12V 1A output using LM317 and TL431

Thread Starter

omerfrkcn

Joined Mar 26, 2021
2
Hello,
I have Lm317 and TL431 and I want to get 12V 1A output using them. 1A should be the limit for me, I have to adjust the current up to 1A with resistors. I can only do 1A current adjustment work with the LM317 circuit, and the 12V output can only be done with the TL431. But I could not combine the two and get what I want. Below is a circuit that outputs 5V 1.5A using both of them, but I could not apply it to my own wishes. Can you help me?
Tl431_question.PNG
 

DickCappels

Joined Aug 21, 2008
7,157
What you have alteady drawn is very close to what you asked for. Connect your LM317 as a current source (adjustable if you want) then close the loop around the output voltage with the TL431-the cathode to the ADJ pin on the LM317 the feedback input to a voltage divider to divide the output voltage down to 2.5v. TL431 anode to ground. Sorry, doing this on my cell phone so drawing a schematic would be uh... difficult
 

Ian0

Joined Aug 7, 2020
1,577
Are you sure about that? You have added an inverting stage between the output and adj, where there wasn't one before. Won't the feedback phase now be wrong? eg. Normal operation - output voltage increase, adj voltage also increase by Vout/DividerRatio. Now output voltage increase - adj voltage (from TL431) decreases. I bet you've made an oscillator!
Isn't the circuit you need the one on page 17 (and the front cover) of the datasheet labelled "Battery Charger circuit".
https://www.ti.com/lit/ds/symlink/lm317.pdf?ts=1616675108265&ref_url=https%3A%2F%2Fstartpage.com%2F
As even the best TL431 can only manage 0.5% accuracy and the normal one is +/-2% those 0.1% resistors are a bit of a waste of money.
 
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DickCappels

Joined Aug 21, 2008
7,157
Feedback in the correct sense because the LM317 is a voltage follower until it goes into current limit (and then it just stops following). Note that the output to the load and the voltage divider for the TL431 is taken from the right-hand side of R1.

Here is how to connect the LM317 as a current source. You can make your current limit adjustable if you use a pot for R1.

1616753346663.png
 

Ian0

Joined Aug 7, 2020
1,577
Yes, you're right about the phase: Vout = Vadj+Vref. I still wonder if it will be stable without a cap between cathode and input on the TL431.
 

crutschow

Joined Mar 14, 2008
26,736
Here's an example circuit which limits the current by adding a series resistor (R4 below) as Dick showed:
Notice that the output current (green trace) stays at 1A until the output voltage reaches 12V (yellow trace).
(Note that if you are trying to charge a 12V lead-acid battery, you need more than 12V).

1616771483937.png
 
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Thread Starter

omerfrkcn

Joined Mar 26, 2021
2
Here's an example circuit which limits the current by adding a series resistor (R4 below) as Dick showed:
Notice that the output current (green trace) stays at 1A until the output voltage reaches 12V (yellow trace).
(Note that if you are trying to charge a 12V lead-acid battery, you need more than 12V).

View attachment 233689
The circuit I will do is for cooling with Peltier. I need this circuit to control the temperature of the peltier with adjustable currents at constant voltage. Thank you for setting up the circuit and sharing your analysis. Here, the current value drops at 12 V is a problem for me. I need a constant 12 V and stable current to drive Peltier. Do you have any advice? It can also be with other components.
 

crutschow

Joined Mar 14, 2008
26,736
I need this circuit to control the temperature of the peltier with adjustable currents at constant voltage.
As I've already said, Ohms law states, you can't have both.
The only way to control the current is to lower the voltage since current equals volts divided by resistance, and the peltier resistance is constant.
The peltier needs a constant current, so you let the voltage be whatever is needed to generate that current.
12V is it's maximum rating, it is not a fixed requirement.

Do you understand that?
 
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