LM317 Constant Current Source Not Stable

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whitehaired novice

Joined Jul 15, 2017
284
I have bread-boarded a constant current source using the attached schematic— the standard example.

My goal is a constant 100 mA source but when I tried to fine tune it by placing a 0-199 mA meter across the output and trimming the resistance, I got flickering readings from about 107 to 95 mA. Fearing to run down my battery (which is four Li-ion cells in series, all fully charged) I switched to trying a 10mA source by increasing the resistor. Same problem—flicker from just over 10mA to about 9. The flicker rate is about 2 or 3 Hz but I didn't measure it.

In desperation I tried connecting a 470 mfd capacitor from ground to everywhere in the circuit, one try at a time. No effect.

I tried a second LM 317 with no effect.

I tried a different power source with no effect.

My guess is that I'm doing something dumb but I sure don't know what.

317.JPG Help appreciated. TIA

YES_SOMETHING DUMB--The power supply for the mA meter was low!!
Please don't reply to this stupit post.
 
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dl324

Joined Mar 30, 2015
9,341
My goal is a constant 100 mA source but when I tried to fine tune it by placing a 0-199 mA meter across the output and trimming the resistance, I got flickering readings from about 107 to 95 mA.
What is the input voltage and what is the resistance of your meter on the scale you were using?
 

rsjsouza

Joined Apr 21, 2014
214
One additional detail that I was facing thos past week with the exact same topology: the 25ohm wirewound potentiometer was bouncing between a few taps of the wire (it was not a multiturn but a 3/4 turn 3W), thus causing a slight variation on the output current.

That was solved by properly getting a fixed resistor divider.
 

AnalogKid

Joined Aug 1, 2013
8,230
That circuit is very simple and very popular, but the datasheet does not go into NEAR enough detail about the requirements for the resistor, especially if it is a pot.

ak
 
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