LM317 CC CV Problem

Thread Starter

yotamamit

Joined Nov 8, 2017
12
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.
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.
And of course made shure it's not the IC itself. Swapped it with another, same issue (Unless I'm realy out of luck and got 2 dodgy 317s).

Current range needed: ~24mA - ~1.2A
Voltage range wanted: ~1.25V - ~24V

Voltage readings:
CC Side:
In->GND: 28.2V
In->Out: 1.4V
Out->Adj: 0V
Out->GND: 26.8V

CV Side:
In->GND: 26.8V
Out->Adj: 1.21V
Out->GND: 1.24V-24.4V

I tried it with different output loads and different loads for the CC IC, and still got no current regulation, just voltage regulation.

Can anyone stop the (probably noobish) problem in the design?
I'm far from new to electronics, but new to power electronics and power supply design.

Schem_708x241_yotamamit.png
 

dl324

Joined Mar 30, 2015
16,731
Welcome to AAC!

CC part of the circuit in question:
upload_2017-11-8_7-0-4.png
the voltage between the output pin and adjust pin of the first LM317 (the CC part) is 0V.
I'd say overload or bad part. Try testing the CC part by itself. If that doesn't work, swap the LM317's.

Were the parts purchased from a reputable seller?

If R3 isn't a high wattage rheostat, you're not going to have much joy.

What's the purpose of D1?
 

Thread Starter

yotamamit

Joined Nov 8, 2017
12
The ICs were purchased from Tayda. I might be very unlucky having two faild parts, because I've already tried swapping the IC.
R3 is a 3W guy.
D1 is just there for reverse polarity protection, just in case I'll be stupid or hasty while wiring.
 

crutschow

Joined Mar 14, 2008
34,063
A 3W, 50Ω pot has a maximum current rating of .24 amps.
You will overload it if you set it for a higher current than that.

What is the voltage drop across the pot as you vary its setting?
What load resistance are you using for the CC test?
 

AlbertHall

Joined Jun 4, 2014
12,331
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.
That's what it should do if you are drawing less current from the power supply output than the CC is set to.
What load did you have on the output for those measurements?
 

Thread Starter

yotamamit

Joined Nov 8, 2017
12
That's what it should do if you are drawing less current from the power supply output than the CC is set to.
What load did you have on the output for those measurements?
Set the voltage output to 4.5V and hooking up a 22R resistor to the output. It drew about 200mA regardless of R3 position.
 

Thread Starter

yotamamit

Joined Nov 8, 2017
12
A 3W, 50Ω pot has a maximum current rating of .24 amps.
You will overload it if you set it for a higher current than that.

What is the voltage drop across the pot as you vary its setting?
What load resistance are you using for the CC test?
It's 0V across it and the 1R resistor.
 

AlbertHall

Joined Jun 4, 2014
12,331
If the voltage across the 1Ω resistor was 0V when the output current was 200mA then something is wired incorrectly as, according to the diagram, the output current flows through that resistor. 200mA in 1Ω should drop 200mV.
 
If the voltage across the 1Ω resistor was 0V when the output current was 200mA then something is wired incorrectly as, according to the diagram, the output current flows through that resistor. 200mA in 1Ω should drop 200mV.
I don't see where you are outputting your constant current--it looks like you are adjusting the current into the second 317. Why would you do that? The adjustable constant current/adjustable circuits I have seen let you adjust one and you let the other fall where it will, according to your load. That is, from one power source you can't adjust both voltage and current at the same time.
 

Thread Starter

yotamamit

Joined Nov 8, 2017
12
I don't see where you are outputting your constant current--it looks like you are adjusting the current into the second 317. Why would you do that? The adjustable constant current/adjustable circuits I have seen let you adjust one and you let the other fall where it will, according to your load. That is, from one power source you can't adjust both voltage and current at the same time.
The CC part is supposed to be used as a current limiter. You can draw less from it, but no more than the regulated current.
Most bench power supplies will feature CV and CC or current limiting.
 

crutschow

Joined Mar 14, 2008
34,063
I don't see where you are outputting your constant current--it looks like you are adjusting the current into the second 317. Why would you do that?
That's the normal way to do it.
That way the voltage drop from the current limiter circuit does not affect the output voltage under non-limited conditions.
 
[QUWOTE="crutschow, post: 1198610, member: 19451"]That's the normal way to do it.
That way the voltage drop from the current limiter circuit does not affect the output voltage under non-limited conditions.[/QUOTE]
Well, I am confused. Many posts use the term Constant Current and Current Limiter interchangeably. But the first part of the circuit shown is a constant current supply--it will attempt to source the set current as long as it is possible. Thus, if it is set to 200 ma and the load is a short circuit it will put 200 ma through the short. If the load is 1 ohm it will put 200 ma through the load, That is, it will output .2 volts. If the load is 10 ohms, the circuit will put 200 ma through it--it will raise the voltage to 2 volts. This will continue till the required voltage is no longer available.
This is not a 200 ma limiter but a 200 ma source..
Now, my name says it -- whitehaired novice--but I'm pretty sure I'm right about this.

Is my face red! I thought I remembered the 317 hooked up as you show it as a constant current generator and was surprised to see it used that way. But memory failed me and it is indeed a current limiting circuit. So, I learned something I needed to learn. So, any luck finding the problem? It might be two bad ICs in a row.
 
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crutschow

Joined Mar 14, 2008
34,063
whitehaired novice--but I'm pretty sure I'm right about this.
You're right as far as it goes.
But if the output second stage voltage is set to say 2V. then when the output resistance gets to 10Ω, the voltage will stop rising for a limit of 200mA, and stay at a constant 2V output for any load resistance above that.
In other words it goes from a CC mode to a CV mode at 10Ω or above.
 
Many thanks for the info about EEVBlog--I spent all morning looking at it and have bookmarked it for future use.It doesn't seem to be searchable and I couldn't find the video you refer to--do you perhaps remember a title or even about how old the video is? I'd like to see it.
 

dl324

Joined Mar 30, 2015
16,731
Many thanks for the info about EEVBlog--I spent all morning looking at it and have bookmarked it for future use.It doesn't seem to be searchable and I couldn't find the video you refer to--do you perhaps remember a title or even about how old the video is? I'd like to see it.
I can't stand that guy's videos. For some reason I find that his voice grates on my nerves like fingernails on a chalkboard... I can't remember if he's one of the "experts" on YouTube that take forever to make a point.
 
I can't stand that guy's videos. For some reason I find that his voice grates on my nerves like fingernails on a chalkboard... I can't remember if he's one of the "experts" on YouTube that take forever to make a point.
My monitor doesn't have speakers, so I'm spared that. If the CC button doesn't work I just skip it. This blog has good CC. Yes, he makes his living doing blogs so he feels he has to use a lot of time. Still, for a novice, there is some interesting stuff there.
 
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