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Fig 10. Ballast Waveforms at Maximum Brightness

Trace

Color

Signal

1

Yellow

Half Bridge Voltage – VS

2

Red

Feedback Voltage at RLS (and DS1 Cathode)

3

Blue

Lamp Arc Current

4

Green

Lamp Voltage

A

Orange

Lamp Power (3 multiplied by 4)

The values of R18 and R19 have been selected so that when the opto-isolator IC4 is completely

off the voltage at the input of the error amplifier, at RDIM connection with the wiper of VR1, will be 1.5 V.

This will set the error amplifier input voltage to 1.5 V when the 0 to 10 V dimming control voltage is a

10 V and maximum output is required. This effectively limits the maximum lamp power because the error

amplifier will regulate the frequency in order to maintain equal voltage at RLS to the input voltage at

RDIM. The run frequency will therefore be above the ignition frequency.

The minimum dimming level can be set by R19 and adjusted by RV1, which prevents the opto-

isolator transistor from pulling the error amplifier input voltage all the way to zero. If the opto-isolator were

on continuously the input voltage to the error amplifier would be so low that the lamp would extinguish or

enter an unstable state of partial ignition. Tests have indicated that the T5 lamp behaves differently when

cold and hot at very low light levels. When the lamp is cold the arc tends to become unstable as the lamp

repeatedly extinguishes and re-ignites, however when the lamp is hot it is possible to dim all the way down

to virtually zero output and a state can be observed where the lamp is in a stable state of semi ignition,

characterized by an faint emission of light along only part of the length of the tube.

In a practical design it is desirable to limit the minimum output level to the minimum current at

which the lamp can maintain stable operation when cold.