11) Follow-on current from AC sources


For a DC power supply, follow-on current occurring in gas discharge tubes for AC sources is easy to understand.
As in figure 12 the only difference is that the power supply voltage (V0) changes with time.
As shown on the previous page, when the power supply voltage is shown as V0(t), the output power is displayed as follows:
With “v” being the voltage at the power out terminal, and “I” the current of the circuit:


v=V0(t)-RI ... (2)


V0(t) varies with time so it appears in the shaded area of the graph. When V0(t) is shown as:


V0(t)=V0sinωt ... (3)


When the power supply voltage becomes 0 (zero cross), there is a short time where the voltage and time range of the power supply output and discharge tube V-I curve do not intersect.
For an AC power supply, because there is always a zero crossing of the supply's voltage, it is easier to stop the discharge than for DC holdover. Near the zero crossing it is impossible to maintain the discharge since the current to the discharge is cut off. The discharge is halted by the ionized gas molecules returning to their normal gas state.
Because the terminal voltage does not exceed the DC breakdown voltage, when the discharge is halted it cannot start again.
However, if the gas molecules remain ionized during this period and voltage is again applied to both terminals of the discharge tube (enters the cycle of opposite voltage), this newly applied voltage will not allow the discharge to end and it will continue in the discharge mode. This is follow-on current for alternating current.
When this type of follow-on current occurs the tube stays in discharge and the glass of the tube may begin to smoke, melt, and possibly ignite.

 

 

 

 

 

 

With 1Ω and 3Ω resistance, results are the same as those in photo 2, as follow-on current is disrupted and the discharge is stopped.
For AC power sources, the resistance value that is connected in series with the discharge tube is small in comparison to DC sources.
If the series resistance is 0.5Ω or greater, it should be sufficient; but for safety a value of 3Ω (for 100V) or greater is recommended. A varistor that acts as a resistor is also recommended and it must have an operating voltage greater than the AC voltage and be placed in series with the discharge tube. Unlike the resistor, discharge will be stopped without follow-on current occurring during the first half of the wave.


Choosing varistor voltage:


For 100 - 220VAC minimum
For 200 - 470VAC minimum


Our DSANR and DSAZR series are made for power supplies and are designed to prevent follow-on current.


At risk applications of holdover or follow-on current:


1) Holdover---Circuits with DC power sources
2) Follow-on---Circuits with AC power sources

 

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