Simple transistor issue
I am working on a project for my robotics team where I need to drive six green, 1-Watt Luxeon LEDs (rated at 350mA). I am using 4 LD1117V33 3.3V regulators (800mA each), and a 2N2222 transistor (TO-92 package) for each LED. I have 1k-ohm resistors going from the PWM outputs of the microcontroller I am using, which has a total absolute maximum current output of 40mA, to the bases of the 2N2222s. Two of the 3.3v regulators are driving two LEDs in parallel each, and the other two regulators are handling one each.
When I bring the outputs to the 2N2222s HIGH, the LEDs light at maximum brightness for about .75 sec and then go very dim. What am I doing wrong? One more note: I forgot to add 10-ohm, 1-watt resistors from each LED's anode to the output of the 3.3v regulators. I have confirmed the microcontroller is not browning out, so it probably lies within the transistors? I think this has something to do with the Hfe values or something...I am not well-versed in transistor calculations so please, I welcome any help from an EE or two!
Thanks![smile.gif](http://forums.parallax.com/images/smilies/smile.gif)
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Giggly Googley!
When I bring the outputs to the 2N2222s HIGH, the LEDs light at maximum brightness for about .75 sec and then go very dim. What am I doing wrong? One more note: I forgot to add 10-ohm, 1-watt resistors from each LED's anode to the output of the 3.3v regulators. I have confirmed the microcontroller is not browning out, so it probably lies within the transistors? I think this has something to do with the Hfe values or something...I am not well-versed in transistor calculations so please, I welcome any help from an EE or two!
Thanks
![smile.gif](http://forums.parallax.com/images/smilies/smile.gif)
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Giggly Googley!
Comments
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Giggly Googley!
What do you mean? Two LEDs to one regulator? If you're talking about connecting the outputs of the regulators I have not connected the Vout pins in any way.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Giggly Googley!
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Giggly Googley!
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Giggly Googley!
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Giggly Googley!
http://www.rason.org/Projects/transwit/transwit.htm
If possible, I would use PNPs, not NPNs, as microcontrollers can sink more current than they can source.
Also, have you tried 5V regulators? I'm not sure what the voltage drop is for those LEDs you're using, but you're cutting it close with the transistor voltage drop plus the LED drop.
And....Here's a helpful troubleshooting tip: Don't try to debug five things at once. Remove 3 of the LEDs and start there.
Mike
If the basic circuit is right, I'm with Mike as to the regulators going into shutdown.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Tom Sisk
http://www.siskconsult.com
·
Also, upand_at_them:
I have measured 2.7-2.8V across each LED as the voltage drop.
What do you mean, "cutting it close"? What could this do?
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Giggly Googley!
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Giggly Googley!
Get the datasheet for the LED (SparkFun has it). Lookup the forward voltage for green. Get the datasheet for the transistor (Jameco, DigiKey, tons of places have it). Lookup the voltage drop. Add the two values together and see if it's less than the voltage regulator output.
Mike
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Giggly Googley!
The current gain (hFE) is usually given as a minimum and is the amount of current flow through the collector (collector to emitter) for a given amount of current through the base (base to emitter). It's usually very variable from transistor to transistor, batch to batch, and manufacturer to manufacturer. Circuit designs take this variability into account. For a switching transistor like this one, you want a minimum since you're trying to saturate the transistor so it's completely conducting no matter how much collector current is being conducted (or off) and the collector to emitter voltage is at a minimum (typically 0.6V in this case)
You're not going to draw too much voltage from the regulators because they have a fixed voltage output (3.3V). If the various voltage drops add up to more than 3.3V, something else will change, typically the amount of current drawn will drop as the LEDs begin to stop conducting.
Post Edited (Mike Green) : 1/26/2010 8:39:43 PM GMT
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Giggly Googley!
Mike: Thanks for the explanation. Disregard my last 2 posts. At this point, unfortunately, I have soldered these components onto a PCB. What do you suggest I change to alleviate this problem, so I can get the maximum light output from these LEDs? The voltage regulators should be changed to 5V and the appropriate power (1W+) resistors added?
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Giggly Googley!
Post Edited (NoBo780) : 1/26/2010 8:47:00 PM GMT
The datasheets for the LED and transistor probably have graphs giving the forward voltage and Vce in relationship to current. That's what you'll use to figure out the resistor value.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Giggly Googley!