Which type of transistor for pulsing LED strip?

Question

Preface

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}

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Components

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.

Power supply

Output: 24 VDC, 5 A (Product page)

MCU

Particle Photon microcontroller (Datasheet)

Transistors

BC517 Darlington NPN 30 V, 1 A (Datasheet)

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Question

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!

Answer 1

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.

Answer 2

https://www.digikey.ca/product-detail/en/stmicroelectronics/STD16NF06LT4/497-4329-2-ND/725195

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.
• 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.