So i'm coming out of my Arduino pin thru a 1k resister feeding the base of a PN 2N2222 I'm using a constant V of 3.2 feeding my setup. Working with 1W led. If I power the led straight from my 3.2v it will pull around 200 mah. If i feed thru the transistor it will only pull 140 to 150 mah.
By far i'm no expert on electronics. Is my 1k resister not pulling the 2n222 to full saturation?
What LED is this? Datasheet? How are you limiting current to the LED?
Even in saturation, a bipolar transistor drops voltage. The greater the current, the greater the voltage drop.
In any case, with a 2N2222, you should drive the base with 1/10th the desired collector current to drive it to saturation. This is according to the datasheet. You are only driving it with about 4mA (subtract 0.6V BE drop, and the Arduino will output less than 5V when loaded).
So for 20mA, consider 4V/0.02A = 200 ohms.
https://www.fairchildsemi.com/datasheets/PN/PN2222A.pdf
VCE is rated at 0.3V at 150mA ICE, and 1V at 500mA ICE.
If the transistor is not overheating, it's probably saturated. You can try putting another 1k resistor in parallel with the existing resistor and if the current doesn't change or the LED doesn't get brighter, the transistor is in saturation.
LEDs are not constant-voltage devices, they are constant-current devices. That means you should use a constant-current source, or approximate a constant-current source with a higher voltage and a current-limiting resistor.
With standard LEDs we normally use a current limiting resistor and we don't worry about the fact that more power is wasted in the resistor than is going to the LED. With 1W or higher LEDs, it's more common to use an efficient constant-current switching power supply.
But... As long as the 3.2V is holding-up, you are probably "safe" with the circuit you have because 3.2V @ 200mA is only 640mW. With the additional voltage drop across the transistor (and the associated lower current) you are running the 1W LED at less than 1/2W.
LEDs are non-linear so a small voltage drop across the transistor can cause a larger drop in current. And, and increase in voltage can cause a dramatically large increase in current.
P.S.
FYI - mAh or milliamp hours is a rating for battery life. A battery rated at 100mAh would run for about an hour at 100mA or about 10 hours at 10mA, etc. Current is measured in Amps (or milliamps or microamps).
If I power the led straight from my 3.2v it will pull around 200 mah.
NO!
Never use an LED without some way of controlling the current, controlling the voltage only IS NOT AN OPTION.
If i feed thru the transistor it will only pull 140 to 150 mah.
This is because there is always some saturation voltage dropped across a transistor and so you are not putting the same voltage across the LED.
Sounds like he's using a 1W LED that requires 3.25-3.3v to get the rated 1W.
Any switching device will have some drop across it. A MOSFET will be better than a BJT, but it will still have some.
You might have better luck if you got a LED rated at 3W; most (but not all - and you can just forget about getting datasheets for affordable high power LEDs) of these will pass more current at a given voltage than the 1W versions.
You know you need to heatsink the 1W LED right?
I'd run it off ~4v and use an AMC7135 for current limiting, switching the negative side with a MOSFET.
controlling the current That is an area the i'm lost. my circuit needs to be as small and light as i can get it. I will be running 8 of the above transistor setups above on one board.
controlling the current That is an area the i'm lost.
Then do something about it. Otherwise you will never get it working.
Here is a simple constant current supply circuit
Can I use any transistors in the above setup? Also Led Current = .7 over R Is that true for all transistors or different according to the transistor
cubmanky:
So i'm coming out of my Arduino pin thru a 1k resister feeding the base of a PN 2N2222 I'm using a constant V of 3.2 feeding my setup. Working with 1W led. If I power the led straight from my 3.2v it will pull around 200 mah. If i feed thru the transistor it will only pull 140 to 150 mah.By far i'm no expert on electronics. Is my 1k resister not pulling the 2n222 to full saturation?
Did you think to try something like 220 instead? If it makes a difference then clearly 1k
was too high.
But then again perhaps you need some sort of current limiting anyway...
