Which voltage range does the filament circuit use, and what controls it?

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The filament circuit is all about heating the tube’s filament to produce electrons. It uses a small heater voltage to warm the filament, and the amount of heating current—and thus how many electrons are emitted—is controlled by the mA setting on the control panel. This is separate from the high voltage that accelerates electrons across the tube, which is governed by kVp, and from the exposure time, which just determines how long the tube is on. So the filament circuit operates at a low heater voltage, with its output set by the mA control.