Electronic – LM317 Voltage Regulator + ammeter = firecracker

failurelm317power supplyvoltagevoltage-regulator

schematic

simulate this circuit – Schematic created using CircuitLab

I don't know what I'm doing wrong. Perhaps someone can explain this. Is there any reason an ammeter would cause a voltage regulator to explode?

I'm trying to understand how to use the LM317 voltage regulator with a potentiometer to reduce 12 volts to a variable voltage between 1.25 and 10.5 volts. The 12V is generated from an ATX PC power supply, and the circuit I am using can be found here: http://www.electronics-lab.com/articles/LM317/

The only difference to the above schematic is that for R1 I am using a 680 ohm resistor, and R2 would be a 5k ohm potentiometer but I haven't even gotten that far yet. After building the circuit, I connected a multimeter to the output of the circuit and shorted what would have been the potentiometer (therefore simulating 0 ohms or full open) and read 12.5 volts. It read steady for several seconds, so I switched from volts to amps on the multimeter, and POW! The LM317 was no more.

Perhaps there is something fundamental I am missing, but what would cause this? Are my calculations (using the above website as a reference) completely wrong? If so, what would I need to do to accomplish my goal of taking a 12V supply and using a potentiometer to vary the output in as wide of a range as possible? Would there be a benefit to having a fuse (perhaps 1 amp) on the input voltage?

Best Answer

If you had a short in place of the potentiometer, you should have read 1.25 volts (or so) at the output of the LM317, not 12.5 volts.

When you changed your meter to read current, presumably moving the red lead to the "10 Amp" socket, the meter would have very low resistance. When you then connected the meter between the LM317 output and ground, a very large current would flow, apparently causing the LM317 to explode.

If I recall correctly, the LM317 is spec'd to deliver 1 amp, with due regard to voltage drop, power dissipation, heatsinks and phase of the moon - that only states the current that it can safely carry under suitable conditions - it DOES NOT limit the current to that value!

The current measurement function of a multimeter should only be used with the meter connected in series with a load whose current drain you wish to measure. It must NEVER be connected directly across a power supply.