Hey folks!
I'm trying to design a 15 volt power supply for a PIC micro programmer. I recently purchased the book "Processing and Customizing PICmicro Microcontrollers". The book came with a PCB for the programmer, so I figured I'd go ahead and build it.

The circuit requires at least 14.5 volts, then regulates the input voltage VIA a 78L12 +12volt regulator.

My ideas:
I don't want to spend $20 on a 15v wall wart and I don't have a transformer that supplies 15 volts so I'm thinking of building a variable power supply using an LM317T variable + voltage regulator. I assume this will be an acceptable circuit, as long as I have 15 volts output.

My question is: Do I connect the negative lead of my output power chord to the bottom right connection, just above ground? Also, should I ground the circuit at this point, to a metal chassis or something. Finally, should I install a small PC fan to blow air across the heat sink? BTW, the LM317T dissipates excess input voltage as heat, and I plan to use 28V input.

Your suggestions are greatly appreciated. Thank you.

Attached Images

lm317.gif (4.9 KB, 305 views)

 

Yes, The negative lead of the output should be connected to GND (pin1 of the 1µ cap). Wheater or not you need a heatsink on your 317 depends on the excess voltage AND the current you draw from it...

The power it dissipates is the voltage drop across it multiplied by the current you draw from it, so if you draw only little current (few mA's) then it won't get hot, nevertheless , installing a heatsink on it won't hurt.

About the chassis, it should only be earthed if it is a metal chassis, a plastic enclosure doesn't require earthing.

 

Thank you EXO

 

hi,

how about a little dash of input / output protection by using diodes :wink:

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