I'm trying to control 3 LED downlights using an arduino. I'm very inexperienced so I'll go through my thought process and you might be able to catch what I'm doing wrong...

I've chosen a 2N5551 NPN transistor based on the following information: NPN transistor because I will be switching higher voltage than what is supplied to the Arduino.

Downlights: http://www.malmbergs.com/frmProductDisplay_new.aspx?item=9974006 They are rated at 350 mA, 3x1.2W / downlight equals a total of 10.8W 10.8W / 0.35 A = 30.86 V (to make sure my math is correct I measured the circuit, 28V and 350 mA) They are driven by a 350 mA CC driver.

So I'm searching for an NPN transistor capable of handling 31V and 350 mA. According to the datasheet: https://www.fairchildsemi.com/ds/MM/MMBT5551.pdf the VCEO is 160V and Ic 600 mA, so far so good?

Here comes the difficult part, calculating the minimum current needed to keep the transistor in saturation (for a newbie like me, transistor datasheets seem to be incredibly inconsistent in their layout and overall hard to read). The guides I've used are telling me to look for Hfe in saturation, Ic/Ib or if it can't be found use a worst case value of 10. Since I can't seem to find the information in the datasheet, I'll use 10.

Base current: 350 mA / 10 = 35 mA (safe value from Arduino seems to be around 40 mA)

Next step, find Vbe: Figure 3 shows a peak of 0.9V at 200? mA (why is the graph not expanded to show up to 600 mA, does the Vbe not rise above 0.9V?) Output voltage of the arduino is 5V, giving us a voltage drop over the resistor of: 5V-0.9V = 4.1V

Base resistor: 4.1V/0.035 = 117 ohm

Feeding the output of the arduino through two 220 ohm resistors in parallel to the transistor base, and the emitter and collector switching the ground for the LEDs, everything is working as expected for 20~ seconds, and then there was smoke...

Any help to solving my problem is greatly appreciated. I have an off-topic question as well, is it possible to PWM a CC driver on the secondary side? Would be real nice to be able to dim these LEDs...


2 Answers 2


While the transistor itself can tolerate 600mA, figure 3 on the datasheet shows that after about 85mA, VCE(sat) increases sharply. If we use something near the last given value on the chart, about 5V or so, we get 1.75W at 350mA. This much power can easily fry a TO-92 given the right circumstances. And the real voltage is likely to be much higher, based on the trend in the graph. Pick a different part, paying attention not only to the static numbers but also the value of VCE(sat) at the desired IC.


This is a perfect application to use a logic level N-Channel MOSFET. This will eliminate the heavy base current requirement coming from the Arduino and make it easier to find a part that has a low RDSON specification.


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