Hello! I'm trying to determine what type of transistor I need in order to pulse an LED strip I have. I have attached a simplified schematic (omitting resistors) as a visual aid to show basic intended operation. (I have very little knowledge about schematic creation, so I apologize—especially for the LED strip and USB power parts. I hope it is helpful.)


simulate this circuit – Schematic created using CircuitLab


Here is some information about my components:

LED Strip

I did not receive a datasheet with the LED strip or identifying part information to locate one. I know the following:

  • It contains two different colored LEDs, alternating down the strip.
  • It has one positive terminal and two negative terminals (one for each color LED).
  • It requires 24 VDC.
  • Multimeter tests show that each negative terminal reads 0.5 A, for a total draw of 1.0 A.

Additionally, here is a photograph of one section of the strip (it can be cut across each section of terminals—the intensity of the flash in the photo appears to shift the colors). Also the back has a prepared adhesive applied.

enter image description here

Power supply

Output: 24 VDC, 5 A (Product page)


Particle Photon microcontroller (Datasheet)


BC517 Darlington NPN 30 V, 1 A (Datasheet)


My goal is to dim each of the two colored LED arrays using PWM from two of the µcontroller's +3.3 V I/O ports. The µcontroller hardware/software is not an issue for me—however, I am having this issue: the LED is always on in a less-than-full-brightness state when this circuit is closed. Even if I disconnect the MCU from the transistor base, and connect a floating wire in its place, the result is the same—but if I disconnect the base completely, the LEDs turn off.

Is this an issue with the transistors I have—and if so, which transistors should I use instead? Whichever transistors I use must be able to switch states faster than 200 µs and ideally faster than 150 µs.

Thanks for the taking the time to read!

  • \$\begingroup\$ LED datasheet link? \$\endgroup\$ Mar 20, 2018 at 0:33
  • \$\begingroup\$ @TonyStewart.EEsince'75 Not available. \$\endgroup\$
    – jsejcksn
    Mar 20, 2018 at 0:34
  • \$\begingroup\$ good Photo graph of strip to show R values and wiring , ground routing \$\endgroup\$ Mar 20, 2018 at 0:34
  • \$\begingroup\$ @TonyStewart.EEsince'75 I updated the question with a photo of one section of the strip. The strip is comprised of identical sections. \$\endgroup\$
    – jsejcksn
    Mar 20, 2018 at 0:47
  • 1
    \$\begingroup\$ Why is 150us so important to light? \$\endgroup\$ Mar 20, 2018 at 2:01

2 Answers 2


Typical problems in these circuits are:

  1. You did not tie the grounds together. A transistor requires a common reference voltage to work. If you do not connect the 5V supply's ground to the 24V supply's ground, then the two voltages are not referenced to each other. Transistors are not isolated from one side to the other.

  2. Wired incorrectly. Double check the orientation. Replace them to test.

  3. You are not pulling the pin down all the way. You may be putting it in a pull up state instead of output ground. This is a software condition.

  4. The transistor, a Darlington, has a high gain, which is why a floating base may allow it to conduct. Floating pins act like tiny antennas. Try adding a pull down resistor from base to the common ground, to force it off. Try 1kΩ and move from there.

  5. The power supply is bad or does not regulate well. 30V collector to emitter will break those transistors.

For 1 and 4. You don't need to do 4, but 1 is required:


simulate this circuit – Schematic created using CircuitLab

The blue lines represent the common ground you should be connecting. R1 is an example weak pull-down resistor to force the transistor off.

  • \$\begingroup\$ I do not understand no. 1. My background is in software, and this is one of my first self-directed circuit projects. Will you please elaborate on numbers 1 and 4? \$\endgroup\$
    – jsejcksn
    Mar 20, 2018 at 1:16
  • \$\begingroup\$ @jsejcksn added some clarifications. \$\endgroup\$
    – Passerby
    Mar 20, 2018 at 1:20
  • 1
    \$\begingroup\$ I connected the MCU ground to PS1 negative and it seemed to solve the problem. I'm not sure how I could connect PS2 to the circuit in any way. (The only interfaces it has are two wall prongs and one USB type A female port.) \$\endgroup\$
    – jsejcksn
    Mar 23, 2018 at 20:40
  • 1
    \$\begingroup\$ The USB port has both Vusb and ground connected, which is normally connected to the rest of the ground on the photon board. So no need to add another. \$\endgroup\$
    – Passerby
    Mar 23, 2018 at 21:48


Darlington is only rated for 1A and will get too hot as it's voltage drop @2.5A is too high.

  • MOSFET Nch is needed for low side switch.
  • Low Vgs (th)<=1V for 3.3V~5V logic gate drive.
  • ESD handling critical.
  • Low RdsOn is need for efficient cool operation. << 100 milliohms
  • Short connection leads necessary for high speed between Gate and Source. (AWG30 magnet wire if not using PCB)
  • Only available in SMD parts. enter image description here
  • Replace connections B-C-E of NPN with G-D-S of Nch MOSFET.
  • keep Drain out and ground return wires close together. Put Low ESR Cap (1~10uF) near drain on 24V connections to gnd.
  • add ~~0.01uF//1k to earth ground to reduce stray noise for dim operation.
  • \$\begingroup\$ I think I will need a considerably greater knowledge foundation in electrical components to fully understand your answer (and I certainly can't yet evaluate it.) Thank you for taking the time to give such a detailed response. \$\endgroup\$
    – jsejcksn
    Mar 23, 2018 at 20:43
  • \$\begingroup\$ The most important part is heat and voltage drop on switch if P=VI=1.5*1=1.5W of heat Darlington and 1.5V for a strip designed to operate 12V +/-0.5V for same brightness that is too much while FET is a voltage control resistance that can be selected much lower. The other thing is transsitor TO-92 is rated for 0.6W at 150'C so it will fry as best case Imax =0.5A so you need a power FET switch for best performance not a small signal Darlington \$\endgroup\$ Mar 23, 2018 at 20:54
  • \$\begingroup\$ Is that true even though each transistor is only receiving 0.5 A? \$\endgroup\$
    – jsejcksn
    Mar 23, 2018 at 21:04
  • \$\begingroup\$ read the datasheet 0.5A is MAX and thats not continuous. \$\endgroup\$ Mar 23, 2018 at 21:22
  • \$\begingroup\$ I'm confused... on the first page in the Maximum Ratings table, the datasheet says "Collector Current - Continuous, Ic, 1.0, Adc". Where are you referring me? \$\endgroup\$
    – jsejcksn
    Mar 23, 2018 at 21:30

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