Photo of PCB including unknown transistor Q5

I need to know the specifications of the transistor Q5 with top marking BC RJ

  • \$\begingroup\$ We need a better image than this. You may need to use a macro lens to zoom in to the part you refer to. \$\endgroup\$
    – user105652
    Commented Jun 27, 2020 at 7:26

1 Answer 1


Tracing the circuit, I got:-


simulate this circuit – Schematic created using CircuitLab

The biasing of Q5 suggests it is a PNP bipolar transistor. The resettable fuse rating of 0.3 A indicates that the transistor must be rated for >0.3 A, and the rating of EC2 suggests a voltage rating of 16 V or higher. The package appears to be SOT89.

I did a Google image search on "PNP SOT89" looking for a marking code similar to "BC" and found an On Semi 2SB1302 with code "BJ". I then searched On Semi's website for other similar transistors, and found the marking code order was the same as the listing order. This suggested that "BC" should correspond to 2SB1120. Finally I Googled that part number and got this:-


  • 2
    \$\begingroup\$ just wanted to say I admire your Sherlocking skills \$\endgroup\$
    – Ilya
    Commented Jun 27, 2020 at 8:22
  • \$\begingroup\$ Special thanks for your reply .Are you sure about this(2sb1120)? By replacing this transistor, it will not damage other parts of the circuit?Are the two(bc rj = 2sb1120) similar? \$\endgroup\$
    – amin nima
    Commented Jun 27, 2020 at 8:24
  • \$\begingroup\$ The reason for tracing the circuit is so that even if I can't identify the exact part, I will know what specifications are required. I know from the marking code ('BC') and the specs that this the right part. This circuit is also well protected by resistances and a fuse, so even if the transistor failed it shouldn't damage other parts. Often an exact replacement part cannot be sourced, so knowing what specs are needed is important for choosing a suitable replacement. \$\endgroup\$ Commented Jun 28, 2020 at 0:18
  • \$\begingroup\$ Thank you very much. I hope you are always healthy \$\endgroup\$
    – amin nima
    Commented Jun 28, 2020 at 10:22

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