Identify component in SOT23-3 and backwards K marking - Page 1
Forum section: electronics / beginners

author: Sergio
author_rank: Contributor
timestamp: May 19, 2021, 09:19:54 am
content: Hello,Im trying to understand an electronic circuit but I cant identify a component so it is getting difficult.I attach a picture of it. It has a BK marking followed by a backwards K. Pin at the right is connected to GND.Can anyone help with this? Does anyone know what this reversed K is?Thank you!!

author: Anthocyanina
author_rank: Frequent Contributor
timestamp: May 19, 2021, 09:46:05 am
content: Not being too familiar with sot-23 markings, it looks to me like it is just B and the "K" markings are diode cathodes, and from the bars on top, it would seem that they represent the pins of the device, making it two diodes with common anode in pin 2, and cathodes in pin 1 and 3. But this is just a guess, and googling it like that gave me no reliable results.

author: Sergio
author_rank: Contributor
timestamp: May 19, 2021, 09:52:46 am
content: Anthocyanina, thank you for your contribution.I though the dashes were some kind of date coding (I read that somewhere).But it make sense to represent the pins and diode cathodes. I will keep investigating this.If any one can suggest anything else... I will appreciate it

author: Zero999
author_rank: Super Contributor
timestamp: May 19, 2021, 11:08:30 am
content: Reverse engineering the surrounding circuitary would give more clues.If you have a universal component tester, you could desolder it and test it. Failing that, use a DVM's diode test function. If it's a pair of diodes, they should have a similar forward voltage. If it's a BJT, the base-emitter junction will have the higher voltage drop. A MOSFET just has a body diode, with an open gate connection and would need more investigation to differentiate from a diode. A JFET's gate will look like a diode, with the source and drain a resistance from 10R to 1k, typically 100R.

author: Sergio
author_rank: Contributor
timestamp: May 19, 2021, 12:22:50 pm
content: This is what Ive got at the moment.It seems obvious that all resistors and trimmer are meant to give a voltage reference to the opamp, but im confused about the SOT23 packageI cant desolder it right now.

author: StillTrying
author_rank: Super Contributor
timestamp: May 19, 2021, 12:29:18 pm
content: BCX71K looks quite possible, a PNP 100mA.https://www.nexperia.com/products/bipolar-transistors/general-purpose-and-low-vcesat-bipolar-transistors/single-bipolar-transistors/single-bipolar-transistors-100-v/series/BCX71-SERIES.htmlhttp://www.marsport.org.uk/smd/mainframe.htm

author: Sergio
author_rank: Contributor
timestamp: May 19, 2021, 01:25:34 pm
content: StillTrying, to be honest, that was my first guess at a coarse search.But, the datasheet says there is a 3rd marking character to determine country of production (p, t, and W) and noone are matching.Also, it does not make sense for me if it was a transistor. What would it be intended for?

author: StillTrying
author_rank: Super Contributor
timestamp: May 19, 2021, 04:15:30 pm
content: I don't know what its use is in what you've drawn as a transistor, or back to back diodes.

author: Sergio
author_rank: Contributor
timestamp: May 20, 2021, 08:20:55 am
content: Just discovered what was te transistor for.And yes, it is a transistor. The BCX71K, as StillTrying suggested and as I had first guessed.The voltage reference to te opamp can be given externally so this transistor takes action to limit the voltage when is overrange.

author: Jwillis
author_rank: Super Contributor
timestamp: May 20, 2021, 08:31:39 am
content: I don't know. Several code listings show it to be MRF957 . SOT23-3 and SOT323 are both common packages and are nearly identical at first glance. You might have to check the actual dimensions . Pin spacing is different yet the bodies are the same .

author: Sergio
author_rank: Contributor
timestamp: May 20, 2021, 09:01:53 am
content: Jwillis, it is for sure SOT23-3. SOT323 is much smaller and I have accurately measured it.Also, if it was a NPN transistor the circuit would not make sense. I simulated it and it works for a PNP transistor.Maybe it is not the BCX71K but it must be a PNP transistor in SOT23-3 package and same markings...

author: nicknails
author_rank: Regular Contributor
timestamp: May 20, 2021, 01:13:57 pm
content: Maybe for overvoltage? Input gets too high, transistor turns on and pull + of the op-amp low. I'm not too good with op-amps, so this is just a guess.