[Solved] Schematic from datasheet, works with TL431-A but not with KA431LZ - Page 1
Forum section: electronics / beginners

author: RoGeorge
author_rank: Super Contributor
timestamp: November 02, 2024, 08:09:44 am
content: - The datasheets for both models look almost identical:TL431https://www.ti.com/lit/ds/symlink/tl431.pdfKA431https://www.onsemi.com/pdf/datasheet/ka431-d.pdf- Don't know the exact manufacturer, both models were recovered from defective PC power supplies- Either model works very well when I test them as a low power Zenner (tested as 2.5V shunt regulator, with the Reference pin tied to Katode pin), so they don't seem defectiveHowever, when I try to add an external transistor to increase the max supported current, KA431 doesn't regulate any more.I am using 2k7 for R1, R2, and 220\$\Omega\$ for R between BE. Decided these values after a design example where the recommended currents are 1mA through the R1, R2 divider, and 10mA through the 431 regulator, though these recommendations are for TL 431 (from pag.6-21https://archive.org/details/TexasInstruments-TI-Data-LinearCircuitsApps-DL/page/n111/mode/2up). Couldn't find similar application notes for the KA variant of 431.The power transistor is a BD680 (4A Darlingtonhttps://www.st.com/resource/en/datasheet/bd677.pdf) but I've tried with a BD238, too (normal PNP, 2A, not Darlington ), and got almost the same behavior as with BD680.No matter the transistor, by replacingKA431LZ with aTL431-A, the same schematic works rock solid.Tested with a Rigol DP832 power supply in constant-current mode (CC with V limited at 7V), and connecting the Zenner circuit directly at the output of the CC power supply. Expecting to measure a constant voltage drop on the Zenner circuit (when constant current is changed from the DP832 CC power supply).With TL 431, the same schematic gives rock-solid 5V no matter the current, from 3mA to 3A.With KA 431 the voltage is about 3-4V, and seems unregulated, varies a couple of volts.Tried so far 3 different KA431, and none of them is working properly (when used with the additional power transistor).Why TL431 is working, while KA431 doesn't, what differences are between the two models, or what am I missing?

author: iMo
author_rank: Super Contributor
timestamp: November 02, 2024, 09:01:30 am
content: Do the KAs oscillate??

author: RoGeorge
author_rank: Super Contributor
timestamp: November 02, 2024, 09:40:52 am
content: It wasn't oscillate, clean trace on a 100MHz oscilloscope, just that it happened to be defective.I guess it was just bad luck, nothing wrong or very different with the KA431 as a chip. Meanwhile I've searched for more 431 on another scrapped power supply PCBs. Now I have two working KA431LZ.

author: magic
author_rank: Super Contributor
timestamp: November 02, 2024, 10:06:11 am
content: What are the voltages on KA431 pins?Hmm, any chance that R and K pins are swapped on this chip? You wouldn't notice in 2.5V zener application...

author: Harry_22
author_rank: Frequent Contributor
timestamp: November 02, 2024, 10:08:25 am
content: Quote from: RoGeorge on November 02, 2024, 08:09:44 amHello!It looks like you are using a mode of operation that lies outside of Absolute Maximum Ratings. Various surprises are possible here.

author: mikerj
author_rank: Super Contributor
timestamp: November 02, 2024, 10:20:00 am
content: Quote from: Harry_22 on November 02, 2024, 10:08:25 amHow is it outside the abs max ratings? The current will only be high if the output transistor of the KAA431 is switched on hard, which it shouldn't be if it's regulating.

author: Harry_22
author_rank: Frequent Contributor
timestamp: November 02, 2024, 11:50:35 am
content: Sorry, I haven't seen this current limit resistor.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 02, 2024, 02:14:35 pm
content: My fault for attaching that schematic without the series resistor, sorry.In my use case there was no resistor, because the DP832 power supply was set for constant current (not voltage), and connected directly to the EC of the transistor. I was testing without the series resistor because the circuit will be used as a floating power Zener diode capable of 3 to 3000mA with virtually no change in its voltage drop.Anyway, it was a matter of defective 431 (got the not working KA431 from old power supplies, and never tested them until now).Replaced the defective 431, and now the circuit works very well no mater which model of 431 is used (tested so far KA431LZ, TL431-A and HA17431VP, all models work fine as power Zener).

author: Zero999
author_rank: Super Contributor
timestamp: November 02, 2024, 05:22:08 pm
content: Quote from: RoGeorge on November 02, 2024, 02:14:35 pmI've not heard of the KA431. The data sheet appears to be the same as the TL431. Presumably it's the same part?It's important to show the power supply on the schematic. If you had your lab PSU set to constant current, then draw it as a constant current source.I simulated it using the TL431 and it works.

author: Harry_22
author_rank: Frequent Contributor
timestamp: November 02, 2024, 06:53:29 pm
content: Quote from: RoGeorge on November 02, 2024, 02:14:35 pmI am rather biased towards real current sources. A couple of excellent fuses were burned by me during testing. It's all about the transition process that happens. The source tries to hold the current, which does not flow while the load is disconnected and increases the voltage to indecent values. At this time, the output capacitor is charging and is not always ready to instantly discharge, bypassing the newly connected load.This means you need also resources to monitor the voltage limit.It is much easier to use a series resistor when measuring a zener diode.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 02, 2024, 07:37:37 pm
content: Quote from: Zero999 on November 02, 2024, 05:22:08 pmProbably not the same die, but similar. At first the TL431 was introduced by Texas Instruments, then 431 became a jellybean part produced by many others. I think the KA431 was made by Fairchild, then Onsemi bought Fairchild. Same as , HA17431 the 3rd brand of 431 I have scraped from defective PC power suppl. HA17431 was made by Hitachi, then Renesas bought Hitachi, or something, one of those never ending corporations merging and acquisitions.Quote from: Harry_22 on November 02, 2024, 06:53:29 pmDid something like that once, some years ago, right after my brand new Rigol DP832 power supply arrived.https://hackaday.io/project/7590-retardo-davinci/log/25376-rigol-dp832-power-supply-set-for-20-ma-can-kill-a-ledThat problem won't happen if the load is connected before turning on the power, and disconnected only after turning off the power.

author: Harry_22
author_rank: Frequent Contributor
timestamp: November 02, 2024, 08:12:14 pm
content: Quote from: RoGeorge on November 02, 2024, 07:37:37 pmI read with interest your article on killing LEDs.This seems to be a common problem with all power supplies. I currently have two R&S NGP800 in calibration, will try to test them with LEDs in your style.I feel sorry for the Littlefuses.