10) Mechanism of holdover

Holdover can occur when the current is supplied to the discharge tube because of varying output voltage and output resistance of the DC power supply.
For current to continue to flow to the discharge tube refer to below, where the relation between the power supply voltage (V0), serial resistance (R), discharge current (I) and the terminal voltage are shown in this linear relation:

v=V0-I.R ...(1)

If voltage V0 is fixed, the slope of the power supply output increases or decreases according to the resistance and may or may not intersect with the V-I of the discharge tube. The power supply shows the linear relation between the output voltage and power supply current. Likewise, the V-I curve of a discharge tube shows the relationship between the voltage and the current.
When static surge electricity is applied to the discharge tube, the shape of the curve shows that the surge is being absorbed during arc discharge.
As the surge ends, the discharge goes from arc discharge to glow discharge and then to a state just prior to glow discharge. At this time, the relationship between the discharge tubes V-I curve and the power supply's output is very important.
As shown in figure 12, with a high resistance in the power supply the output (pink line) and the discharge tubes V-I curve (red) never intersect, so current will not flow from the power supply and follow-on current will not occur.
When the output of the power supply (pink) intersects with the V-I curve of the discharge tube (red), it is possible for the current from the power supply to flow into the discharge tube. When the surge ends, the current should decrease from arc discharge to the pre-glow state, but instead, the power supply will continue to flow when it intersects in the glow or arc discharge region. This condition where the power supply continues to allow current into the discharge tube is called holdover.
Figure 12 shows how the power supply continues supplying current to the discharge tube when its line intersects the discharge tubes V-I line in the glow or arc discharge regions.
To prevent holdover from occurring, it is important to keep the V-I line of the power supply from intersecting with the V-I curve of the discharge tube.





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