# Calculate transistor current output

I want to light up an LED strip via an Arduino (ESP8266)

The Arduino’s GPIO has a maximum output of 12mA. It is not enough to power a 5m length LED strip.

An obvious solution is to use a transistor to increase the current. I’ve followed an internet tutorial that used a TIP31 transistores, but my lights were too dim. The solution provided on this answer (RGB LED strip is too dim) is to halve the resistance to 200ohms and even that may not be enough because the current needed is about 2A per channel (6A total). They pointed to me that I could use a Darlington pair to increase even more the current and I’ve decided to try it out but before I could try someone said that it could burn my LEDs.

I can’t replace this LED strip in less than 3 weeks (shipping to my city is hell) and I would like to do it safely.

So um trying to figure out which resistor and which transistor I should use.

Some formulas I already know:

• amps needed per channel: (#LEDS / 3) * 20mA => 2A
• watts needed: V * I => 12V * (#LEDS / 3) * 20mA => 24W * 3 => 72W => 90W to be safe

This is the suggested circuit that someone said it will work and someone else said it would burn my LEDs

So I’ve decided to learn how to choose a transistor and a resistance, I’ve found this datasheet for the TIP31 but I can’t decode its math.

Can someone help me with that?

• You could always put a current limiting device somewhere in there. A plain resistor will work, or a more advanced solution with a transistor and some feedback circuitry (essentially using a linear regulator as a current limiter) Commented May 11, 2018 at 14:28
• In any case, you want to look at the DC current gain rating, as well as the base-emitter forward voltage rating. Those will tell you most of what you need to know. Commented May 11, 2018 at 14:35
• I’m sorry, I’m new to electronics, can you elaborate? I’m having trouble exactly with that, calculating DC current gain based on datasheet information. I’m unable to grasp what should I be using to calculate the emitter‘s output current Commented May 11, 2018 at 14:46
• The emitter current is simply the sum of the base and collector currents. You can usually treat it as just being equal to the collector current, as the base current is quite small. Commented May 11, 2018 at 14:49
• You typically do not design based on just the DC gain of the transistor. This varies a lot and will be unstable. Depending on the Vf of your LEDs, you may also be dissipating too much power in Q2 and burn it up. If your goal is to keep the circuit simple, a power resistor in series with D1 may be a solution. Commented May 11, 2018 at 14:54