Identify SOT type - Page 1
Forum section: electronics / repair

author: mikec
author_rank: Contributor
timestamp: December 12, 2020, 12:37:20 am
content: Hi,I am trying to figure out what the device shown in the picture is:I can only make out theSCpart of the code, the rest of the code seems to be at the point where it let out the smoke.I was reading the link:https://www.eg.bucknell.edu/~dkelley/eceg350/lab/Reading_Transistor_Markings.pdfthis suggests thatSCwill refer to a NPN HF TransistorIs that correct? Can I assume an NPN transistor at least from that part of the code?Regards

author: fzabkar
author_rank: Super Contributor
timestamp: December 12, 2020, 05:08:00 am
content: "U4" would suggest that it is an IC rather than a transistor, while the proximity of the two capacitors suggests that it may be an LDO regulator.A continuity test with a multimeter should determine the ground and Vin pins. The Vout pin must connect to the Vcc pin of one or more ICs. You could then consult the datasheets for those ICs to determine the value of Vout.

author: DaJMasta
author_rank: Super Contributor
timestamp: December 12, 2020, 07:48:59 am
content: Agree that it's probably a regulator, often with SMD code websites they will give you permutations involving the part of the code you can read - but my searches didn't turn up anything that seemed particularly likely. Knowing that it's a standard SOT-23 can help differentiate, as well as the parts around it and its apparent function. The above method works great if it's an LDO, because then outside of specific cases, you can probably use almost any replacement in the same package. Yes there's potential for oscillation or PSRR requirement or whatnot, but in 95% or more of cases, any old LDO in that package will likely work fine if you find the right voltage.

author: mikec
author_rank: Contributor
timestamp: December 12, 2020, 01:01:48 pm
content: Thanks Fzabkar and DajMastaOk good catch with the U4, makes alot more sense now. Your right, one of the pins goes to the Vdd of a number of other ICs.I've uploaded another picture to show what I have probed so far. The only chips I could identify were the TI 8:1 Analog switches.Based on what the information I have and the likely hood this is a LDO as pointed out. I am guessing the pin that I have markedAwould bethe input to the LDO. That leaves me with trying to work out the Output voltage atBwould be. Unfortunately the TI Analog switches have a pretty widerange of input, so I may need to dig more (its less then 12Volt anyway).If it trace that is going to the LCD display is the power for this separate board AND if it is not doing any additional regulating then based on the LCD controller chip (Holtek HT1621B) i can narrowthe output voltage to between 2.4V and 5.2Vdc, I will need to double check this.I should note as well that currently I have 21ohm resistance measurement fromBtoGND, I suspect this could be what destroyed the LDO.. though I would have thoughtthe LDO could have saved itself from a high current draw (depending on the output voltage, say assume 3.3V)?I should say this is a Powerbank (electriQ Multifunction USB 20100mAh), I was switching through its Voltage output modes with nothing connected when it just died!!!I think if we are happy this is an LDO and happy with the pinouts in the attached picture, I might just get a few 3.3V LDO and 5V LDO. I would start by putting in the 3.3V and see if theunit responds at all?Regards,

author: fzabkar
author_rank: Super Contributor
timestamp: December 12, 2020, 09:43:01 pm
content: Would it be worth examining the inputs of the analog switches to determine their upper range? Hopefully they're not all sitting at 0V.

author: mikec
author_rank: Contributor
timestamp: January 28, 2021, 12:18:51 am
content: Well its taken a while for me to get back to this problem! For the sake of closing out this thread I will update here now.I replaced the regulator with a 3.3V regulator, but it heated up very quickly and there was no live in the unit! As i suspected off course that 21ohm "short" from the output of the regulator to GND needed to be investigated.I tracked it down to U1, the IC in the picture that had its markings removed. I removed U1 and confirmed it has 21ohm between its Vcc and GND.I really hoped it wouldn't be that part, unfortunately its one of those occasions where i shouldn't have taking it apart as it was still under warranty!!It seems this is a rebranded powerbank where VolPower are the original manufacturer (P65 laptop powerbank 20000mah is the product model).