Holdover can occur when the current is supplied to the discharge tube due to varying conditions of output voltage and output resistance of the DC power supply. What are the conditions that allow current to continue to flow to the discharge tube?
The relation between the power supply voltage (Vo), serial resistance (R), discharge current (I) and the terminal voltage are shown in the linear relation below:

\[ v = Vo - I \cdot R ・・・(1)\]

If voltage Vo is fixed, the slope of the power supply output characteristic line increases or decreases according to the resistance and may or may not intersect with the V-I characteristics of the discharge tube. The characteristic linear line of a power supply shows the relation between the output voltage and current of the power supply. Likewise, the V-I curve of a discharge tube shows the relation 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 characteristics are very important.
As shown in the figure 12, with a high resistance in the power supply, the output characteristic line and the discharge tubes V-I characteristic curve never intersect. Therefore, current will not flow from the power supply and follow on current will not occur.
However, when the output characteristic line of the power supply intersects with the V-I curve of the discharge tube, 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 where 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.
The figure 12 below shows how the power supply continues supplying the current to the discharge tube when its characteristic line intersects the discharge tubes V-I line in the glow or arc discharge sections.
To prevent holdover from occurring, it is important to keep the V-I characteristic line of the power supply from intersecting with the V-I curve of the discharge tube.