LM317 Driver - No output voltage

panic mode

Joined Oct 10, 2011
2,135
not everything... your resistor seem to be 330Ohm instead of 10Ohm
if the value is 330 Ohm that means LM317 output current is 1.25V/330 Ohm = 3.78mA
the circuit in video is made to output 125mA
 

dl324

Joined Mar 30, 2015
14,467
Welcome to AAC!
I do everything like on video but i dont get any output voltage on lm317t
(1-12v tested on power supply)
Did you measure the voltages on the 3 terminals to see what the regulator was doing? LM317 aren't guaranteed to operate with less than a 10mA load current.

Is black connected to your power supply and red connected to the load?
1651185284376.png
 

crutschow

Joined Mar 14, 2008
29,799
In the constant current configuration the constant-current-output is from the ADJUSTMENT pin, not the OUTPUT pin.
No.
The ADJUSTMENT pin may be connected directly to the output but the current is still from the OUTPUT through R1.
The ADJUSTMENT pin is just connected for reference, and has less than a mA of current coming out.
It maintains a constant 1.25V across the resistor to generate the constant-current.
 

MrSalts

Joined Apr 2, 2020
1,762
Yes, black to plus of power supply red to load
so its probably bad resistor? I want to get 150-200 ma
What size is that resistor? I see orange orange red (3300 ohms) - correct? This means your current will be 1.25v/3300 = some microamps.

you should be using a 10 ohm resistor for 125mA. Or 5 ohm for a 250mA.
 

Thread Starter

Lgzaqq

Joined Apr 28, 2022
4
So you're only getting 0.00038 Amps (0.378 milliamperes).

same answer was given in post #2.
i have one more question
I see 0.25 watt and 1 watt resistors
Does it matter in this build?

Connecting two 2.2om 0.25w resistors give Me 4.4om 0.5w?
 

crutschow

Joined Mar 14, 2008
29,799
The datasheets all show the constant current connection point is the adjustment pin. Semantics aside, I will defer to their recommendation.
If you are referring to my answer, there are no "Semantics" involved.
I agree the connection point is the Adjustment pin.
But the current comes from the Output pin, not the Adjustment pin.
 

dl324

Joined Mar 30, 2015
14,467

Jon Chandler

Joined Jun 12, 2008
192
If you are referring to my answer, there are no "Semantics" involved.
I agree the connection point is the Adjustment pin.
But the current comes from the Output pin, not the Adjustment pin.
The problem is that post #4 incorrectly said the output to the LED should come from the output pin – this is incorrect for a constant current regulator.

In post #6 and with the figure in post #7, I pointed out that the output (to the LED) needed to be connected to the Adjustment pin, not the output pin. I was not trying to explain the inner workings of the LM317, just clarifying where the output connection had to be made. This is the central point in this thread.

Your reply in post #8 is technically correct. But it only adds to the confusion about the application under discussion here.
 
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