Dimming CCFL:s with PWM or amplified analog voltage

Question

First post here. I have been trying to get this project going for a while, but I now think it is time to ask for help.

I want to build an array of cold cathode fluorescent lights (CCFL:s). The brightness of each of the lights should be individually controllable with a digital signal (serial or DMX), to be able to control them like a screen with tubes as pixels.

So far, I have been successful in dimming the lights with a 16-bit DMX to PWM-interface, but the solution is quite bulky. I am modulating the 12V DC signal that goes into the CCFL inverter. I want to know how I can improve this solution by considering different options.

1. Normal DMX with 8 bit resolution does not create a nice enough effect with the dimmer I am using (it is in fact meant for dimming LEDs). The different steps are too noticeable. I have so far been able to get a nicer effect by using 16 bit PWM-drivers, but the desired dimming effect occurs just over a small portion of the resolution. Could I get the desired effect from a PWM-dimmer that uses a different dimming “range”?
2. Instead of using PWM, I have been thinking about dimming the lights with an analog signal between 0-12V, as is done in this video. There are two ways that I have thought of that could make analog dimming possible. One is staying with DMX and getting DMX to analog converters, or getting serial DACs to provide the desired voltage dimming. The problem in both of these cases are the amperage limit of conventional DACs and DMX to Analog-converters. The converters usually can withstand about 20mA, and the CCFL:s require about 150mA each to be fully powered. What components or circuits can I use that can supply an analog voltage signal at this amperage level?

While reading up on the above I have come in contact with electrical components called “emitter-followers” and “op-amps”, but I would be happy to be pointed to some resources where I can read up on specifically how to use them for this kind of applications.

Answer 1

1.Normal DMX with 8 bit resolution does not create a nice enough effect with the dimmer I am using (it is in fact meant for dimming LEDs). The different steps are too noticeable. I have so far been able to get a nicer effect by using 16 bit PWM-drivers, but the desired dimming effect occurs just over a small portion of the resolution. Could I get the desired effect from a PWM-dimmer that uses a different dimming “range”?

The reason the range of steps is apparently small for the dimming range is that the response of the human eye to light intensity is non-linear.

2. To get this amount of drive current, you could use an op-amp with an emitter follower inside the feedback loop:

^{simulate this circuit – Schematic created using CircuitLab}

The specific components are not what I would use (it will take me a little while to decide the specifics), but in the circuit above, you drive the V dimming control node from your DAC (or some other voltage source); the output voltage will be the same as the input voltage as this configuration is known as a voltage follower (provided you keep within the parameters of the datasheet such as the common mode range and Vout vs. V+ (at the opamp power pin) to name but two.

The transistor within the feedback loop provides a higher current path to your CCFL for dimming than the opamp itself could support.

I would suggest a new question if you need help with choosing components as more information will be required (the voltage range of dimming, power rail(s) available at least).

Answer 2

Control of CCFLs is not nearly as obvious as you think. Unlike LEDs, driving CCFLs is not remotely straightforward, and doing so by varying the inverter input voltage suggests that you are applying a completely inappropriate mental model to the problem.

I suggest you read Jim Williams' classic and exhaustive treatment of the subject.