I was troubleshooting a circuit and found that a transistor BC547 is getting very hot and after a day or two it is completely burned out while keeping the circuit on continuously. Then i measured some voltages and currents around it and providing here. I also referred to datasheet of the transistor but couldn't solve the issue. Vce = 10.2V, Vcb = 9.5V, Vbe = 0.7V, Vc to gnd = 22.0V, Ve to gnd = 12.1V, Vb to gnd = 12.7V, Ic = 20mA, Ie = 20mA, Ib = 6.3uA, Here all the values are way below the max values. Then why is the transistor heating up. Do i need to check anything else. ANY HELP.

  • 6
    \$\begingroup\$ Add a schematic! \$\endgroup\$
    – winny
    May 22 '17 at 12:56
  • \$\begingroup\$ Schematic is quite hard to trace. forgive me for not providing that. \$\endgroup\$
    – Raj Kiran
    May 22 '17 at 13:38
  • \$\begingroup\$ You'll need to add more info, emitter at 12V, is it a power supply or amplifier or... \$\endgroup\$
    – sstobbe
    May 22 '17 at 14:12

Well, you have 10.2 volts between collector and emitter and 20 mA flowing. That's a power of over 200 mW and it will get quite warm. If you don't have any circulating air (natural or forced) the transistor will just get warmer and warmer until is dies. There is also to consider that you might be operating this device in a naturally warm environment and this just adds to the woes.

Thermal resistance of the device is specified by Fairchild as being 200 degC/ watt so it will certainly rise at least 40 degC just on the power dissipation alone and, like I said, if there is no circulating air (or other cooling mechanism), it will continue to get hotter.

  • \$\begingroup\$ Well, the power through collector is 500mW max as given in the datasheet. Isn't that ok. \$\endgroup\$
    – Raj Kiran
    May 22 '17 at 13:51
  • \$\begingroup\$ Yes, 500 mW is the upper limit but there is also the internal junction temperature and if you don't apply enough colling then you can't rely on 500 mW. There is also a derating factor on power dissipation above certain temperatures and this closes things down even more. \$\endgroup\$
    – Andy aka
    May 22 '17 at 14:32
  • \$\begingroup\$ 500mW ABS max at Ta=25 degrees C, which is a detail that matters. At 200mW I am not at all surprised that the thing is heating up noticeably, that is more then I would be happy running a TO92 at. That said, it should probably not be failing unless you have some other condition that pushes it further. \$\endgroup\$
    – Dan Mills
    May 22 '17 at 14:37
  • \$\begingroup\$ Ok, i have a new circuit of this kind. Let me measure the similar parameters for that transistor. After that i will get back to you. Thanks for your suggestion. andy aka. \$\endgroup\$
    – Raj Kiran
    May 24 '17 at 4:33
  • \$\begingroup\$ so what is the mW range so that the transistor do not get heated. Is it mentioned in datasheet. dan mills \$\endgroup\$
    – Raj Kiran
    May 24 '17 at 4:38

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