RE: Reply#7 constant current source:
Working with 1W led
PR = 1 Watt (Given, by OP, since PR = PLED)
VR = 0.7 V (Given by circuit design)
IR = PR / VR = 1 W/0.7 V = 1.428 A
(since P = I*V > I = P/V )
ICollectorBC547 = 100 mA = 0.1 A
1.4238 A > 0.1 A
Conclusion: The BC547 transistor can not handle the 1 W needed by the OP's LED.
Easy thing for me to do is up my V to 3.5V then Then add a 1 Ohm resistor in line to limit the current to 300mA
raschemmel:
Conclusion: The BC547 transistor can not handle the 1 W needed by the OP's LED.
Depends on "+V supply", what's not across the LED and R will be across VCE.
cubmanky:
Easy thing for me to do is up my V to 3.5V then Then add a 1 Ohm resistor in line to limit the current to 300mA
The road to Hell is paved with good intentions.
What about Mike's circuit do you dislike? "Too many" parts?
Depends on "+V supply", what's not across the LED and R will be across VCE.
1 -I don't know the forward current for the led, only the power dissipation.
2- How much current does a 1 Watt led draw ? ( I don't know)
My calculations were based on ILED = IR
If that's wrong, then fine. Show how to calculate the current through R ? (for a 1 Watt LED)
I don't mind being corrected. I wouldn't post it if I did. You and I both know that posting anything on the forum is like jumping into pool full of sharks waiting for a feeding frenzy so it's best to dot your i's and cross your t's. Be my guest. Correct my calculations and show what the current would be. Frankly,I don't think we have enough information.
It seems to me, my mistake was here:
PR = 1 Watt (Given, by OP, since PR = PLED)
since PR is based on VR and we don't know VLED, so we really can't know ILED, we don't really know what PR is. We really only know PLED and without VLED and ILED ,we can't know IR.
And as far as:
The road to Hell is paved with good intentions.
see you in Hell.... ;D
What about Mike's circuit do you dislike? "Too many" parts?
Did I say I didn't like the circuit design ?
I'm under the impression the hell and dislike comments are directed to cubmanky.
Oops !
Oh well...I guess it's too late now.
Little harm done. I doubt Runaway Pancake will be too bent out of shape (I assume that shape is a perfect circle. Bwahaha!)
cubmanky:
Also Led Current = .7 over R Is that true for all transistors or different according to the transistor
Sufficiently true for your purpose for any silicon bipolar transistor.
Russell.
cubmanky:
Can I use any transistors in the above setup? Also Led Current = .7 over R Is that true for all transistors or different according to the transistor
Minor differences but generally in the range of 0.65 to 0.7V.
raschemmel:
How much current does a 1 Watt led draw ? ( I don't know)
We are generally talking about a white LED with a running voltage of 3.3 to 3.5V. Which for one watt means 280 to 300 mA. To control it, you are going to need a transistor with a dissipation of at least a watt - just to make sure. A BD135-BD137 sounds about right.
So, 0.7V, 300 mA, resistor value is 2.3 ohms (2.2 will do). Now, note that if you can saturate the transistor, VCE will be in the order of 0.3V, so this circuit requires a minimum drop of 1V, so you need a 4.5V supply. 5V will be pretty much "spot on" for this.
The 3.3K resistor needs to be more like 220 ohms for this application. (You may then wonder what the original circuit would be useful for? With the original transistors and 3.3k, it would be appropriate for driving a number of LEDs in series at 20 or 30 mA from a 12V or 24V supply.)
Grumpy_Mike:
Then do something about it. Otherwise you will never get it working.Here is a simple constant current supply circuit
So I throw the above setup in my schematic, Using a pn22222 would I not now have .2v drop across the transistors?
More info for the job.
input 3.3v at 3A
LED is a 1w VF is 3.0 to 3.6 at 350mA
I would like to run the LED around 200 to 250 mA
Also using the Arduino Pro Mini 3.3v version
