[Kevin] doesn’t stock zener diodes anymore. Why? Because for everything he used to use zeners, he now uses TL431 bandgap voltage references. These look like zener diodes but have an extra terminal. That extra terminal allows you to set the threshold to any value you want (within specifications, of course). Have a look at the video below for an introduction to these devices and a practical circuit on a breadboard.
Inside, there’s a voltage reference, an op-amp, and a transistor, so these are tiny 3-terminal ICs. The chip powers itself from the load, so there are no separate power supply pins.
Note that just before the five-minute mark, he had a typo on the part number, but he corrected that in the comments. He goes on to put a demonstration schematic in KiCad. Once it was all worked out, it was breadboard time.
As always, there were a few real-world things to resolve, but the circuit worked as expected. As [Kevin] points out, the faux-zeners are about four for a dollar and even less in quantity. A zener might be a few pennies cheaper, but unless you are making thousands of copies of your circuit, who cares?
We don’t see zeners as often as we used to. As for the TL431, we’ve seen one torn apart for your amusement.
The TL431 is in about 90% of all offline switching power supplies, in the feedback loop from the secondary to the primary side, through an optocoupler.
Whenever I open one, I search for, and usually find it.
True that!
Ken Shirriff wrote up a beautiful explainer.
https://hackaday.com/2014/05/26/ken-shirriff-explains-the-tl431/
https://hackaday.com/2016/01/08/super-cheap-lifepo4-battery-charger/
(Scroll down to the photo on the bottom for a surprise!)
TL431 costs around 10ct in moderate (10+) quantities. I also desoldered some from old power supplies, but that was a very long time ago. It is a very nice part to have though, so consider helping yourself to a section of cut tape with these things.
Also note they are not all the same. There are differences in tolerances, temperature drift and other parameters.
A very important issue with this part is that it’s not always stable. It works with a very small output capacitance, and it works with a big output capacitance, but there is a smallish range of output capacitance for which this chip is not stable and WILL oscillate.
I’d say it’s more like 99%, but sometimes it has different marking. The optocoupler is also the same, but there is a bit more of variety, when it comes to its markings. This is true especially for all ATX supplies…
Don’t expect a circuit based around an op-amp to be able to clamp fast voltage transients.
What would?
A diode….. ever heard of a transient voltage suppression diode?
Besides that. Not all diodes are fast diodes.
TVS diodes are essentially a subset of zener diodes with beefier current capacity from the larger surface area, thicker wire bonds, and more metallization on the die.
Not all diodes are fast diodes, agreed. Fast diodes are the correct devices for clamping fast voltage transients.
TVS diodes are made for this. There are unipolar and bipolar variants, and PCB routing is also important. Do not make a T-split to go to the TVS, but route from the connector to the TVS, and then from the TVS to your IC. The other side of the TVS should be connected directly to your GND plane. TVS diodes work with very fast transients, ans for such transients proper HF design is a must. Adding a small resistor between the TVS and the “outside world” (connector) also helps, but chokes are more suitable.
This is really where the “programmable zener” lie shows its teeth; a true zener/avalanche diode has essentially no response time, limited by lead inductance and junction capacitance. The control loop here makes the device useful at the microsecond scale and up. Which is still quite good for signal clamping purposes.
I often used them in place 78xx and LM317’s. Despite having hundreds of zener’s in my junk drawer that were passed down to me, I didn’t use them often except for the odd Vref. I recall seeing them used as regulators in a lot of older schematics. It’s a nice little semiconductor in a TO-92 package but I never stopped to research how they actually worked. Thank you for the video.
With modern low-power electronics it’s easy to forget that sometimes there’s a need to dissipate non-trivial amounts of power at higher voltages. I just checked, and Mouser still sells 100-volt 5-watt zeners. Hitting those specs with a TL431 and ‘helper’ components might cost more money, and probably more board real estate as well, than the two-legged ‘OG’ zener.
Then again, a 100V 5W stud-mount zener from the same source costs about 50 bucks, so maybe a TL431 and a big pass transistor would be cheaper. But it might also be less reliable.
Now get off my lawn! ;-)
With modern low-power electronics it’s easy to forget that sometimes there’s a need to dissipate non-trivial amounts of power at higher voltages. I just checked, and Mouser still sells 100-volt 5-watt zeners. Hitting those specs with a TL431 and ‘helper’ components might cost more money, and probably more board real estate as well, than the two-legged ‘OG’ zener.
Then again, a 100V 5W stud-mount zener from the same source costs about 50 bucks, so maybe a TL431 and a big pass transistor would be cheaper. But it might also be less reliable.
