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I am trying to use LM317T as a current regulator and the setup is from its data sheet. I couldn't get it to work.

I want a constant output current of 10uA when input voltage is changed from 3V to 2V. Since the internal reference voltage between output and adj is 1.25V, I put a resistors of 125kohm from adj to output.

I am getting very high currents like 3mA and I can't figure out why. I appreciate all your help.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ IIRC, you need capacitors on the input and output of the regulator, close to the pins. Without them it might oscillate. \$\endgroup\$ – Leon Heller Mar 2 '17 at 13:46
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Datasheet check!

IADJ: Adjustment pin current

  • Typical 50 µA
  • Maximum 100 µA

So... it will not work.

For an accurate 10µA you will need to use a different topology.

  • Opamp-based current source
  • FET/JFET/MOSFET as a pass element, controlled by opamp. Bipolar could also be used, depending on required temperature drift.

Choice depends on factors you will need to mention, like how much parasitic capacitance you can tolerate at the output of your current source, voltage compliance, available supply voltage, etc.

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  • 2
    \$\begingroup\$ You also need a higher input voltage. The LM317 needs 3V above the output as an input, and for a current regulator you add the reference voltage to that (4.5V total) and that is the minimum input voltage. The 3V input is just too low for the LM317. It might actually regulate the current if it had enough voltage to operate on. \$\endgroup\$ – JRE Mar 2 '17 at 14:05
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The LM317 requires amplifier bias current. That means there must be a few milliamperes into the input, and out of the output, for normal operation as a regulator. The data sheet expresses this as a 'recommended output current' of 10 mA or more.TI LM317 data and also as (this gets confusing, I know ) "Minimum load current to maintain regulation" 3.5 mA typical and 10 mA maximum.

The '3 mA' current that you see, powers the internal circuitry so it can perform its useful function, i.e. control its output terminal, whether for voltage or current.

If you want to make a smaller current source, a more complex circuit will be required.

schematic

simulate this circuit – Schematic created using CircuitLab

The op amp has to sense near the positive rail. The output current is $$ 1.25V \times R2 \over {R1 \times R3} $$

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Try the LM134/LM234/LM334 range of dedicated adjustable current sources, this is still on the low end of their range but they are rated for down to 1uA. http://www.ti.com/lit/ds/symlink/lm334.pdf

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