating, and discriminate against static and powerline magnetic fields.

author: chrono68
author_rank: Contributor
timestamp: January 24, 2024, 07:44:50 pm
content: Quote from: Ian.M on January 24, 2024, 06:48:55 pmSay this might be an idea. Wonder if injecting 100mA would have the short finally show up on an IR camera?Or I might need a little bit more on the hall sensor. Connect the output to a meter and look for the one with the largest rise?

author: chrono68
author_rank: Contributor
timestamp: January 24, 2024, 08:34:52 pm
content: So I soldered some 26awg wire to the CCD legs and set a supply to 80mA CC and it didn't heat up very well (Lot of copper on this board) so I cranked it up to 150mA just to see and tada!Image isn't very good I have to dial it in more but even just the quick shot I took to post here got me within 4 LEDs (of 26).Side note: FLIR's post processing software is amazing. It lets you adjust the thermal range after the picture is taken and do things like overlay with the regular photo.If I can get it dialed in, then that's perfect but I want to know more about the hall effect sensor solution. It would certainly be a cheaper method than buying a FLIR.

author: Ian.M
author_rank: Super Contributor
timestamp: January 24, 2024, 09:33:38 pm
content: Pro:https://www.power-mag.com/pdf/feature_pdf/1327592496_TTI_Layout_1.pdfDIY:http://circuitszoo.altervista.org/current_probe.html

author: chrono68
author_rank: Contributor
timestamp: January 24, 2024, 09:39:46 pm
content: So it's like a really small current clamp minus the clamp.

author: Ian.M
author_rank: Super Contributor
timestamp: January 24, 2024, 10:49:41 pm
content: Yes. I've just done a quick test switching 100mA down two adjacent conductors of a ribbon cable, with a slim TO-92 package '49E' hall sensor (@5V supply) from an Arduino sensor kit. With the sensor end-on I got approx 100mV swing when I switched the current on/off. With the face of the sensor against the conductor pair, about 1mm away I got approx 40mV swing.Here's Diodes Inc.'s datasheet for theirAH49Esensor, which has approx. 1/3 the sensitivity (typ 1.6mV/Gauss) of the AllegroA1324(5mV/Gauss) one in the DIY link above.

author: chrono68
author_rank: Contributor
timestamp: January 25, 2024, 07:22:57 pm
content: Well today's update is great news. LED with an internal short showed up around 200mA. Got to use the IR camera one more time and dialed in the focus and got that sucker.Also got approved to get a used Aimtti I-Prober 520 to see if it can detect the short through the height of the LED package. To switch current could I use a bog standard signal generator?

author: Ian.M
author_rank: Super Contributor
timestamp: January 25, 2024, 07:48:57 pm
content: You'd probably need some sort of power amplifier on the signal generator output. You also want a series resistor to turn its voltage output into an approximate current source and some (switchable) clamping to limit the voltage it can put out to lets say +/- 500mV, enough to drive plenty of current through a short, but not enough to drive significant current through silicon junctions. At that point, building a dedicated signal injector you can just pull out the draw and use is probably preferable vs having to hook up signal genrator, amplifier, PSU and the current injector circuit.

author: chrono68
author_rank: Contributor
timestamp: January 30, 2024, 03:17:46 pm
content: I purchased this HIKMICRO PocketC (Same as the Pocket 2 but no wi-fi module) from Amazon here:https://www.amazon.com/dp/B0CL6JNF2XAnd a Macro Lens Adapter here:https://www.amazon.com/dp/B0BHP17WX1Didn't spend too much time without the macro lens attachment but with the attachment on the LED popped out like a sore thumb. As much as I want to personally pursue the hall effect options, I think we're down to a cost-effective method and chapter can be closed. I appreciate the advice @Ian.M and putting up with me!