Confusion Re: QTR/QTRX Current Draw

Doc: 0J13/4 says this: “Driving a CTRL pin low for at least 1 ms turns off the associated emitter LEDs, while driving it high (or allowing the board to pull it high) turns on the emitters with the board’s default (full) current, which is 30 mA for QTR and QTRXL boards.”

product/4247 says this:
"* Full-brightness LED current: 30 mA (independent of supply voltage)

  • Max board current: 125 mA"

This is a 7 sensor board. How does the “LED current 30m” relate to "maximum board current: 125mA?

Is 30mA the required current for all 7 LED’s or each LED? If the max current for each, how come total current isn’t 7 x30 for 210 mA?

Thank you, Bob

Hello, Bob.

I am sorry for the confusion; the documentation does not currently describe how the LED current relates to the maximum board current very well (we are working on adding clarification). Essentially, the LED current is mostly intended to be used as a brightness specification; the emitters are generally driven in pairs, with the two emitters in each pair connected in series. So, the total board current is not the LED current times the number of LEDs as you might otherwise expect; it is usually closer to half that.

For the #4247 in particular, there are 7 emitters, which means there are 3 pairs in series and one extra, so the total LED current at full brightness is 3 × 30\text{mA} + 30\text{mA} =120\text{mA}, and the additional 5mA difference in our max board current spec accounts for the rest of the circuit.