Fridge Timer UpdateProto board fixed !
See schematics below.
Well the firsts phase was quick, it appeared the transistor that drives the relay coil had died. It's B-E emitter junction had died open. OK maybe bad luck. So I replace it but wary, I solder a "socket" for the new transistor. Well, a piece of female 0.1" strip header, that is...
I am glad I did. 2 seconds later the new transistor dies of the exact same fate !
OK so there is indeed something wrong... but what... only way to fry the B-E junction is too much base current... but that's limited to 2+ mA or so with my 4.7K pull-up, and that resistor measures just fine...
Anyway after much experimenting and probing around... turns out the relay coil had become a short. hFE must have been a bit more than the 75 specified in the datasheet, which means the transistor was able to draw way more than the 800mA it's rated for (I noted the power rail dropped from 12V to 6V when the coil was energized ! ).
I think I damaged the relay when I soldered the wires to the coil terminals. Being upside down it was a pain to get solder to stay in place to stick to the wires. So I had to make several attempts. I remember one of the terminal "wobbling" in its plastic base... meaning the plastic softened enough for the pin to move...
Looking through the clear case of the relay, I can see that pin internally, leaning towards, touching the coil enameled wire. Maybe it was so hot that it melted the enamel and shorted the coil. The latter measured only 3+ ohms .... far from it's original 220 ohms...
So I replaced that relay with the one in the picture below. This time with thinner terminals that can go through the pad holes, so I could solder it onto the board, eliminating the risk of overheating the terminals. I wanted to avoid soldering a relay to the board as it now means I have to solder wires to the board hence bring mains to the board, safety and all that. So I hope everything will be alright.
After that, the board came to life just fine, rely is clicking away.
So I can now fit the board into the project box and add the power socket and give it a try !! Right=, right ?!!! NO !!!! It's 00H50 and freaking tired, I can barely see what I am typing, never mind what I was soldering just a minute ago ! I am going to bed ! !!!