Story alert.
Working on a little SMPS that was given to me as dead (no output) and I plugged it in before opening it up and couldn't get any AC/DC voltage readings.
The device the SMPS powered I confirmed was ok by using the bench supply initially current limited, found normal operation at about 1A and since the SMPS was 3A, thought that checks out and it's back to crack open the SMPS.
The SMPS pcb was a bit of an eye opener. It had two tiny wires going to the 240V mains socket, one broke off or had already been broken before I got in, the earth connector on the mains socket went nowhere so that was a waste of time. The worst problem was lots of shitty soldering and a couple sets of SMD resistor bodges on the output. Convenient lack of protection and filtering on the mains input.
Anyway I tested the primary and the mains input through the fuse, bridge rectifier, every transistor and diodes both ways and any type of weirdness, most caps for shorts, low ohm resistors for opens. Hooked it up and measured the voltage of the filter cap that was good. Discharged it and took a quick check of the transformer and then moved onto the secondary.
Secondary looked like two diodes in a half wave setup, one filtering cap, a more elaborate control circuit than anything. Looking at the bodges one SMD resistor on the output was reading weird so I removed it and found it had a friend added underneath, added up to 1K so thought fair enough and put them back in. Checked optocoupler nothing shorted or open, checked output and not shorted.
I did find one free floating SMD resistor embedded in the solder on the positive of the output, wondering if it was stuck on the assemblers iron and they added it in. I tacked that open on another spot for later retrieval. Reworked a lot of the PCB as it was a mess and the joints looked really bad, too much solder on some of the bodges and a lot of flux before I got there.
Time for a test flight, hooked up the mains and measured the main filter cap - good. Checked for DC on the output and across the output cap - nothing.
Thought ok I'll load it down 10mA and see if it starts up because at this stage the secondary was dead and everything else looked good to go. Touched the load on the output a couple of times for a second or so it didn't kick in nothing seemed to be melting. Then left it on for a few seconds.
BANG!
Well she started up alright and this is the point and moral of the story, always wear eye protection. The fuse while it didn't explode in my face it was brutal and left a nice couple of inches of carbon flash at ground zero. I'm glad my fingers were down at the output end away from the action as well.
The next thing is I should have really used the dimbulb tester circuit for some current limiting, but things were looking so promising
, it could have saved me having to now additionally find and order some micro sized fuses for it.
So you got this far to the end of the story? What could be the problem with the SMPS? What did or didn't happen and what did I forget? I have my own ideas of course but it makes for discussion.