LM317 Gives me from 0.6v to 3.7v with 9v battery! Help! (SOLVED) - Page 1
Forum section: electronics / beginners

author: Mint.
author_rank: Frequent Contributor
timestamp: January 14, 2012, 03:33:55 am
content: LM317 Gives me from 0.6v to 3.7v. I keep on trying and trying to get my lm317 power supply to work properly. Right now it just doesn't want to cooperate with me. I need some help. Can somebody please be so kind and make the circuit on a breadboard and take a picture of it or a step by step tutorial. My R1 is 120 ohm and R2 a 1k pot.

author: amspire
author_rank: Super Contributor
timestamp: January 14, 2012, 04:02:15 am
content: Have you got the correct pinout?It is easy to make a mistake as the LM317 has the ground and output pins swapped compared to the standard 7805/LM340 type positive regulators.If the pins are right, can you give us the voltages on the 3 pins to ground with the pot set to, say, 110 ohms?Richard.

author: Unknown
author_rank: Unknown
timestamp: January 14, 2012, 04:31:06 am
content: Maybe you don't have enough juice. What is the voltage on the input side?

author: Unknown
author_rank: Unknown
timestamp: January 14, 2012, 07:34:16 am
content: Check the LM317 data sheet for dropout voltage at whatever current/temperature you're using. This is the minimum voltage between input and output. For low currents, this might be something like 1.5 V, which means that with 5 V input, the maximum output voltage will be about 3.5 V, regardless of the set resistors. There's also a minimum output voltage (without supplying a negative voltage) of 1.2 V, because this is the voltage of the internal reference.

author: Armin_Balija
author_rank: Regular Contributor
timestamp: January 14, 2012, 09:54:53 am
content: I'd suggest looking at the datasheet because they usually have a typical setup with all resistor, pot, and cap values preset to test the voltage regulator out. Dropping 5.4 volts is excessive and doesn't seem right. Maybe you switched up your ground and input pins because the LM317 has them swapped.

author: Mint.
author_rank: Frequent Contributor
timestamp: January 14, 2012, 10:25:14 am
content: Quote from: amspire on January 14, 2012, 04:02:15 amHey Richard,I changed my layout a bit, now it gives me from 3.2 volts (why not 1.25v I don't know) all the way up to 7.2 volts. If I am correct the regulator should drop 2 or 3 volts, so shouldn't my max Vout be around 6 volts?Anyways the experiment that you asked me to do, I done it with the new setup and the results are as follows.Writing on the regulator facing mePin 1 or ADJ 2.67vPin 2 or Vout 4.40vPin 3 or Vin 7.84v**As you can see the 9v battery is a bit discharged.

author: amspire
author_rank: Super Contributor
timestamp: January 14, 2012, 11:49:25 am
content: Something is not right.If you have an 8volt supply, then for the output between 1.25V to 6V, there should always be 1.25V between the adj pin and pin and the output pin.With 2.67V on the adjust pin, 4.4V out should be impossible.Any chance of a photo of the test circuit?Things that could be happening include:Regulator is oscillating. What sort of capacitor are you using for the 0.1uF and is it mounted right next to the regulator?Multimeter is not reading low voltages correctly. If you read the voltage of a 1.5V alkaline battery, does it read about 1.5V?Regulator is still not connected right. When you are working out the pins from the data sheet, do you have the heatsink on the side away from you?Regulator is damaged. Do you have any spare regulators?Pot is not connected right. When it is turned down to minimum, does the ADJ pin go down to almost 0V?Richard.

author: Zero999
author_rank: Super Contributor
timestamp: January 14, 2012, 12:49:56 pm
content: Quote from: Mint. on January 14, 2012, 10:25:14 amIs that with respect to 0V?The voltage on the adj pin should be 1.25V with respect to the output (pin 2).

author: Mint.
author_rank: Frequent Contributor
timestamp: January 14, 2012, 07:33:45 pm
content: Quote from: Hero999 on January 14, 2012, 12:49:56 pmI measured all of them with ground of my meter connected to the ground of the battery.

author: IanB
author_rank: Super Contributor
timestamp: January 14, 2012, 07:42:00 pm
content: OK, since this is an experiment I need to do, here are my results from a quick bit of breadboarding.In my circuit R1 is 100 ohms and R2 is a 2k pot. I have a 47 uF electrolytic on the Vin side and 100 uF on the Vout side. I put a reverse biased diode between Vin and Vout to protect the regulator.This is the voltage at the minimum setting:This is the maximum voltage where the LM317 dropped out of regulation:The battery in the picture provides about 10 V.You should maybe check your circuit construction one more time, and perhaps try a different LM317 in case the first one is damaged.

author: Mint.
author_rank: Frequent Contributor
timestamp: January 14, 2012, 08:06:16 pm
content: Whoops sorry forgot to change the rotation

author: Mint.
author_rank: Frequent Contributor
timestamp: January 14, 2012, 08:07:13 pm
content: * ignore the massive heat sink

author: Unknown
author_rank: Unknown
timestamp: January 14, 2012, 08:09:21 pm
content: R1 (the resistor) appears to be connected between the adjust pin and Vin, it should be between adjust and Vout.

author: amspire
author_rank: Super Contributor
timestamp: January 14, 2012, 09:15:51 pm
content: Quote from: alm on January 14, 2012, 08:09:21 pmThat should fix it

author: Mint.
author_rank: Frequent Contributor
timestamp: January 15, 2012, 01:08:58 am
content: Perfect! It worked, thank you very much! It seems that I got confused by the schematic slightly.Output 1.25 to 7.2!