Propeller Proto Board 32212: will 12V hurt it?

I've dug out an old proto board and it says "6-9VDC" right next to the DC jack, but how strict is that upper limit anyway? I'd like to run it off 12V. Will that break something?

If they're serious about that 9V thing, what's a good way to drop 12V to within the acceptable range?


  • Mike GreenMike Green Posts: 22,917
    edited 2018-05-11 - 03:58:45
    There's a 10uF capacitor across the input to the 5V regulator that will probably get ruined with 12V input. Safest thing would be to pick up an LM317 adjustable voltage regulator and configure it for 9V output. Download a datasheet for it to see what resistor values are needed. You'll need a 0.1uF input capacitor and the 10uF input capacitor on the 5V regulator will also work as the output filter for the LM317 if the distance between the two is less than 6". For R1 = 240, R2 will be roughly 1.5K (see the datasheet for the formula and schematic and double check my calculations).
  • Thanks, Mike. Is that 10uF cap all that important? If I apply 12V and it gets ruined, so what? Just wondering how little work I can get away with. #lazy
  • Mike GreenMike Green Posts: 22,917
    edited 2018-05-11 - 04:59:30
    The input capacitor is optional so you could remove it. You only need it if the regulator is more than a few inches from the input supply. You can always get a 10uF 25V or 30V electrolytic capacitor and solder it across the power input terminals of the protoboard.

    A lot depends on how much current you're going to draw through the 5V regulator. The only heatsink is the PCB itself and there will be a 7V drop through the 5V regulator that will have to be dissipated as heat. If you add another regulator in the chain, they'll share the heat dissipation. I'd set the LM317 to output 7V. If you're drawing 1A, that's 5W for the LM317, 2W for the 5V regulator, and 1.7W for the 3.3V regulator. The LM317 could have a "real" heatsink and that would keep everything else cooler. If your current draw is a lot less, this is mostly moot. Remove the input capacitor if it's only rated for 10V and substitute a higher voltage unit. Keep it if it's 16V or better.

    You did ask about dropping the 12V to within the spec'd range.
  • Mike Green wrote: »
    You did ask about dropping the 12V to within the spec'd range.

    Oh, totally. Doesn't mean I'm not going to look for a shortcut ;)

    Anyway, you make a good point about the heat dissipation, so I'll bite the bullet and go find an LM317 somewhere. Thanks again!
  • Four diodes in series will give you around 3V drop.... heat dissipation depending upon current drawn is usually the limiting factor with these simple linear regulators.
    You could use 1N400x rectifier diodes.

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  • Oh, man, just when I've decided to do the right thing. :)
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