Why is my TIP120 burning up? [SOLVED]

Hi, so I'm building a led rgb strip controller that will be controlled via bluetooth communication between an android and an ATTINY84. I wanted to add a "sync to music" functionality so I made an electronic switch using a transistor that when a pin on the attiny was on, it would send current to the base of the tip120 and connect the power of the led strip to a tip31c (a popular bass syncing transistor). When the switch is off, the power of the led strip is just connected to the positive of the 12v power supply. When the switch is off everything works fine, but when the switch is on the tip120 will get very hot and the whole circuit stops working. However, this same setup was working when I tested this project using an Arduino Uno board. I've placed a diode between the emitter and collector of the tip120 (negative pointed towards the collector) and I have a 10k resistor between the pin on the attiny and the base of the tip120 (I've also tried changing this value, nothing works).

Here is the complete circuit as it appears on my breadboard:

Here is a small section of the circuit showing how the tip120 interacts with the circuit:

I tried looking at other forums and one of them suggested using a logic level fet. Do you think this would solve my problem?
Or is there something wrong in how I'm wiring this circuit?

I'm also fairly new to electronics so bear with me in case I don't fully understand all the technicalities. Any response would be greatly appreciated.

Your image is great, but can you provide us with a schematic ?

I looked at your pictures of spaghetti. I am hungry. Is the emitter of the TIP120 connect to ground?

Mmmm... TIP120... Wasn't Noah using those as wedges to hold the rabbit's cage doors shut? They were obsolete back then, and they are obsolete now (for certain values of "obsolete").

There are a million and one better choices than a TIP120.

It's like choosing am LM714 and wondering why it goes "hissssss"...

emitter to ground, collector to +12V, thats the problem

You really should use a schematic drawing program.
Your collector to emitter problem would jump right out!

Is there a free, easy, online schematic drawing program ? Where?
Thanks, Jack

Is there a free, easy, online schematic drawing program ? Where?
Thanks, Jack

Press the Schematic button in Fritzing?

Is there a free, easy, online schematic drawing program ? Where?
Thanks, Jack

Draw on paper, take photo, post photo. Probably the least effort.


Ok so I drew up the schematic here:

emitter to ground, collector to +12V, thats the problem

Correct me if I'm wrong, so when the pin is active, is there a short circuit occurring when the collector and emitter connect? Can you suggest any way to wire this up so that I can connect the tip31 to the power of the lights safely using an electronic switch?

I need to connect the emitter on the tip31 to the +12v on the led strip, when the pin on the attiny is active, is what I’m trying to do.

There are a million and one better choices than a TIP120.

Could you please name some? I'm new to this.

Some comments:

  1. You are using some MOSFETs to control the LEDs. That is good. What isn’t good is that they are power MOSFETs, not logic level MOSFETs. You should be looking for IRLxxx not IRFxxx MOSFETs, or other MOSFETs where the RDSon is very low at the voltages you will be switching at - ideally a few milliohms at <2V.

  2. I don’t know quite what you are trying to achieve with the TIP120. If you want to control the power ON/OFF then you should consider using a P-channel MOSFET to control the +12V line. That P-channel MOSFET should be driven by either an N-channel MOSFET (logic level) or a simple small signal NPN transistor.

I can’t recommend something to replace the TIP120 until I know just what it is you ate trying to do with the TIP120.

When you apply +volts to the base of teh TIP120, it basicly, turns on, applying near ground to the collector. But you have the collector hooked to VCC, so you get a burnout.
You probably want to hook the collector to the ground of the led board, so when you turn on the TIP120, you apply ground, and it lights up.
BTW, in your diagram I noticed the battery symbol was backwards. I assume It is really turned the other way in real life.

I can't recommend something to replace the TIP120 until I know just what it is you ate trying to do with the TIP120.

The TIP120 is acting as an electronic switch basically. Its not an ON/OFF switch, it just connects the power of the lights to the tip31 so the lights will sync to music coming from the audio in stereo jack. I have android ui, with a button that says "music", and when you click that button, the android tells the attiny84 (via bluetooth) to digitalWrite pin 0 to HIGH (the pin connected to the base of the TIP120). So that when the pin is active, the +12v lead on the led strip is connected to the emitter of the tip31, and flashes in sync with the music's base.

Except that is all wrong.

What you have there is the music causing PIN0 to get shorted to ground, and PIN0 causing the 12V to get shorted to ground.

I don't see where the music is being used to control the lights anywhere.

To control whether the lights have power or not you have to interrupt the power signal. Shorting it to ground is the worst possible idea. Don't worry, I have seen it before, and done by people that should know better.

Control the power using a P-channel MOSFET between the 12V and the LED strip. Control the P-channel MOSFET using a logic level N-channel MOSFET from a separate pin on the MCU.

Then decide how you are going to interface the MCU to the incoming music...


You are trying to make a high side switch... not a good place for NPN's to be honest. And a High Side switch should have no connection to GND as you have done... since done your way... the part must conduct a near short circuit... and it will release the smoke.

Find a PNP power transistor and create a High side switch, since what you assembled is just wrong. A P channel mosfet would also work as a High Side switch.

High Side Switch

high-side load switche
What is all that stuff. The OP wants to turn on the transistor, and turn it off. Don't matter high side, low side, left or righr side. A NPN transistor with the emitter connected to ground is a workable device.