Author Options:

Thermal Flashlight scaling help Answered

Hello all,
I'm dont have much basic knowledge about electronics and I got a problem.

I've built this exact Instructable: https://www.instructables.com/id/Build-a-Thermal-Flashlight-Light-Painting-with-T/?ALLSTEPS
I want to drive a 1m RGB LED strip with it so I've changed the NPN transistors to stronger ones that handle 0.5 A, it's the only components that are different. These are the exact NPN transistors: https://www.elfa.se/elfa3~eu_en/elfa/init.do?item=71-014-58&toc=0&q=BC337-25

Problem: the circuit works fine with ONE RGB LED but it does not work with even the smallest bit of RGB-strip (3 emitters). Goal is to drive a 1m strip and that consumes 14.4W @ 12V. Im using 12V 1A adapter for the Arduino and another 12V 2.5A for the LED-strip.
What am I not understanding?


Oh btw, if your NPN transistors get crazy-hot after a while they are probably to weak right?

These are 0.5A, have some 1A I could change to.

They ain't heatsinked, and/or they ain't turned on properly. Try Mosfets instead.

What do you mean they ain't turned on? How does one go about turning a transistor on or off properly?

I do have a 1k resistor between the transistor and the LED-strip, is that even necessary seeing the LED-stip has it's own resistors?

I'll get me some Mosfets, seems like the easiest solution.

NPN (and PNP) transistors are NOT switches, they are amplifiers. In other words, they multiply the current entering or leaving the base by some number and TRANSfer that to the collector emitter. If a transistor has enough current driving its base, the output will be carrying as much current as it can, if you don't it will let some current flow, and develop a much higher voltage across the collector-emitter than you would like,because the power dissipation (how hot it gets) = Vce x Ic.

Measure the voltage across your collector-emitter, if it isn't less than a couple of volts, its not saturated, and will get hotter than you expect.

No, you shouldn't have ANY resistor between the LED and the transistor.

So for example, if your load is 1A , and the transistors gain is 1000, you need 1/1000A in your base, to saturate the transistor. If you can only give it 0.5mA, it won't saturate, and you'll see half the supply on the transistor.

Thanks for the detailed info Steve, clears a few things up!
I'll keep experimenting.

Problem was the power supply grounds, what I did was use the 2.5A supply for the Arduino and then just hooked the + from the LED-strip to the + on the Arduino power connection (+ solder spot on the board). Works like a charm now!

The LED's seems to be in parallel, striped it and checked.
Thanks for all the input and I'll be sure to have a look at that tutorial.

Are you trying to drive the LEDs in series or parallel I can’t tell from the photo. LEDs are current driven and if they don’t get enough current they won’t work right.

Looks like the Arduino can't drive your transistors properly. Have you connected the power supply grounds together ?