LM317 voltage drop when load is applied...

I'm trying to use a LM317 to provide power to a Sparkfun Serial GLCD (which needs 6-7 volts). I have it circuited so that R1 across the Vout and Adj is 100 ohm and R2 connected to Adj and GND is a 1K trimmer set to about 430 ohm. By the LM317 calculator is should get 6.6 volts on Vout with a 12 volt supply. I check with my meter and I am getting 6.6 volts with no load.

The problem is when I connect the GLCD (which draws 80mA) to the regulator, the voltage drops to roughly 4.25 volts and the display doesn't work well. The only thing I can think of is that the 100 ohm resistor (R1) is causing the issue... the recommended R1 is 220 or 240 ohm. I don't have a 220 or 240 on hand to test this but I was wondering if anybody else had this problem?

Thank you.

I think you have things wired up right and it should work. Can you confirm how much current is coming from your 12V supply? Does it have a current readout, or can you use a DMM to measure the current going in to the LM317? It should be ~100mA with the GLCD attached, and at that current you shouldn’t be having a problem.


The Ruggeduino: compatible with Arduino UNO, 24V operation, all I/O’s fused and protected

Will do when I get home.

I can say that the 12 volt supply is 8 new AA batteries.

RuggedCircuits:
It should be ~100mA with the GLCD attached, and at that current you shouldn't be having a problem.

Unless it's a LM317L.
How about 1-10uF on the output?

I do not have caps on the input/output... probably not a bad idea.