Why i am killing my tip120 ?

Try connecting the wire connected to the Arduino D3 pin to ground instead of the Arduino pin. Does the LED still light?

@cjdelphi: That is not what the datasheet shows. Pin 1 is the base.

cjdelphi:
you're not blowing them, they work still no doubt, collect D3 of Arduino to the Base Pin (middle/2 pin), Emitter to GND, Collector to your LED's GND (short lead) and the Anode + side goes straight to your 5v rail.

According to the datasheet, the base pin is the pin 1, right ?

Pin 1 is the base. Did you try the test?

SurferTim:
Pin 1 is the base. Did you try the test?

Yes, and the led is still light

ah yes sorry my bad, I looked at the Darlington first, then i went back to a similar datasheet i had open and looked, then i went back and realized.

take a look at the attachment....

confusion.png

ah yes, my amazing circuit design skills :smiley:

fair enough, so what resistors you using? to connect the transistor? i'd use a 1k for the base, and then 300 ohms to limit the LED current, also while you're at it, move it from 3v to 5v, it's possible if you're not
using a red led, it will be dimly lit, even less so with the voltage drop through the transistor.

Sorry but i don't understand your schematics :astonished:

^^ so just swap to a 5v source and let us know the resistor values.

I put a 1K resistor between D3 and the base pin, also put a 200 ohms resistor for the led

Why do i need to change the voltage to 5v ? 3,3v is not correct ?

What, where's the voltage in (5v supply directly from USB) pin? i see 2 2v pins, surely not 1 is Vin? :fearful:

different LED's require different voltages, Red being the brightest with the least (something to do with the band gap, wavelengths and things I never understood)

White for example will light dimly from a 3v cell coin, but a Red will shine really brightly... so if you used a white led, combine it with a voltage drop 3.3v, you're at the limit's a 5v would be better that's all.

Are you certain that is a TIP120 (NPN), and not a TIP125 (PNP)? Under these conditions, the TIP125 would keep the LED lit no matter the voltage on the base. Look at the polarity of the collector-emitter diode in the product schematic. Left is NPN, right is PNP.

Just checking...

Oh crap! they are labeled TIP 135 :astonished: But i ordered some TIP120!! The box is labeled TIP120!

That is the problem. It is being lit by the collector-emitter diode. That is why you are seeing 0,5v collector-emitter.

Thanks for the help, next time i'll check everything :grin:

Something does not sound right. It looks like it should work.

For that TIP120, the VCE @ IC=20ma would be about 0.5V. So your measurement of that value is fine, but it indicates the darlington is on, as does the fact that the LED is on.

The diagram looks right but I'd change some of the values. Since the gain is so high (more than 1000 for a TIP120), that 1K resistor on the base could/should be more like 100K for turning on an LED. Also, the LED resistor should be about 75ohms given the that supply voltage is 3.3V. Try those values.

Also, have you verified that the signal from D3 is actually going high and low? If it is, then check over your wiring again.

Another thing you could try, since the TIP120 is huge overkill for this situation, is a simple little NPN transistor like a 2N3904 or 2N2222. if you try the little transistor, use 4K7 for the base resistor and still use 75ohms for the LED resistor.

Take a look on top, i had a problem with my supplier. I ordered some TIP 120 and i had some TIP 135 (PNP). The "funny" thing is that the box was labeled TIP120 but inside, there was some TIP135, and i didn't check it... Was my fault :disappointed_relieved:

I know that the TIP120 is much more than necessary in this case, but the led was here only for a test purpose. In the end i'll put 2 led lights for an amount of 40W :smiley:

...i had some TIP 135 (PNP).

Yes, I see that now. I don't know why, but the 2nd page did not show up when I read this thread the first time. Maybe a few packets were dropped on their way to me.