Electronic ballasts with 2-wire dimming

The company where I work has used several types of dimming electronic ballasts; mainly for florescent bulbs. They all seem to have about the same 3-modes of operation:

with control inputs (Vio+ / Gry-) open max bright shorted min dim

pot (

Your guess describes it pretty well - it's the 10V intensity-control standard, and many industrial lighting companies offer it. But one issue, one generally wants to run multiple fixtures from one control, and can do so by wiring them in parallel. This means the control has deal with a wide range of sourcing currents, and yet set a 0 to 10V clamp voltage as a function of its pot rotation. IIRC, clamping to below 0.5 or 1V shuts off the lights, so the control circuit does have a little bit of voltage headroom to operate from. I imagine it's an interesting little circuit.

This control input must be electrically isolated from the ballast so using a small control transformer, the primary is driven by switching frequency and current limited, maybe 100KHz, The secondary is half wave rectified and controlled by the user causing the primary to limit the positive swing at the secondary control voltage. This primary limit voltage then controls the current in the lamp. Higher limit voltage, higher lamp current and more intensity. At least this is one implementation that I have done many years ago IMMSMC. YMMV Harry