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Topic: LM317T Voltage Regulator problem (better photos) (Read 1 time) previous topic - next topic


Aug 24, 2012, 11:35 am Last Edit: Aug 24, 2012, 12:04 pm by RobertEagle Reason: 1

I'm in a little dilemma with a voltage regulator. It's called LM317T
Here you have a link to it's datasheet:http://www.fairchildsemi.com/ds/LM/LM317.pdf
This is an example of a circuit: http://www.zen22142.zen.co.uk/Circuits/Power/lm317reg.gif

The R1 is 360 Oh and the R2 is 519 Oh. What I want is a voltage of 3V. In this case the math is saying that it should output 3.05, which is okay.

I've made the connections but what's giving me is almost always lower with 2 volts than the input. The resistors are fine, the bonds have no problem.

What is the problem with my wiring?
Here are some photo samples:

Thank you,

PS:The green resistor is the 519 Oh one, and the other one is 360.


The voltage regulator has a drop out voltage (see page 3 of the data sheet) - It takes some voltage for itself.
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Aug 24, 2012, 12:32 pm Last Edit: Aug 24, 2012, 12:36 pm by RobertEagle Reason: 1
But then why the regulator doesn't stay at a fixed voltage?

(By the way, ignore the fact that the input line isn't connected to the 3rd pin of the regulator. In reality it is)


R2 should be grounded from PIN1 of LM317T, I think you are grounding it from PIN2 of LM317T.



Make sure that the LM317T is properly wired.  I suggest you remove all other components from the breadboard to give you room to work.

Ensure that you have the 360? between pins 1+2.
Ensure that you have the 519? between pin 1 and ground.
Ensure that you have the power input at a minimum of about 5V and connected to pin 3 and ground.
Ensure you take the output from pin 2 and ground.
[/b]Most importantly, ensure that all the grounds are connected.[/b]


Aug 24, 2012, 01:24 pm Last Edit: Aug 24, 2012, 05:23 pm by RobertEagle Reason: 1
Yep. I've finished the checking procedure. All the components are staying at the proper position and are connected to each other.
All I get when I measure with the multimeter is the voltage input minus the dropout voltage.
I evenly switched the regulator with another one of the same model.

EDIT: I found what was the ,,problem". As long as the resistors are leveled with few ohms the output voltage will vary very much, but when there are bigger resistors(like 100k, 800k) there is almost no problem.


Ok.  What is the voltage between PIN3 of LM317T and GND?  Replace the R2 with a wire and check the output voltage at PIN2 and GND, it should be around 1.2V.  If you are not seeing 1.2V, then check the wiring or LM317T.


It's 5V. Yeah. It gives me like you said.
It's because of the resistors. As I said, the bigger they are, the smaller the error.

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