I am using an LM317 to create 8VDC-9VDC from a 12VDC bench supply. I am confused because the output pin always has approx. 11 v no matter how I configure the adjustment resistors, while I find my correct, adjusted voltage on the adjust pin instead. I can watch it changing as expected while I adjust the pot. I have tried fixed resistors and a pot, I have tried it with and without decoupling caps in the input and output, and I have tried it with both an LM317 and a LT1085IT. I have also tried it with R1=220, R2=1100; R1=1K, R2=5.1K etc for various combinations to give me to output I want. What am I not getting? I have successfully used LM317s before and I don't recall having this issue. Schematics and data sheets all tell me the output shout be on the output pin but I find it on the adj pin. Thank you for any advice.

 

Hello,

What is the value of R1 on your breadboard?
I see a resistor of 1K on your breadboard.
Many circuits use 120 ohm for R1.

Bertus

 

I have also tried it with R1=220, R2=1100; R1=1K, R2=5.1K etc for various combinations to give me to output I want.

 

Next question, where did you purchase the regulator from?
What voltage do you read on pin #1?

 

Is that a real LM317? What is its provenance?

 

Hello,

It sound like that the ground connection of R2 is loose.
That way the regulator is floating and will almost pass the input voltage.

Bertus

 

Maybe you didn't meet the minimum load requirement which can be as high as 10mA. What happens if you apply a 1k or so load to the output?

 

If you always use R1=120Ω you will never fail to meet the minimum load requirement.

 

Next question, where did you purchase the regulator from?
What voltage do you read on pin #1?

Probably Mouser or Digikey. I read varying voltage on pin 1(adj) depending on how I adjust the pot.
Pin 3 (Vin) shows 12.06V, pin 2 (Vout) shows 11.08V.

 

Hello,

It sound like that the ground connection of R2 is loose.
That way the regulator is floating and will almost pass the input voltage.

Bertus

You know, it could be my bench supply's ground is funky. I will try another.

 

Maybe you didn't meet the minimum load requirement which can be as high as 10mA. What happens if you apply a 1k or so load to the output?

I tried putting a 1K resistor on the output. I can't remember what happened but I was disappointed. I'll try that again.

 

Is that a real LM317? What is its provenance?

The one pictured is the LT1085IT . I am awaiting a new batch of LM317s. It's very odd that I had the same anomaly with two different regulators, which is what made me think I was cracking up. This should be very simple but it has me stumped. The bench supply is the only thing I have not yet swapped out; I will try a different one.

 

It might we worth using a continuity tester to check that the breadboard internal tracks are working correctly. I spent a while scratching my head trying something simple of one of my breadboard until I found that an internal tracks along one of the 5 hole rows was broken.

 

Hello,

Rebuild the circuit on an other part of the breadboard.
As @Dave Lowther said, a bad spot on the breadboard can happen.

Bertus

 

Pin 3 (Vin) shows 12.06V, pin 2 (Vout) shows 11.08V.

What is the adjust pin voltage? What value is R1?

 

It might we worth using a continuity tester to check that the breadboard internal tracks are working correctly. I spent a while scratching my head trying something simple of one of my breadboard until I found that an internal tracks along one of the 5 hole rows was broken.

That’s a great suggestion. I’ve swapped everything else. I’ll try a new breadboard and power supply

 

That’s a great suggestion. I’ve swapped everything else. I’ll try a new breadboard and power supply

If any of the connections are bad, you can find them using standard troubleshooting techniques. You start by measuring voltages, not starting over. When you find something that isn't right, determine why it isn't right.

 

That’s a great suggestion. I’ve swapped everything else. I’ll try a new breadboard and power supply

I found the error on mine by checking the wiring with a continuity tester.
For example:

Check continuity between point A and B point, then check between point B and point C.

 

Check continuity between point A and B point, then check between point B and point C.

Or measure the voltage on those 3 component leads.

 

Problem solved -- It WAS bad components! I suspected a bad LM317 when the first one misbehaved, so I tried an LT1085IT which exhibited the same misbehavior. I concluded I was doing something wrong although this is very simple. I rounded up an LM317 in a TO-3 package and it worked correctly. Yesterday my new TO-220s arrived and everything is right with the world again.
Now my only question is, were these two bad components pure coincidence, or did I damage them? Maybe my power supply was creating spikes when I powered it on. I have treated myself to a new bench-top supply I have never zapped any ICs before. Curiouser and curiouser. Thanks everyone for your suggestions.