TL431 component test + impact of bad TL431 in PSU

[ddewaele]

Hi,

I pulled a TL431 adjustable zener from a "broken" PSU, not really sure if the TL431 is still in working order.
I did a diode test between the kathode and anode and that checked out fine, voltage drop on one end, OL on the other.

But I wanted to see it in action in a circuit so I hooked it up like this :

Code: [Select]

12v       ------> 330ohm resistor ----> REF pin
Anode pin -------> GND


Now on the cathode pin I was expecting to see a 2.5v output voltage but that wasn't the case. I was seeing something like 6v.

I then pulled another TL431 from a working PSU and inserted it in the circuit above and voila ... 2.5v output on the pin.

Would that imply that the first TL431 is defective, and what kind of impact would such a component have in a switch mode power supply ?
From what I understand it is used in the feedback circuit together with the opto-coupler and will ultimately control how the switching of the power transistor occurs.

Is it possible that wrong readings coming out of the feedback circuit could cause some issues on the hot side of the power supply ?

[mikerj]

That's not how a TL431 works, it's a shunt reference.  Tie the cathode and the reference pins together, put your current limiting resistor between your supply and the cathode/reference connection, ground the anode and you should have 2.5v across the device.  330 ohms will limit the current to ~29mA which is within the capability of the TL431 but is a quite a lot higher than you'd typically run this device at as a voltage reference.

[ddewaele]

That's not how a TL431 works, it's a shunt reference.  Tie the cathode and the reference pins together, put your current limiting resistor between your supply and the cathode/reference connection, ground the anode and you should have 2.5v across the device.  330 ohms will limit the current to ~29mA which is within the capability of the TL431 but is a quite a lot higher than you'd typically run this device at as a voltage reference.

That's how I wired it up , I see now how my description was not clear. Apologies for that. Here's how it was wired up (taken from Ludic Science : ) :



But REF and cathode were indeed tied and I measured . Tried different resistor values but I think it should always output 2.5V

But like I said I have 1 TL431 that doesn't do that .... it was outputting something like 6v. What kind of an impact would that have in a PSU.

[magic]

Then it's broken.
If a PSU uses it on the secondary to regulate output voltage, the output voltage will be regulated too high.

There is also the possibility that it's some cost-reduced TL431-like chip which simply cannot sink 30mA. Try higher resistance. Those regulators work correctly down to 0.1mA of current or something like that.