The Setup
I have an RGB LED controller that is mostly a black box to me (I assume it's using PWM to produce color transitions). It's connected to an assortment of common anode RGB LEDs (they're all tiny PCBs with the LED chip + a resistor connected to each cathode). They are all connected to the controller in parallel.
The Question
Why is it that when I take one of the LED modules and disconnect only the anode (positive) wire, it will continue to dimly glow?
My beginner-level brain can't figure out how that could ever be possible. What could be happening in the controller that could result in this? Some sort of induced current even though this is a DC circuit? Could it be related to the PWM signals produced by the controller? Magic?
Best Answer
LEDs, like all diodes, have reverse leakage current.
Because of this leakage, with the common anode left open, if one or more of the cathodes is at a positive voltage (that is, the 'off' state) the common anode will bias to some midpoint voltage, which will leak to the diodes that are in the 'on' (cathode grounded) state. This leakage can be enough to make the forward-biased LEDs glow dimly.
Try measuring the unconnected common anode with the cathodes in various states. You will see that it will be some voltage other than GND if one or more of the cathodes is at +V state. Bear in mind however that even measuring that point will change the behavior due to the impedance of the instrument.