LM317 CC CV PS Problems. - Page 1
Forum section: electronics / beginners

author: yotamamit
author_rank: Newbie
timestamp: November 06, 2017, 11:04:29 am
content: Hey folks, I'm having a problem with my LM317 CC CV Power supply.The CV part works perfectly, yet the CC part does absolutly nothing.(Schematic attached)The dashed line over the 1uF cap is there because I tried with and without it, didn't do a thing.The thing that I can't seem to solve and find out what's wrong with it is that the voltage between the output pin and adjust pin of the first LM317 (the CC part) is 0V.At best it got to 0.25V When there was no feedback.And to make it even more wierd (at least to the LM317-inexperienced me) is that they AREN'T shorted. Tested with DMM a few times, and it just reads the resistance of the resistor and pot, and the board isn't shorting them. It basicly just works as a 1.45V voltage dropper...And of course made shure it's not the IC itself. Swapped it with another, same issue.Can anyone stop the (probably noobish) problem in the design?

author: eblc1388
author_rank: Frequent Contributor
timestamp: November 06, 2017, 03:35:46 pm
content: To test out the constant current function of the circuit, you would need to apply a certain loading to the PS output.For example, set the output voltage to 12V and then connect a 10 to 22 ohm resistor as loading. Now monitor the voltage across this resistor when you adjust the 50 ohm current limit variable resistor in the CC part of the LM317T.There should a point on the variable resistor that the output voltage starts to drop. This is the current limiting point and shows that the CC circuit is working correctly.

author: Gurumurthy
author_rank: Regular Contributor
timestamp: November 06, 2017, 04:40:21 pm
content: Hi,you have designed current limiting function variable from 24mA(1.2V/51ohm) to 1.2A(1.2V/1 Ohm).Iout=Vref/RIout=1.2V/RThe output voltage of first current Regulator 317IC, Vout=Vin-1.2V. This Voltage will be the input voltage of Second Voltage adjustable 317 regulator. Hence the second regualtor act as a load for Current limiting first regulator. Now you have to connect a load in second voltage regulator by using ohms law.example:Vout=12VRL=500 to 1 ohm, according to 50 ohm resistor at the current regulator. So that you cxan vary 24mA to 1Ampere.

author: yotamamit
author_rank: Newbie
timestamp: November 06, 2017, 09:05:10 pm
content: That's what baffled me. To test it I hooked up a 22R 5W resistor to the output, which was set to 4.5VIt drew 200mA regardless of the potentiometer's variations.

author: yotamamit
author_rank: Newbie
timestamp: November 06, 2017, 09:07:37 pm
content: Did it all already, without success.The constant current function just doesn't work, which leads me to think that there's a design problem

author: orolo
author_rank: Frequent Contributor
timestamp: November 06, 2017, 10:50:17 pm
content: The first 317 is acting as a constant current source, and the second as a constant voltage regulator. This means the 317s are trying to convey an fixed current I, at a fixed voltage V, into a resistor R. Unless V = I·R happens, this is not possible, and at least one of the regulators will have to give in to Ohm's law, and fall out of regulation.

author: rstofer
author_rank: Super Contributor
timestamp: November 07, 2017, 12:28:23 am
content: Dave has a video series and the first episode covers your style of PS.

author: hugo
author_rank: Regular Contributor
timestamp: November 07, 2017, 01:47:13 am
content: Quote from: yotamamit on November 06, 2017, 11:04:29 amYou should test it with a lower input voltage like 10V (15V) DC and see what happens ...