I have designed a dimmer controller using an output op amp from TI : OPA211. And I have some questions.
Below is the explanation how the controller work.
Every ballast is a current source that feeds 0.125-0.300 mA current. The controller is a current sink that sinks this current so that the voltage on the line drops to the wanted control voltage level.
For 0-10V Ballasts, a dimmer is required, which is connected to the control terminal of a ballast (Violet, Grey) I have a figure to send to you if the explanation is unclear to you.
The dimming interface (between our controller and the output stage of a ballast) is a power supply and a current limit resistor R. The controller should be equivalent or similar to an adjustable Zener ( not a resistor).
So it is always required that a dimmer controller sinks current. The design below makes the understanding easier. In most cases, a dimmer is just a passive circuit powered from ballast. For our control card which is self powered, it also need to be current sink. Otherwise we won't get correct control voltage. The picture below is a just equivalent circuit. Please keep in mind that the current limit resistor and Vcc is variable from ballast to ballast.
This current from the ballast flows into the output transistor stage of our opamp. Depending on the level the op amp is set up on one of his inputs, it will open its output transistor more or less, sinking the current and setting a voltage across Violet to Grey between 0 to 10V.
The ballast will sense this voltage and provide a correspondent current to lamps. 0 V or below 0.5V between violet and grey terminals means minimum dimming level usually 5% of full brightness. 10V or higher than 9V means 100% of full brightness. For ex: when a Ballast provides lower current to lamps and keep this current constant, the lamp voltage is slight higher, the lamp power (lamp current multiply lamp Voltage) is lower, then the brightness is lower.
Now, if two such of controllers work in parallel to the same one ballast, you will notice the following behavior:
If one of the controllers is set up for 3V output, the other one can get at the output up to 3V maximum, even you try to control him from minimum to maximum. So his output is limited by the value set from the other controller.
So, it is like you have two op amp in parallel, with the outputs tied. When one sinks current and the output voltage between grey and violet drops to 3V, the other one can vary its output between 0V and 3V.
Do you know why? What limits the output range of the second?
