# LM317 Driver - No output voltage

#### Lgzaqq

Joined Apr 28, 2022
4
Im trying to make simple laser diode driver from this video
I do everything like on video but i dont get any output voltage on lm317t
(1-12v tested on power supply)

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• 1.1 MB Views: 23

#### panic mode

Joined Oct 10, 2011
2,135
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

#### Jon Chandler

Joined Jun 12, 2008
192
The black wire is V+ input, and the red wire output?

#### sagor

Joined Mar 10, 2019
695
Wired wrong, output is center pin: (youtube video is misleading as well)

#### 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?

#### Jon Chandler

Joined Jun 12, 2008
192
In the constant current configuration the constant-current-output is from the ADJUSTMENT pin, not the OUTPUT pin.

#### Jon Chandler

Joined Jun 12, 2008
192
From the TI datasheet.

#### 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.

#### Jon Chandler

Joined Jun 12, 2008
192
The datasheets all show the constant current connection point is the adjustment pin. Semantics aside, I will defer to their recommendation.

#### Lgzaqq

Joined Apr 28, 2022
4
Yes, black to plus of power supply red to load
so its probably bad resistor? I want to get 150-200 ma

#### 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.

#### 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?

#### DickCappels

Joined Aug 21, 2008
8,710
Power = E^2/R
Power = 1.25^2 / 10
Power = 160 milliwatts.
You can use a 1/4 watt(0.25 watt) resistor.

#### 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
so its probably bad resistor? I want to get 150-200 ma
Yes its 3.3k
Your problem was already pointed out by @panic mode. You're using the wrong resistor value.
$$\large R = \frac{V}{I} = \frac{1.25V}{200mA} = 6.25\Omega$$
I see 0.25 watt and 1 watt resistors
Does it matter in this build?
It does.
$$\large P = I^2R = 200mA*200mA*6.25\Omega = 0.25W$$
You should use at least a 1/2W resistor.

#### 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.

#### BobTPH

Joined Jun 5, 2013
4,907
This us why we use schematics instead of words.

Bob

#### crutschow

Joined Mar 14, 2008
29,799
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.
Okay.
I missed what post #4 was saying and see know what you meant.