I am making a constant current source as an adjustable. The maximum current flowing is 1 A. I fixed the current at 570 mA, using a BDX53 BJT. BJT collector emitter voltage is 10 V. This circuit also has a square wave but not completely within the transistor's base, collector and emitter voltage. But it fixes square wave to constant wave with the aid of capacitor and coil. So load voltage is a constant current as well. My problem is a BDX53C heating up over time while BDX53C powers only 6 W.

How can I fix it?

  • 4
    \$\begingroup\$ Electrical engineers communicate with circuit diagrams. Please add a circuit diagram to your question. \$\endgroup\$
    – Janka
    Jan 8, 2020 at 19:57

1 Answer 1


https://www.onsemi.com/pub/Collateral/BDX53B-D.PDF shows Thermal resistance, junction to ambient, to be 70 deg C per Watt. I would expect things to get pretty hot at 6 Watts. You can try active cooling or heat sinks.

You're going to need to drop Vce if you want to keep using this chip.

  • \$\begingroup\$ Sorry ı have forgotten to read using cooling. But is it normal? I think its not... Because bdx53c's max watt is a 60 w. what is the 70 degree c per watt ? If it could, thats normal ... \$\endgroup\$
    – Naci
    Jan 8, 2020 at 20:17
  • \$\begingroup\$ You can get up to 60W dissipation by using an extreme heat sink. And it will still get quite hot. \$\endgroup\$ Jan 8, 2020 at 20:47
  • \$\begingroup\$ so which is faster, pc817 or tlp627? I need to fast optocoupler for my circuit \$\endgroup\$
    – Naci
    Jan 9, 2020 at 18:42

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