Identifying mystery SOT23 component in voltage reference circuit - Page 1
Forum section: electronics / projects

author: cowana
author_rank: Frequent Contributor
timestamp: December 16, 2013, 01:01:23 am
content: I'm repairing a digital power supply which has lost it's accuracy (by 80% overnight).It's a nice design - all through hole, all standard name-brand components. Five opamps, an ADC, a DAC, and an AVR to drive it.However, there's one thing confusing me - the ADC (a Burr-Brown ADS7841) has an external VREF pin, to use as a reference. This is connected to a couple of resistors, a trimmer potentiometer - and an unknown SOT-23 component.The component is marked as '43R' - however in my searching I haven't come across anything. The silkscreen designator is 'ICxx', which indicates it's not a transistor (which are marked as Qxx).I attach a photo of the board, close up of the component, and a schematic showing how it is connected - the VREF produced for the ADC has to be a DC voltage somewhere between 0 and 5v (although probably towards the higher end of that. Maybe VREG is meant to be 4.096v to go with the 12 bit ADC? I don't know...Any thoughts to what this component might be or could do? It's in a strange configuration!Cheers,Andy(Datasheet for ADC:http://www.ti.com/lit/ds/symlink/ads7841.pdf)

author: c4757p
author_rank: Super Contributor
timestamp: December 16, 2013, 01:27:43 am
content: It's a ZR431 (just a TL431 with a different name). Datasheethere.A search forvoltage reference sot-23 "43r"made quick work of this one. What the hell were you searching for?Weird that they're trimming such a huge portion of the reference voltage when it's a 0.5% unit. Haven't they heard of digital calibration??Also weird that they chose a SOT-23 reference when everything else on there is through-hole and 431s are available in TO-92 in vast quantities.Quote from: cowana on December 16, 2013, 01:01:23 am*cough* CrapXon *cough*

author: minime72706
author_rank: Frequent Contributor
timestamp: December 16, 2013, 02:58:12 am
content: Quote from: c4757p on December 16, 2013, 01:27:43 amIt's still hard to argue with the presence of a Burr-Brown part, though!Ah the good-old TL431. I wonder how many of those have been manufactured ... and I wonder how many of them are in the average first-world home.

author: electronics man
author_rank: Frequent Contributor
timestamp: December 16, 2013, 06:27:54 pm
content: It doesn't apear that it is available in other packages. And may I ask how old is the unit.

author: SeanB
author_rank: Super Contributor
timestamp: December 16, 2013, 07:02:23 pm
content: Probably they had a reel of them left over from a previous project and so they wanted to use it. They figured soldering 1 SMT in by hand was doable by the production line without too much hassle, and then the new design would be all SMD and automatic assembly if there was big enough demand.

author: minime72706
author_rank: Frequent Contributor
timestamp: December 17, 2013, 01:58:12 am
content: Quote from: electronics man on December 16, 2013, 06:27:54 pmYou talking about the TL431?The TL431 appears in lots of packages, but there are also off-brands that use similar but different part numbers. I've seen "KA431", for example. The specific part number he has isn't too important.

author: amyk
author_rank: Super Contributor
timestamp: December 17, 2013, 12:58:55 pm
content: Quote from: minime72706 on December 17, 2013, 01:58:12 amNote that '431s come in different accuracy/tempco grades, so they're almost but not exactly 100% interchangeable.

author: cowana
author_rank: Frequent Contributor
timestamp: December 17, 2013, 02:27:32 pm
content: Quote from: electronics man on December 16, 2013, 06:27:54 pmIt's pretty new (as test equipment) goes - date code on most chips is 2005. There's one other SMD chip on the entire board (the DAC, which isn't available in through-hole) - which was obviously being soldered by hand (evident by lots of flux and hand done joints) - so I guess doing the TL431 by hand wouldn't be too much extra effort in the production line.Quote from: c4757p on December 16, 2013, 01:27:43 amIt is weird - there *is* calibration in the firmware (scale and offset for the ADC readings) - trimming such a large part of the reference voltage makes it very sensitive to adjustment...

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: December 17, 2013, 03:47:19 pm
content: Quote from: c4757p on December 16, 2013, 01:27:43 amThis is a typical answer from someone who doesn't even know what a REAL LCR meter isIf it doesn't have a single button that does it all for you and a BIG display to show the results, their lost

author: echen1024
author_rank: Super Contributor
timestamp: December 17, 2013, 05:14:30 pm
content: What type of power supply is that?

author: cowana
author_rank: Frequent Contributor
timestamp: December 17, 2013, 05:57:23 pm
content: It's a no-brand, 18v@5A 'Programmable DC Power Supply'.https://www.eevblog.com/forum/testgear/very-strange-failure-mode-in-programmable-dc-power-supply/