Electronic – Multiple infrared LEDs in parallel or series

infraredledraspberry pitransistors

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.

Schematic of the two circuits

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.

Best Answer

Series is always the best, you waste less power on the resistors, your BJT needs to handle less current.

Also the forward voltage of LEDs may vary quite a bit, which will make some LED draw more current than others and generate different brightness. This will not happen when in series as they will all share the same current. Brightness is directly correlated to the current, not the voltage.

If you can (have a higher voltage rail than 5V if it's battery-powered), put everything in serie.

Higher voltage is always easier to handle than higher current (within reason).