I am designing a LED Headlight for my car. But I am confused about my BJT, I dont know if it is suitable or not. The overall design is given below.

My diagram

The problem is, when I run the ckt under engine off (power source is the battery, 11.86v) then there is no problem, even all the components are ice cool. But when I start the engine (power source becomes the dynamo, 13.5v) then in 2 seconds I start to see smoke from the BJT and just under 5 seconds it is blown. I see a small hole up on the emitter pin. Can anybody tell me what could be the reason?

Besides, if you want datasheet, all the model numbers are given in the diagram. If you want to know, I couldn't find any data sheet for my LEDs so I measured the amps on my own, I am giving you a link of those LEDs.


  • \$\begingroup\$ Look at the datasheet \$\endgroup\$
    – PlasmaHH
    Jul 27, 2015 at 13:54
  • \$\begingroup\$ What exactly are you trying to achieve with the transistor? And why do you have that diode between the base & collector? \$\endgroup\$
    – brhans
    Jul 27, 2015 at 13:55
  • \$\begingroup\$ I have looked at the BJT's datasheet and there is nothing wrong, its max amp is 3-6A and I am giving 1.05A at max. Vce is 30V and I am giving 13.5 at max ( after LEDs drop it becomes 8.6v). Each LED can consume up to 200 mA, but I am giving only 70 mA at max. At 70 mA they doesn't even heat up. \$\endgroup\$ Jul 27, 2015 at 14:00
  • \$\begingroup\$ I am using the transistor to control the dipper and normal light. I want 10 LEDs to glow in normal mode and all 15 LEDs to glow in dipper mode. The fact is the power socket is 3 pin ( one negative two positive). When I change the mode the socket pin configuration is also changed. That's why I am using a BJT to control it. The diode is just for safety. \$\endgroup\$ Jul 27, 2015 at 14:03
  • \$\begingroup\$ The diode is applying your power supply voltage directly to the base of the transistor, and with no series base resistor its no surprise that the poor BJT is leaking its magic smoke. You're applying 12-14v across a pair of semiconductor junctions which don't expect much more than 0.6-0.7v each. \$\endgroup\$
    – brhans
    Jul 27, 2015 at 14:25

1 Answer 1


I don't get why the transistor is necessary, just use a couple of diodes:


simulate this circuit – Schematic created using CircuitLab

When the low beam wire is on, only the low beam LEDs light up, when the hi beam wire is on, all LEDs light up (D4 is there to prevent backfeeding to the car system - my car would not care about that, but a newer car might).

  • \$\begingroup\$ Thanks for your efforts, but the fact is when I switch from low to high beam, one + is fixed but the position of the other + and - are changed. So if I simply use some diodes, where would I connect the ground? Its like at low beam the ground is in the bottom and at high beam the ground is on the top right. Please see the image I have attached, see the sockets for both diagram and you will see it. \$\endgroup\$ Jul 27, 2015 at 15:56
  • \$\begingroup\$ Besides, I can not use a relay because there is not enough space inside the holder I am trying to attach the lights with. \$\endgroup\$ Jul 27, 2015 at 16:00
  • \$\begingroup\$ @WahidMasud, I have modified the schematic to suit the positive-pin-fixed socket your car has. Now the LEDs light up depending on which one of the changing pins is negative. \$\endgroup\$
    – Pentium100
    Jul 27, 2015 at 18:27
  • \$\begingroup\$ Wow I should have thought about it, its really very easy! Thanks for the efforts, you have saved a lot of my works! \$\endgroup\$ Jul 27, 2015 at 18:43

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