Assistance request with lab PSU adjustable V/C module/ammeter hack

Discussion in 'General Electronics Chat' started by Canobi, Mar 8, 2015.

  1. Canobi

    Thread Starter New Member

    Mar 4, 2015
    Hi all

    Started an EE course recently to plug the large gaps in my electronics knowlage base, though I have many years of practical experience and basics under my belt.

    Among other interests, I have a passion for making all sorts of things, and since the course is online, it will all be done from home, including the various circuit experiments. As such, I suddenly have a need for an electronics lab.

    I have a few of the very basics already like variable temp iron, couple of test meters etc, so my first port of call was to make a quick and simple regulated lab PSU.

    Purely as an exercise, and as a stand in till I get/make a proper one, I went with an ATX hack as this is well trodden ground it seems and there's plenty of them out there to use as reference.

    A lab PSU was a good project for me as as I happend to have a new/unopened micro ATX PSU I bought by accident, an old RS exposure box (circa 1980's_not working), and an equally old metal project box kicking around.

    The exposure box became the main chassis case housing the ATX and load resistors with room to spare, and the project box was a perfect control console for the displays with room inside for other things.

    To smooth things out I opted to use adjustable voltage regulator modules on each rail to give me preset voltages of: +3.3v, +5v, +12v at 2A each, and brought the -12v rail to the front for combining with any of the above rails for flexibility.

    I'm now trying to setup an output rail using an adjustable V/C module to give a variable out of +0.8v to +24v and bought a duel display to go with it.

    All was well until it came to hooking up the ammeter.

    Firstly, I'll be getting a second V/A display as I found out what happens when one tries to use the same power line your trying to monitor (they really don't like that very much, rookie mistake).

    I've since added a wall wart as my isolated supply for the display and stuck that in circuit with the mains input after removing the wart guts from the casing (the gubbins inside were quite small and easy to fit).

    OK, so the voltage part of the display I get, and it was working fine, but I'm having trouble working out where I put an ammeter so I can read the modules current output (ie how much current the module's actually providing at any given trim pot setting).

    This setup would be ideal dor me as I play with LEDs a lot. I could then dial in the correct V/C and plug in the LEDs directly knowing how much load will be needed in each case/setup, it would also help with choosing required brightness levels for some applications too.

    However, after much trawling and coming up blank, this is starting to look harder than expected because if I have it right the normal options are:

    1. Putting ammeter in series with module input.

    Though I'm guessing this will display how much current is drawn by the module, not how much it's giving out.

    2. Putting ammeter in series to module output.

    Likely not to display anything until a load is added to the PSU rail, and then displays what is being used by the load.

    If the two above are correct, I may need assistance in finding a way to hack the module.

    My first thought was to find the current adjust output track, split it and bridge it with the ammeter (depending on physical possibility, ie track width/length/accessibility), but my guess is there may be something I'm missing due to lacking knowlage.

    Any ideas on this are most welcome :)

    In case anyone is interested, here's a couple pics of my PSU as it is: