Using TL431 for constant current output

何子衿

When this circuit is measured, the voltage across R32 cannot be stabilized at 2.5V. The measured result is 1.9V. When the resistance value of the load R33 is changed, the voltage across R32 will also change. The current flowing through R33 will also change with the load. R33 is the load and its resistance value is not constant. When the load resistance value changes, how to change the circuit to keep the output current constant?

maychang

"Using TL431 for constant current output"

I don't know who designed this circuit. Did they get the feedback wrong?

何子衿

maychang posted on 2022-10-12 18:45 『Use TL431 for constant current output』 I don’t know who designed this circuit. Did they reverse the feedback?

Boss, is there any problem here? I found the circuit online, but it doesn't have the constant current effect.

maychang

He Zijin posted on 2022-10-12 18:53 Boss, is there any problem here? I found the circuit online, but it doesn't have the constant current effect

"I found the circuit online, but it didn't produce a constant current effect."

Of course there is no constant current effect. On the one hand, the circuit is wrong, and on the other hand, you have actually verified it.

maychang

He Zijin posted on 2022-10-12 18:53 Boss, is there any problem here? I found the circuit online, but it doesn't have the constant current effect

"I found the circuit online, but it didn't produce a constant current effect."

Half of the stuff on the Internet is wrong.

何子衿

maychang posted on 2022-10-12 18:55 "I found the circuit on the Internet, but it didn't produce the constant current effect." Half of the things on the Internet are wrong.

Can TL431 do constant current output? How to do it?

maychang

He Zijin posted on 2022-10-12 18:53 Boss, is there any problem here? I found the circuit online, but it doesn't have the constant current effect

Return to the first post electrical schematics.

The operation of TL431 is that when the voltage of REF terminal (the horizontal line in the middle) exceeds 2.5V relative to ANODE terminal (anode of diode), the current flowing into CATHODE terminal increases. In the figure, if the voltage across R32 exceeds 2.5V, the current flowing through R31 increases, the P-channel MOS tube tends to be more conductive, the drain voltage of the MOS tube rises, and the voltage across R32 is larger, that is, the current flowing through R32 increases.

So the feedback of this circuit is reversed. It should be negative feedback, but it becomes positive feedback in the figure.

何子衿

He Zijin posted on 2022-10-12 19:00 Boss, can TL431 do constant current output? How to do it?

But this is the application circuit in the specification, and then I changed it to my own kind. Can this circuit be made?

maychang

He Zijin posted on 2022-10-12 18:53 Boss, is there any problem here? I found the circuit online, but it doesn't have the constant current effect

I don't know what you are going to use this circuit for. If it is for a product, give it up as soon as possible and use another circuit instead.

maychang

He Zijin posted on 2022-10-12 19:02 But this is the application circuit in the specification book, and then I changed it to my own kind. Can this circuit be made?

The datasheet says that an NPN transistor is used, which is correct. The datasheet says that this circuit can work properly.

You may want to use a MOS tube instead. If you want to use a MOS tube instead, you can use an N-channel MOS tube.

However, there is no benefit in using MOS tubes in this circuit, and the cost will increase. The circuit is good in the specification.

何子衿

maychang posted on 2022-10-12 19:02 I don't know what you are going to use this circuit for. If it is used for products, give it up as soon as possible and use other circuits.

I want to use this circuit to measure ground resistance. I want to use L9110S to output a constant current to flow through the ground, and then detect the voltage at both ends to get the resistance value.

何子衿

He Zijin posted on 2022-10-12 19:09 I want to use this circuit to measure the ground resistance. I want to use L9110S to output a constant current flowing through the earth, and then detect the voltage at both ends to get the resistance value

Now I just don't know how to give L9110 constant current input

何子衿

maychang posted on 2022-10-12 19:06 The specification uses NPN transistors, which is correct. The circuit in the specification can work normally. You may want to use MOS tubes instead. If it is...

Sir, but I tried the circuit in the specification and it still doesn't work. I connected different loads to the output, and the current is not constant.

I tried this

maychang

He Zijin posted on 2022-10-12 19:21 Boss, but I tried the circuit in the specification and it still doesn't work. I connected different loads to the output, and the current is not constant. I...

"When I connect different loads to the output, the current is not constant."

Any constant current source has a certain operating range, beyond which it will no longer maintain constant current. You need to tell me within what range you are "connecting different loads".

何子衿

maychang posted on 2022-10-12 19:25 "When I connect different loads to the output, the current is not constant." Any constant current source has a certain working range. If it exceeds the range, it will no longer maintain...

Resistance value is 10R-200R

何子衿

maychang posted on 2022-10-12 19:25 "When I connect different loads to the output, the current is not constant." Any constant current source has a certain working range, and it will no longer maintain beyond the range...

Also, I connected a 1K current limiting resistor to 12V, and then removed the load. The voltage across R32 was 1.9V. According to the voltage regulator of TL431, this place is 2.5V. I don't understand why it is wrong.

maychang

He Zijin published on 2022-10-12 19:27 Resistance value is 10R-200R

The load is about 10 ohms and the constant current can be maintained, but the load is 200 ohms and it is not. If the load is 200 ohms and your R32 is 51 ohms, the constant current should be 49mA. When 49mA flows through the 200 ohm resistor, the voltage across the load should be 9.8V. 9.8V plus 2.5V is 12.3V, which exceeds the power supply voltage of 12V, so the constant current cannot be maintained.

maychang

He Zijin published on 2022-10-12 19:27 Resistance value is 10R-200R

There is another problem with your circuit: R31 is 5.1 kilo-ohms. In order to make the emitter current of Qd4 reach 49mA, the base current must be 0.5mA. Let's add some margin and calculate 1mA. If the voltage across R31 is 1V (already very small), the current flowing through R31 cannot reach 1mA. In addition to the 1mA supplied to Qd4, TL431 also needs at least 1mA current to work properly. Therefore, R31 is really too large.

maychang

He Zijin posted on 2022-10-12 19:37 maychang posted on 2022-10-12 19:25 "When I connect different loads to the output, the current is not constant." Any constant current source has a...

"Also, I connected a 1K current limiting resistor to 12V, and then removed the load. The voltage across R32 was 1.9V. According to the voltage regulation of TL431, this place is 2.5V."

The voltage across R32 is 1.9V, so the current flowing through R32 is 37mA. Can you calculate how much voltage 37mA current will generate on your 1k ohm resistor?

I guess you measured 1.9V across R32, which is incorrect.

maychang

He Zijin posted on 2022-10-12 19:09 I want to use this circuit to measure the ground resistance. I want to use L9110S to output a constant current flowing through the earth, and then detect the voltage at both ends to get the resistance value

Does the L9110S chip in the picture need a constant current power supply?

It seems that the ground resistance is not measured in this way.