I have some older Luxeon high power LEDs and bought some drivers (spec below) to power them. They work fine for what I need.
However, I am going to be controlling these using TIP120 circuits (one to one) from an Arduino and it occurs to me that I don't know if I should be controlling power flow to the driver or to the lamp itself. I'll be doing both on/off and PWM.
Might sound like (and be..) a dumb question but when I stopped to think of it I wondered about which is least stressing on the hardware, delayed start and/or hysteresis. Is there a best practice or rule of thumb here?
The drivers are no doubt constant-current drivers - this means their output terminal will rise to whatever voltage will yield that current through the LED or LED-string. This voltage is not very well defined (depends on power supply voltage, LEDs, temperature...)
So I'd suggest using the darlington transistor to switch power to the driver (low-side switching), since then we know the voltages will be what we want (emitter at ground, base via resistor to Arduino pin, collector to negative supply to driver...
These drivers don't necessarily react well to having their outputs switched off. They rely on the power consumption of the LED to stabilize the circuit -- you'll notice they frequently specify the number/wattage of the LEDs they are designed for.
I'd suggest digging around the DX site and reading the forum posts regarding dimming techniques for similar drivers.