Now get off my lawn! ;-)
“a 100V 5W stud-mount zener from the same source costs about 50 bucks”
That’s because stud-mount zeners have been exotic stuff for some decades now. The same thing in a more conventional case (1N5378B) costs <0,50€ each in small quantities and about 0,1€ if you buy a full reel.
With modern low-power electronics it’s easy to forget that sometimes there’s a need to dissipate non-trivial amounts of power at higher voltages. I just checked, and Mouser still sells 100-volt 5-watt zeners. Hitting those specs with a TL431 and ‘helper’ components might cost more money, and probably more board real estate as well, than the two-legged ‘OG’ zener.
Then again, a 100V 5W stud-mount zener from the same source costs about 50 bucks, so maybe a TL431 and a big pass transistor would be cheaper. But it might also be less reliable.
Now get off my lawn! ;-)
431s are useful little buggers. I have about 100 in various grades in my parts bins. But I do not use them (directly) for ADC references. Dragons be there.
Worked at a power supply company for more than a few years, learned to love the humble TL431. You will find TL431’s in more than a few of my designs, wherever a stable reference voltage is required.
With modern low-power electronics it’s easy to forget that sometimes there’s a need to dissipate non-trivial amounts of power at higher voltages. I just checked, and Mouser still sells 100-volt 5-watt zeners. Hitting those specs with a TL431 and ‘helper’ components might cost more money, and probably more board real estate as well, than the two-legged ‘OG’ zener.
Then again, a 100V 5W stud-mount zener from the same source costs about 50 bucks, so maybe a TL431 and a big pass transistor would be cheaper. But it might also be less reliable.
Now get off my lawn! ;-)
I would be surprised if this could be used as a noise generator like a reverse biased zener can.
Dr. José Marcos Alonso, University of Oviedo, Gijon, Spain has put together a two-part YouTube video about creating and testing a TL431 behavioral model in ADI’s LTspice simulator for the venerable TL431 adjustable shunt regulator:
LTspice #26: How to create a TL431 model (I)
https://www.youtube.com/watch?v=UoVRj3j__8g
LTspice #27: How to create a TL431 model (II)
https://www.youtube.com/watch?v=YsZ7IgzklLQ
All files are now available at the repository:
https://github.com/marcosalonsoelectronics/LTspice-27
Dr. José Marcos Alonso (1967)
https://www.amazon.com/stores/author/B08413HBP9/about
https://www.youtube.com/c/MarcosAlonsoElectronics
https://www.amazon.com/stores/author/B08413HBP9
Diodes Incorporated manufactures the AS431 drop-in replacement for the TL431. Diodes Inc. also provides a detailed datasheet for the AS431, as well as a downloadable SPICE model. Diodes Inc. produces the AS431 in SOT23, SOT25, SOT89 (two Versions) SMT packages as well as the venerable TO92 three-lead through-hole mount plastic package. The TO92 AS431 is readily available in unit quantities for around $0.30 USD each.
Diodes Inc. AS431 2.5V ADJUSTABLE SHUNT REGULATOR TO-92 Qty-1 @ $0.30
https://www.diodes.com/part/view/AS431
https://www.digikey.com/en/products/detail/diodes-incorporated/AS431BZTR-G1/7724957
AS431 SPICE Model from Diodes Inc.
https://www.diodes.com/spice/download/4963/AS431.spice.txt
Cheap TL431 chips TO92 package on Amazon, 100 pcs/order (YMMV):
[1] Chanzon TL431 TO-92 Positive Adjustable Voltage Regulator IC, Three Terminal Positive Regulator for Power Management (Pack of 100pcs), 4.7 out of 5 stars 235 ratings, $7.99 ($0.08 each before tax & shipping)
https://www.amazon.com/Regulator-Encapsulate-Adjustable-Accurate-Reference/dp/B083TQL5K8/
[2] TL431 TO-92 Voltage Regulator 1-100mA TL431ACL Three-Terminal Shunt Regulators (Pack of 100pcs), 4.5 out of 5 stars 550 ratings, Black Friday Deal, $6.29 with 30 percent savings ($0.063 each before tax & shipping)
https://www.amazon.com/ALLECIN-TL431-Regulator-Three-Terminal-Regulators/dp/B0CBK29VSF/
Zeners support a much higher Vin than does a ‘431, though if you need to, you can work around that by using a zener to coarsely regulate Vin, and follow that with a ‘431.
If you need a lower voltage, there’s the TLV431B : 1.24-6V at 0.5%, though they have a lower permissible Vin.
Also, within the specs of other parts in a switching wallwart, being able to locate the 431 and identify its divider can allow you to modify the output voltage, up or down (like getting 27V out of a 24V adapter, because you need the extra volts – but you can’t blindly drive voltage up without risking some component isn’t rated for it).
The ‘431 is a handy part worth having in your kit.
sounds a lot like any 3 pin adjustable regulator.