Go Down

Topic: Transistors and Resistors (Read 1 time) previous topic - next topic

Finnius

Thanks fungus! Cleared up a few things.
I ran a formula and worked out i should use a 2.5k ohm resistor for the TIP120 running a 1W LED. Does this sound in the ballpark?

Bit wary of running a led straight off the power supply - so i saw some cheap 1w led drivers on ebay. They run on 12v. Do you think i could switch one of these with another resistor and an external power supply?

Thanks for the help

fungus


Thanks fungus! Cleared up a few things.
I ran a formula and worked out i should use a 2.5k ohm resistor for the TIP120 running a 1W LED. Does this sound in the ballpark?


What formula, what numbers?

2.5K will probably work with a Darlington but really, it's not critical. The two dangers are:
a) Resistor value too high - doesn't fully saturate the transistor
b) Resistor value too low - lets more than 40mA out of the Arduino pin

The gain of a transistor depends a lot on temperature, voltage, etc. so trying to get it 'perfect' will often cause problem (a).

You're not designing a NASA space probe so it's better to be 100%  sure the transistor is fully saturated.


Bit wary of running a led straight off the power supply - so i saw some cheap 1w led drivers on ebay. They run on 12v. Do you think i could switch one of these with another resistor and an external power supply?


A proper power supply is always good but why 12V? I assume the LED runs a 3.6V so dropping 12V->3.6V is a big leap (and creates more heat).

Can you use a 5V power supply? A nice little regulated 5V supply can power both the LED and the Arduino and the drop from 5V to 3.6V for the LED is more manageable.


What I would do,

a) Get a little supply like this one: http://www.ebay.com/itm/140729265471

(nb. I applied my rule of never running eBay stuff at maximum rating, 2A feels about right for this job)

b) Run your LED at 70-80% power with something like this: http://www.ebay.com/itm/310578065040  (nb. I don't know the exact value because I don't have your LED datasheet, but that's probably not far off).

Even better, step up to a 3W LED and run it at 60% power with a slightly lower value 10W resistor.

nb. 60% power usually means about 80% light - LEDs produce more heat than light when you get near their limits.

(all eBay items picked at random, I don't know the sellers)
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)


fungus


It doesn't have an operating voltage, i was just going to run it at 5v originally from the arduino 5v pin?


It says "DC forward voltage 1.4-1.7V"

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

Chagrin


This is my LED: http://www.ebay.com.au/itm/1W-Infrared-IR-940NM-High-Power-LED-Bead-Emitter-with-20mm-Star-Base-/221063642644?pt=LH_DefaultDomain_0&hash=item33786b7e14&_uhb=1#ht_4040wt_1163


As an aside, 5mm LEDs with 100ma forward current (e.g. TSAL5100) are quite a bit cheaper than those larger, star emitter types. You shouldn't have any problem finding those from local distributors and won't feel as bad burning up a ~$.25 part as compared to a $5 part.


Go Up