# LM317 voltage drop problem in constant current configuration

Joined Jul 22, 2021
4
Hello peeps,

I have been working on a 10mA constant cource that supplies atleast 5V at output. I have been using the LM317 regulator for creating the constant current source of 10mA but the voltage drop is too high. I read in the datasheet of LM317 that it has a 1.25V drop. I have been using a 9V DC input. I have attached the datasheet for LM317, the spice models I have created as well for your reference. It would be great if anyone could help me sort out the problem. I need to arrive at a circuit that outputs 5V at 10mA. Any help and comments are appreciated much.

Thank you

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#### boostbuck

Joined Oct 5, 2017
414
If you want to output 5V at 10mA, then your load will need to be 500 ohms.

10mA across 10 ohms is only .1V, with the rest dropped across the regulator and resistor, as you see in your sim.

Joined Jul 22, 2021
4
If you want to output 5V at 10mA, then your load will need to be 500 ohms.

10mA across 10 ohms is only .1V, with the rest dropped across the regulator and resistor, as you see in your sim.
Hello, Thank you for your input. I shall check it out.

#### AlbertHall

Joined Jun 4, 2014
12,292
I have been using the LM317 regulator for creating the constant current source of 10mA but the voltage drop is too high.
You will have 1.25V across R1, but the chip itself needs a minimum of 3V between input and output, so for a 5V output you will need an input voltage of 5V + 1,25V + 3V = 9.25V

#### BobTPH

Joined Jun 5, 2013
8,060
Leann Ohm’s law. You cannot have both a constant current and a constant voltage with a variable load.

Bob

Joined Jul 22, 2021
4
Thank you for the input

#### Bordodynov

Joined May 20, 2015
3,116
See