Well I had an interesting fault the other day at work, one of the boards I built had a fault.
It's a display board using a MAX7219, feeding four large 7 segment displays, and 16 individual LED's. The fault was one lot of segments (and obviously two of the 16 LED's) didn't work. So initially, I added a bit of code to the software, to display all the segments - and they all worked?.
So presuming the chip must be duff, I un-soldered it, fitted a socket (I thought it was a good idea
), and replaced the chip. Exactly the same?? - have to dig deeper!.
Next I located the pin for the offending segments on one of the 7 segment LED's and used a multimeter on ohms to measure from that pin to each of the others in turn, and I found one that it read short to. I then checked round visually (again) as well as possible, but couldn't find a short anywhere.
So next I dug out my milli-ohm meter D: - something I was going to build, but then I found they are quite cheap from AliExpress so bought one.
So I measured across the offending pins of the LED I'd been measuring on, 0.36 ohms, tried the next one, 0.33 ohms and so on - then I tried directly on the MAX7219 - which read 0.065 ohms - lowest by far.
I next consulted the PCB layout to see where the tracks ran, and knowing the short had to be near the MAX7219 started working along those tracks from the chip using a magnifying head visor - and low and behold, I managed to spot a tiny short between the tracks. A careful cut with a fine scalpel, and all was well.
I knew that milli-ohm meter would come in handy!
It's a display board using a MAX7219, feeding four large 7 segment displays, and 16 individual LED's. The fault was one lot of segments (and obviously two of the 16 LED's) didn't work. So initially, I added a bit of code to the software, to display all the segments - and they all worked?.
So presuming the chip must be duff, I un-soldered it, fitted a socket (I thought it was a good idea
), and replaced the chip. Exactly the same?? - have to dig deeper!.Next I located the pin for the offending segments on one of the 7 segment LED's and used a multimeter on ohms to measure from that pin to each of the others in turn, and I found one that it read short to. I then checked round visually (again) as well as possible, but couldn't find a short anywhere.
So next I dug out my milli-ohm meter D: - something I was going to build, but then I found they are quite cheap from AliExpress so bought one.
So I measured across the offending pins of the LED I'd been measuring on, 0.36 ohms, tried the next one, 0.33 ohms and so on - then I tried directly on the MAX7219 - which read 0.065 ohms - lowest by far.
I next consulted the PCB layout to see where the tracks ran, and knowing the short had to be near the MAX7219 started working along those tracks from the chip using a magnifying head visor - and low and behold, I managed to spot a tiny short between the tracks. A careful cut with a fine scalpel, and all was well.
I knew that milli-ohm meter would come in handy!
