transistor and current limiting resistor for LEDs on 12V

[T3STY]

I'm working on a UV exposure device that uses 320 UVA LEDs. The LEDs are connected as series of 4 LEDs, resulting in a total of 80 series connected in parallel. Each LED is rated 3V 30mA so I can power up the series using a 12V power supply, but...
1) I want to turn on and off the LEDs using a pin on a PIC, which doesn't have the necessary power supply to turn on more than a single LED. While I read on the web I found that I can use a PNP transistor with base connected to the PIC, Collector connected to the power supply and emitter connected to the device I want to power. Is that right?
2) I have already built a prototype board (although, that one uses only 40 series, half the one I intend to build) and I tried to power it on using a 12V 2A power supply: it didn't work as expected, the LEDs were turned on but they only had very low brightness instead of the full brightness they should have. The power supply has a voltage regulator so I tried to give a bit more. When I reached 12.8V the LEDs were having a proper brightness, but at that voltage the LEDs were slowly heating.
Did I calculated right that a series of 4 LEDs , each LED rated at 3V 30mA, can be properly supplied with 12V? If that's right, why didn't LEDs turn on properly at 12.0V ?
3) The current limiting resistor... which value should I give the resistor so I am able to turn on the 80 series at the proper brightness?
I made some tests using different resistor values on the same 12.8V power supply that turned on the 80 series, but even the lowest resistor value (47 Ohm) would not let the LEDs turn on at maximum brightness.. actually they only have half brightness.

 

[crutschow]

You connection is not correct. A PNP must have a negative collector to emitter voltage for proper operation.

If you want to drive a grounded load, then you would need an NPN, (PIC to resistor to base, emitter to ground), which drives a PNP (emitter to the plus supply, base to the collector of the NPN, collector the the LED anodes, LED cathodes to ground through a suitable resistor).

If you can drive a high side load then you just need an NPN with the PIC (resistor to base, emitter to ground, collector to LED cathodes, LED anodes to the plus supply through a suitable resistor).

You must drive LEDs with a current limiting resistor, you can damage them if you don't. If you want to drive 4 LEDs in series with each having a 3V drop (is that their spec?) at 30mA then you could use a 15V supply with a (15V -12V) / 30mA) = 100Ω in series with each set of 4 LEDs.

 

[T3STY]

Thank you for the answer!
I got it about the transistor. I understood I need a NPN transistor as the PIC pin will output logic 1 (near 3.3V) when I want to turn the LEDs on.

About the current limiting resistor, did I correctly understood that I actually need a higher power supply and a resistor for each series? What if I simply use the same 12V DC power supply to power only 3 LEDs (with the same 100 Ohm resistor) ? I actually don't want to buy a 15V transformer, I already have one for 12V.

 

[crutschow]

.........................

About the current limiting resistor, did I correctly understood that I actually need a higher power supply and a resistor for each series? What if I simply use the same 12V DC power supply to power only 3 LEDs (with the same 100 Ohm resistor) ? I actually don't want to buy a 15V transformer, I already have one for 12V.

Yes, you need some way to limit the current as LEDs are diodes and basically current operated.

And you could use 12V to power 3 LEDs in series with the same 100 ohm resistor.

 

[T3STY]

Thank you very much for help!

I still wonder though, why the 40 series of 4 leds didn't light up at maximum brightness when I powered them with a 12V power supply, but they did when I used a 12.8V power supply... maybe the LEDs are rated 3.2V? The seller said they're rated 3.0V @ 30mA but it would make sense for them to be rated 3.2V, it would explain why they light up right on 12.8V... but then they were heating up - is that the result of the basic diode that uses as much current as possible?

 

[crutschow]

Thank you very much for help!

I still wonder though, why the 40 series of 4 leds didn't light up at maximum brightness when I powered them with a 12V power supply, but they did when I used a 12.8V power supply... maybe the LEDs are rated 3.2V? The seller said they're rated 3.0V @ 30mA but it would make sense for them to be rated 3.2V, it would explain why they light up right on 12.8V... but then they were heating up - is that the result of the basic diode that uses as much current as possible?

Yes, a diode has a very low dynamic impedance so a small change in voltage can make a very large change in current. That's why you need a series resistor (or a constant current source) to drive an LED.

 

[T3STY]

Thank you very much! Now more things make sense (like, why the amperometer I used was changing value so fast when I was changing voltage..).