Puzzled with the result of LM317 constant current

So, LM317 can be used as a constant current regulator by adjusting the voltage in the Adj pin. The internal voltage reference is 1.25V, and so using a 1.25/0.900 ~= 1.4Ohm resistor and a constant current source of 12v, one would output a constant current of 893mA.

simulate this circuit – Schematic created using CircuitLab

At least in theory, and according to every site I read on the internet; however, I just did that and measured the current passing to the led: approx. 200mA!

What am I doing wrong?

Addendum: I'm using these type of leds, and I'm measuring the current passing between the limiting resistor R1 and the LED1.

• What is the forward voltage of the LED? How do you measure the current? What is the power dissipation at the LM317? – venny Sep 12 '15 at 12:56

Since I cannot easily post images in comments... "there's your problem" as the Hyneman would say.

The voltage drop across Rs is only guaranteed if certain conditions are met:

• minimum voltage drop from input to output [of LM317's pints] of 3V, and
• minimum load current of 10mA, and
• power dissipation below 20W.

In this case the first condition wasn't met.

If your LED lamp assembly is 10W at 900mA it needs about 11.1V to operate. The LM317 current source needs 1.25V in the resistor and a further 3V headroom to operate, put it all together and your circuit needs a supply of at-least 15.35V to behave as you intend.

At that voltage and current the LM317 will be burning over 3.8W so you'll be wanting a heatsink on the LM317... You're probably better off with a switched-mode led driver, you can get an assembled PCB on Ebay etc for a few bucks.

• Ahhh, yes, I was exactly googling about the voltage drop on the LM317. So the problem may be the input voltage! The thing is that I want PWM control the LED, and I was using a trick I read with a transistor connecting Adj to Gnd. Need to search for PWM-capable LED drivers... Nonetheless, I wanted to understand the problem, so, thanks :) – Hugo Sereno Ferreira Sep 12 '15 at 13:07
• yeah, if it's not over-heating. – Jasen Sep 12 '15 at 13:08
• I'm using a fairly big heatsink, so I don't think it's due to overheating. – Hugo Sereno Ferreira Sep 12 '15 at 13:10
• Actually, now that I think about this, I'm not so sure that I completely understood the 200mA figure. Why does the LM317 decides to output a lower current instead of an higher current at a lower voltage? Is that a consequence of the limiting resistor working under a lower voltage? – Hugo Sereno Ferreira Sep 12 '15 at 18:27
• like most electric equipment, it doest work correctly if the supply voltage is too low. – Jasen Sep 14 '15 at 7:59