Help with SOT-89 component identification - Page 1
Forum section: electronics / beginners

author: giosif
author_rank: Frequent Contributor
timestamp: March 17, 2019, 10:02:54 am
content: Hello,Can someone please help me identify the SMD component in the picture attached?I'm trying to reverse engineer the schematics for the power section of a Tektronix TX1 DMM and this component is in that part of the circuitry.I am suspecting it is some kind of voltage detector, but I would like to identify the exact make and model.Thanks!

author: RES
author_rank: Regular Contributor
timestamp: March 17, 2019, 08:47:01 pm
content: Only 9B gives two devices from this chart:http://www.dl7avf.info/charts/smdcode/c9.html#TOC3 SOT-89 µPC1943T NE Nec06113 SOT-89 NJM79L03UA JRCThe µPC1943T is an adjustable precision shunt regulator. (NEC) (http://www.chipfind.net/datasheet/nec/upc1943.htm)??

author: amyk
author_rank: Super Contributor
timestamp: March 17, 2019, 10:29:52 pm
content: Quote from: RES on March 17, 2019, 08:47:01 pmThe marking is probably not 9B, but 302. You can often confirm your guesses by looking for pictures from sellers who are selling the part. A UPC1943T looks like this:http://www.eric1688.com/product.aspx?id=384410The marking for the NJU7201U30 looks similar, 30A instead of 302:https://www.aliexpress.com/item/10pcs-lot-NJU7201U30-NJU7201U30-TE1-30A/32841594775.htmlI suspect it's an NJUxxxx part.

author: floobydust
author_rank: Super Contributor
timestamp: March 17, 2019, 10:51:14 pm
content: The zero and oh are a different character, it could be OB9 not 0B9. I was thinking a Torex LDO.

author: amyk
author_rank: Super Contributor
timestamp: March 18, 2019, 02:41:08 am
content: Quote from: floobydust on March 17, 2019, 10:51:14 pmTorex don't have 2x3 character markings, from what I've found:https://www.aliexpress.com/item/XC6203P332PR-SOT-89-TOREX/32344792830.html

author: giosif
author_rank: Frequent Contributor
timestamp: March 18, 2019, 11:11:53 pm
content: Thank you all for the responses!Yeah, I tried finding the component specs myself, before asking for help here (and usually am lucky enough to find it) but, like yourselves, I didn't quite get the exact part for this one.In any case, I think I figured out its purpose in the circuit and that is to clamp the power supply to the DMM, in case more than 3V is supplied (the output of this component drives the base of an NPN transistor whose collector is connected to the positive of the power supply and emitter to the negative of the supply).On to the next part of the circuitry, trying to understand why the DMM is drawing 5mA with the rotary switch in the OFF position.This meter is rather peculiar, as the OFF position is a "soft" power off - i.e. some parts of the circuitry are still drawing power - but, still, 5mA is quite a bit (another, working Fluke 185 is drawing something in the uAmps in the OFF position).

author: Microcheap
author_rank: Frequent Contributor
timestamp: March 19, 2019, 04:07:19 am
content: When I need to decode a smd marking I use this chinese website:http://www.smdmark.com/en-USSearching for "302" the only SOT89-3 found is a 3V regulator FH62R302https://www.datasheets360.com/pdf/-533934800570334198Does it make any sense in the circuit?

author: giosif
author_rank: Frequent Contributor
timestamp: April 21, 2019, 11:30:32 am
content: Hi,Thank you for the links and sorry for the late reply!I since moved on with the investigation to other parts of the circuit, so don't recall 100%, but I think it is indeed a voltage regulator.In any case, after hours of swearing, cursing, pulling hair, etc. I was finally able to identify the culprit for the indecent current draw on the Fluke 185 in the OFF position.I must have de- and re-soldered a dozen or so of components, until I eventually got the bugger: a capacitor turned resistor - C504 (sitting in the are of the PCB which is normally underneath the LCD).Once I replaced the cap, current consumption in OFF state went down to about 40uA, as it should.Quote from: Microcheap on March 19, 2019, 04:07:19 am

author: giosif
author_rank: Frequent Contributor
timestamp: April 21, 2019, 08:57:38 pm
content: BTW, for reference, the value of C504 is supposed to be about 100nF.