For a bird camera project, I would like to power 4 infrared LEDs from a Raspberry Pi as the light source for the Pi's NoIR camera.
The IR LEDs (TSHF6410) have a forward voltage of 1.4V and a maximum forward current of 100mA.
I came up with two different circuits: one with all 4 LEDs in parallel, and one with two strings of two LEDs in series.
Which circuit would be the best choice?
In the first circuit, with all LEDs in parallel:
- LED current: 77mA = (5V - 1.4V) / 47Ω
- Transistor current: 308mA = 4 * 77mA
In the second circuit, with all LEDs in parallel:
- LED current: 100mA = (5V - 2 * 1.4V) / 22Ω
- Transistor current: 200mA = 2 * 100mA
For the LED current, the second circuit is exactly at the maximum rating for the IR LED. Would that be a problem? Do I risk damaging the LED?
For the transistor current, both circuits are within the limits of the 2N5551 transistor (600mA). For the second circuit a 2N3904 transistor (200mA), might work as well - or is this again too close to the limits?
Does the higher current of the first circuit also mean it consumes more power? So in that regard the second circuit would be the best choice? Are there any other advantages/disadvantages?
PS: I'm not sure whether it matters, but I intend to control the brightness of the LEDs via PWM.