TL431 question

[Dannyx]

Good day folks. I'm having a bit of a puzzle trying to figure out something. I saw this video where the guy explains how you can test TL431's but sadly doesn't explain the principle of operation.

He connects the GND of the PSU to the ANODE, then the REF and CATHODE via a 1k resistor to 5v. He measures the voltage after the diode and gets 2.5v. I know 2.5 if the reference of a TL431, so it can't be an arbitrary number. How does the math work here ? I know a 431 is an adjustable zener - that I get, but I can't work out the numbers here.

I looked up the datasheet and had a look at the internal diagram: it's basically an op amp driving a NPN transistor. The REF goes to the non inverting input and there's a 2.5 Vref on the inverting input. So the chap in the video sends 5v into the non-inverting input, but what happens to that 2.5Vref then ?
Sorry for this being a stupid question, since it's a standard building block components, but that's precisely why I want to learn about it more, plus his test seems fairly efficient and bulletproof. Cheers.

[Dannyx]

Re: TL431 question

Actually I got it now, since there really isn't any math involved. It was the principle of operation that confused me, but I got it: when REF connects straight to the positive rail, the transistor becomes fully conductive, allowing full current to flow through, thus bringing down the voltage at the cathode to 2.5v (since at that point the non-inverting input takes over and stops the transistor, since both inputs are equal), which is what the chap demonstrates shows. The value of the resistor doesn't influence this - it's just to prevent overcurrent through the 431's transistor. I think that's it.

[PeteS in CA]

Re: TL431 question

A couple of tips if you experiment with TL431s and TL430s.

Pay attention to the minimum "zener" current. If the current is too low the TL431 will oscillate. I've seen this with other shunt references, too.

If you connect the REF pin to a voltage that can vary the TL431 can be used like a comparator (with K being the "output"). But pay attention to the minimum and maximum output voltages in the spec sheet. IIRC, the "Low" can be as high as 2V; The "High" is probably a volt or a little more lower than the positive rail.

It's a very versatile IC, one that was created in the 1970s and too good/cheap to be made obsolete.

[rievax_60]

Re: TL431 question

Also, the TL431 will draw an increasing current of up to about 500ua well before the Ref pin reaches 2.5v. The TL431 needs to be allowed to draw a minimum of 1ma before any driven circuitry responses.
For example, if the TL431 is driving the LED in an opto coupler, a resistor of about 1K needs to be placed across the LED to achieve proper regulation.

[redwire]

Re: TL431 question

It behaves like a zener diode - but if you remember a TL431 has an op-amp stashed inside of it, that needs its own power ~0.5-1mA, then the IC is easier to understand.
ATL431 is improved, low Iq version ~35uA that is stable with most load capacitances.