Hey,

I soldered 7 exact same high side switches (followed this explanation) (for a 7 segment display, I bought common cathode ones, stupidly) onto a board like this. 6 of them work pretty much as expected (11.8V output when the NPN base is 5V, about 0.6 when it's logic LOW/connected to GND), but one doesn't work properly. No matter what I connect the NPN base to, the output voltage seems to gradually climb towards 1V and then drop again. I think I had an accidental connection between my 12V and the PNP collector. I fixed that and carefully checked for more accidental connections, and then swapped out both transistors without success. I even checked that the resistors are still working as expected. What could still be broken here?

Thanks in advance,
Ben

 

Hello,

I think that the circuit on the following page will work better:
http://www.w9xt.com/page_microdesign_pt12_hv_pnp_switching.html
In that circuit the transistors will be cut-off better.

Bertus

 

Thanks for the reply.
I know that the adding another resistor there would be better, but I thought I could save myself some work since I don't need terribly fast switching and the LED forward voltage is 8.6V, and I tested it to work okay with the displays. But I just can't figure out what is going wrong on that one section.
Ben

 

Solved it - I had a really non obvious bad solder joint at the 10k resistor. It looked just fine from the outside, maybe a bit not shiny enough, but the multimeter measured an almost infinite resistance at the bottom of the board, while measuring 10k at the pins. I think the solder was contaminated by molten plastic or something, making it have too much resistance/not bind to the leg correctly.