Radiography at ECC: The X-ray Circuit

The x-ray circuit is composed of two major secondary regions or circuits:
1. the filament circuit (outlined in red on the diagram below), and
2. the high voltage or high tension circuit (outlined in blue)

The filament circuit gets its power supply directly from the autotransformer and supplies current to the filament of the x-ray tube. The major components of the filament circuit are:
1. the mA selector
2. the focal spot selector
3. the step-down transformer

The Filament circuit is switched on when the prepare button is pressed. This sends current through the filament wire causing thermionic emission which produces a space charge. Prepping for too long before each exposure will put undue stress on the filament wire. So keep your prep time optimal (just right--which usually means short).
The mA selector functions as a rheostat (which is a variable resistor--you adjust the resistance to vary the amount of current flowing in the filament circuit). The amount of filament current determines how hot the filament wire gets. (More current--more heat---more electrons boiled off the filament wire--higher mA).

The focal spot selector is a switch that allows current to flow through one filament wire at a time. When you select the large focal spot the filament current flows through the longer filament wire, and when you select small focus the filament current is switched to flow through the small filament wire.

The step-down transformer steps down the voltage in the filament circuit. In the diagram to the left, there are two sets of windings or turns in the step down transformer (in yellow). There are more windings or turns on the upper part of the transformer (the primary side), than on the lower side (the secondary).

The step-down transformer steps down the voltage, but steps up the current. Therefore, in the filament circuit we get low voltage and high current.

Remember, the filament current has to be high to ensure adequate thermionic emission. This is how we control the quantity of electrons boiled off at the filament.

The mA selector and focal spot size selector are located in the primary portion of the step-down transformer. (see diagram above)

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The high voltage portion of the x-ray circuit (shown in blue in diagram 1 above), controls several important aspects of x-ray production. In the high voltage circuit you will find components such as:
1. the pre-reading kV meter--that indicates the kilovoltage that will be applied to the x-ray tube once the x-ray exposure starts (the potential difference between cathode and anode)

2. the exposure switch that closes to start the x-ray exposure and opens to end the exposure. The operator (you) start the exposure, but the amount of time the circuit stays closed is determined by the timer. (So you start it, but you don't end it).

3. the timer controls the amount of time the x-ray exposure lasts.

This is the sequence of events of an x-ray exposure: Once you press the exposure button the high voltage circuit is closed. This means that a high potential difference is set up between the cathode and anode. The electron cloud (space charge) that was produced during the prep phase of the exposure is pulled across form the cathode to the anode based on electrostatic attraction. (negative electrons attracted to positive anode).

The electron flow from cathode to anode is called tube current. As long as tube current is flowing x-rays are being produced.

The tube current will flow as long as the high voltage circuit is closed.

4. the step up transformer--steps up the voltage by a fixed ratio. If the turns ratio (number of secondary windings divided by number of primary windings--Ns/Np) is 1000 the voltage in the secondary portion of the transformer will be 1000 times the primary voltage.

The region highlighted in yellow below is the primary of the step-up transformer.