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I have a NPN transistor (2N3904), I would like to be able to turn on my LED panel - 37v (don't know the amps) with my Arduino Uno, which gives output of 5v, 0.2 amps.

As far as I understood, the Arduino will be connected to the base of the NPN transistor. if I put a diode or a resistor between the Arduino and the NPN transistor's base, following the tutorials it "should" work, but I don't want to burn my Arduino down.

I don't understand, if the transistor is a 'switch', when I give electricity to the base, why is there electricity coming into the base from the collector? shouldn't it go straight to the emitter? how can I know how big of a resistance to put, between the Arduino and the base of the transistor? In addition, should do 'GRD' from Arduino go to the LED's negative terminal? Thanks


marked as duplicate by JRE, Peter Smith, Daniel Grillo, W5VO Apr 14 '16 at 6:21

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  • \$\begingroup\$ @WesleyLee not quite, the person is using the transistor under 5v, I need to use it under 37v \$\endgroup\$ – user3578847 Apr 11 '16 at 23:39
  • \$\begingroup\$ what are the specs to your LED Panel? its really important to know how many Amps it will draw. 37V at less than 200mA sounds strange for a LED panel.. \$\endgroup\$ – Wesley Lee Apr 11 '16 at 23:42
  • \$\begingroup\$ Im almost certain your panel uses a dedicated constant current circuit, and likely has an enable pin somewhere. \$\endgroup\$ – Passerby Apr 12 '16 at 0:22

IF your LED Panel draws a good bit less than 200mA (say 100mA~150mA), I'd suggest the following:

1 - As per datasheet 2n3904 hFe is minimal 30 (in practice more). In Ib = Ic/hfe, we get:

Ib = 150mA/30 = 5mA

2 - VBE max at closest match (50mA) is 0.95V, say 1V (in practice less).

So 5V - Vbe = Rb * Ib, gives ~4V = Rb * 0.005mA

Rb = 800R

3 - Schematic:


simulate this circuit – Schematic created using CircuitLab

Note that you get uncomfortably close to 2n3904 limits: Vce 40V, Current 200mA, etc..

So I'd actually suggest the following:


simulate this circuit

I just used this particular mosfet because its available at Sparkfun and gives better margins (60V, 30A). You can use anything that suits your needs. (R2 optional)

  • \$\begingroup\$ Thank you very much for the answer. Quick question, what does 'Load' resistor mean? also, what if my LED panel draws more than 200mA? will that burn my Andurio? Thanks! \$\endgroup\$ – user3578847 Apr 12 '16 at 0:15
  • \$\begingroup\$ I used a resistor but it represents your load, the LED Panel. If it draws more than 200mA your transistor will possibly have a VERY short and unhappy life. I'd use the second option. \$\endgroup\$ – Wesley Lee Apr 12 '16 at 0:18
  • \$\begingroup\$ With second option using two resistors, I can be 100% safe? \$\endgroup\$ – user3578847 Apr 12 '16 at 0:23
  • \$\begingroup\$ 100% safe is very hard to guarantee, I can say that it ~can~ be very durable. If you are really worried take a look into some simple techniques to protect MCU pins, such as clamp diodes, etc \$\endgroup\$ – Wesley Lee Apr 12 '16 at 0:31

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