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Suppose the DC motor has a peak current of 500ma and after that it drops to 100ma, the transistor can switch continuously 200ma.

If I limit the current of the transistor from the base current to 100ma, for some reason could the transistor consume a higher peak than that when starting the motor or would it be limited to 100ma?How effective is limiting hfe gain against inrush currents?

  • \$\begingroup\$ if the base current is 100 mA, the motor current will be effectively unlimited. I'm not familiar with the BC548, but if it's anything like the general purpose transistors I've used, it has a β on the order of 100, so the collector current would be limited to about 10 A. \$\endgroup\$
    – Hearth
    Jan 13, 2021 at 4:45
  • 2
    \$\begingroup\$ "If I limit the current of the transistor from the base current to 100ma" - Do you mean limit Collector current to 100mA by applying the necessary Base current to get it, or apply 100mA to the Base? \$\endgroup\$ Jan 13, 2021 at 5:06
  • \$\begingroup\$ Suppose the hfe is 100 to 12v and 100ma of collector, on the base I applied 1ma \$\endgroup\$ Jan 13, 2021 at 10:34
  • \$\begingroup\$ If you never supply the 500mA that the motor needs at the start then the motor may not turn at all. \$\endgroup\$ Jan 13, 2021 at 13:34
  • \$\begingroup\$ Limited the motor with resistor to work at 100ma maximum I had satisfactory performance! My concern is to protect from inductive peaks in very short times that destroy my transistor! I know that I should dimension the circuit, but I would like to limit the components too, I understand today that a transistor limited to 100ma of current between collector and emitter would eliminate the use of a resistor \$\endgroup\$ Jan 13, 2021 at 14:28

1 Answer 1


The Hfe value is one of the least stable or predictable metrics or a BJT, datasheets often gives ranges of 50 to 200. Current gain varies widely from one device to another and also with current, temperature etc. That said, a BJT will do an effective job of limiting the inrush current but it’s not possible to predict with any degree of accuracy what the current limit will be.

  • \$\begingroup\$ Exactly, the bjt varies its hfe from the same model to the other, but using an "ideal" model: 100ma of collector at 12v VCE, HFE of 100 in these conditions, and base current of 1ma. If the load current is more than 100ma, or the load is short, the transistor would effectively limit it to 100ma or for a short time it will peak above 100ma (3.4.5x greater than 100ma etc) ? my concern is with very short spikes exploding my transistor \$\endgroup\$ Jan 13, 2021 at 10:39
  • \$\begingroup\$ @MarceloAugusto Do you just want to simulate or is this a real project? If only simulation then your proposed fix will work. In real life it has several problems. \$\endgroup\$ Jan 13, 2021 at 13:35
  • \$\begingroup\$ Elliot Alderson would be a problem for real life, I know there would be variations in voltage, temperature etc, however, I really wanted to know the effectiveness of the transistor in limiting the current from the hfe gain and base current, is there a possibility in us seconds or nano seconds for the transistor to pass a much higher current and burn? \$\endgroup\$ Jan 13, 2021 at 14:24
  • \$\begingroup\$ The short answer is no, the transistor will not pass excess current for any measurable period of time. Constant current supplies operate on similar but subtly different principles and you can abruptly apply a dead short without causing any problems. \$\endgroup\$
    – Frog
    Jan 14, 2021 at 7:02
  • \$\begingroup\$ @Frog thanks!that's what I needed to know! \$\endgroup\$ Jan 15, 2021 at 0:16

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