Problem with using TL431 voltage shunt regulator

[GUGLHUPF]

Hi

I am trying to get a TL431 voltage shunt regulator to work without luck and wonder if anyone can help me ( I know little about electronics but until now thought I can read circuit diagrams.....)


The circuit is a built like the reference circuit in the datasheet with a general resistor (220OHM 5W) and a voltage divider to the shunt regulator (both R1 and R2 have same value of 2000OHM 1/4WATT).

I put 5V, 12V and 24V on the circuit but get always 1V on the output. I think the shunt does something or I would expect 1 -> 2V -> 5v but the output voltage is steady at 1V. Also when I unplug the shunt the LED I have on the output starts glowing.

According to the formula I wanted to get 5V Vout=(R1+R2)/R1 Vref with Vref=2.5V according to the datahsheet.

Would someone know what I am doing wrong?
Thank you for your help.


[This is just an initial test circuit. The final goal is to put a bridge and a varistor in front and a optocoupler in the back and use a microcontroller to check that my 24VAC sprinkler valve is on but I am stalled on the first step it seems.]

 

[mvs sarma]

negative tl431

Try to check the pin reversals. As you put the chip facing you and pins downwards, pins counted left to right in the order
1. Reference
2. Anode
3. Cathode.
The anode will face 220Ω resistor and it is also output and one side of R1. cathode (pin3) will be common negative and also R2 other lead. R1,R2 junction to got to pin1. R2.

one side of 220Ω will be +ve input.

Mostly I suspect pin reversals of the IC.

Check up and it should work and withe set up you get 5V output if you provide atleast 8V as input.
Sarma

 

[GUGLHUPF]

tl431 negative

Thank you.

I did draw the circuit outline as it is on my breadboard. At a voltage of 24V the LED starts glowing lightly. The power supply has +12V and -12V that's why I labeled it with two numbers. The potential delta is 24V.

On the 220Ohm resistor I am measuring 22.3V. On the LED I measure 1.7V. On the 2k resistors I measure 0V.

I was thinking about it a bit more. Could it be that the LED causes this behavior. I think I read somewhere that a LED nails the voltage so if this LED puts the voltage to 1.7. Again when I remove the shunt the LED is shining very bright.

A) With this circuit could someone explain me what I am seeing please? I would love to understand this behavior.

B) If I drop 22V on a 220Ohm resistor it has to dissipate 2Watts, right? the resistor is a 10W resistor but still it gets very hot at 20% of it's allowed value.

I do apologize, I posted this to the wrong forum. I meant to use the beginner forum but I am not well versed with user groups. Apologies for this.

 

[laktronics]

tl431 usage

Hi,
Just reverse the voltages, cathode has to be positive and anode negative, being a zener. I suppose, your diagram shows TL431 viewed from top side and not pin side, you should always mention this in the drawing to avoid confusion.
Regards,
Laktronics

 

[Siniša]

tl431 +negative

In addition to reversing input power, remove that LED at the end.

 

[GUGLHUPF]

cathode go to positive tl431

Oh Boy. Thank you all so much!!!!

That did it. Little do I know that Cathode is connected to positive here. Sorry about that.

Thank you so much for the all the help!!!