TIP127 not turning off.

Hello All,

I am trying to build a prototype of scanning device for which i need to drive some high power leds and control Camera, and i am currently stuck at the very first and basic step, that is driving high power LED :frowning:

Goal :
To drive around 8 High power LEDs, each turning on one by one in sequence for brief amount of time ( 30 - 40 secs ).

Current setup:
I have followed the circuit from THIS tutorial PFA the schematic of my circuit.

LED in the schematic are EPISTAR 3W
LM2596 in the schematic is Variable voltage Step down power module

** Note : I am using 3.3K omhs resistor at the base on both transistors. instead of 2.2 as suggested in the tutorial since i dont have them handy right now.

I am running blink sketch on arduino.

Problem :
LED stays continously on and doesnt turn off.

Can anyone please let me know what am i missing or doing wrong ?

info on high side switching using TIP 127 seems to be very scars on internet, i wasnt able to find much except for a few ( 2-3 ) threads in here and the above tutorial.

I also noticed everyone suggesting either using TIP 120 or FET's to drive these leds, but for now i would like to go with TIP 127 since i have them laying around and getting TIP 120 would take quite some time ( It takes around 2-5 days to get the items at my place from online sellers. )

** Note : I struggling newbee in electronics and honestly sometimes a lot of things just go over top of my head :frowning:
So please bare with me
So i might

Forget about that Youtube video.
I watched it for a few minutes, and already noticed several 'mistakes'.

Dump the TIP127s. Scarse, because nobody wants to use them anymore.
Mosfets rule in this century.

Yes, you could use a buck converter, set to 5volt, to power the LEDs.

You could switch the LEDs low-side (in the ground line) with a TIP120.
~0.75volt loss across the "transistor switch" at that current, but that's acceptable in this case.
Use a 1k base resistor (not 2k2 or 3k3) for full saturation of the transistor.

You MUST use a current limiting resistor in series with that LED.
Volt drop across the CL resistor will be "5volt (supply) - 3.3volt (LED) - 0.75volt (TIP120)" = 0.95volt.
CL resistor value = 0.95V / 0.66A = 1.44ohm. Nearest value > 1.5ohm.
Power in resistor = 0.95 * 0.66 = 0.6watt (will get hot). Nearest > 1watt.

600-700mA LED current = 2watt (not 3watt as in the link), but better stick to that value.
Wise to NOT buy those LEDs, but the ones already mounted on a star base, for easier cooling (they get hot).

Post a diagram, showing the TIP120/LED/resistors, so we can check.

Some will probably say that you should use constant current drivers.
But a resistive dropper is cheaper/easier than eight CC LED drivers, and should be ok for the short time you're driving the LEDs.

thanks wawa for the details.

i was hoping i would be able to use the 127s :frowning:

i checked on stores and couldnt find 120s but i did find 122s, i havnt checked the datasheets for 120s and 122s yet ( as it is i dont i will be able to make out much from the datasheets coz i am still learning ), but i hope i can use the 122s instead ?

also it would be nice if you can suggest a good mosfet i can try to search for it as well

TIP120-122 are the same, just selected for max collector voltage (60, 80, 100volt respectively).
So yes, no problem to use the 122s instead.

You can use the 127s (PNP). You just have to add eight small signal NPN transistors as well (as in the video).
Makes your circuit more complicated.

Any LOGIC LEVEL n-channel mosfet that can switch at least 1Amp can be used.
They don't drop the 0.75volt calculated for the TIP120, so LED CL resistors have to change as well.
Mosfets don't really have an advantage in this circuit.

Ok I have ordered some TIP122s, will test them and post the details once i get them.

ok i got the 122s and tested with everything seems to be fine now,

also i got the 127s working, while wiring the 122s i relliazed i hadnt connected the grounds correctly :frowning:

So please bare with me
So i might

I might not.

Thanks for posting the result though.