Response to quick rise time of static electricity is good.
Same quick response in dark or light
High Insulation Resistance: Greater than 108Ω
Low Capacitance: Less than 1pF
No polarity
About Follow-On-Current
1. What is follow-on-current?
Follow-on-current is literally something that will continue to flow; in this case it is the phenomenon where current in a discharge tube continues to flow.
Normally surge absorbers are in a state of high impedance. When a surge enters the absorber it will drop to a low impedance stage, allowing the surge to bypass the electronic circuit it is protecting. After the surge has passed, the absorber should return to a high impedance condition.
However, when the absorber is in a low impedance state, if there is sufficient voltage on the line to keep current flowing, even when the surge ends the absorber remains in a discharge state and does not return to a high impedance state. In this case we see current continuing to flow.
This is the phenomenon known as follow-on-current.
Surge absorbers that display this follow-on-current phenomenon are discharge type and semiconductor switching absorbers. A characteristic of these absorbers is that during surge absorption (bypass) the operating voltage (remaining voltage) is lower than the starting voltage.
The advantage of this is that during suppression the voltage is held very low, so as to reduce stress on the equipment being protected, but there can be a problem when the line current of the equipment is sufficient that it continues to drive the absorber when the voltage is at a low state.
Below then we will go into detail about the follow-on-current mechanism. The discharge and power source ■ Characteristics for the discharge tube as well as conditions of follow-on-current will be explained.
2. What are the V-I properties of discharge tubes?
The micro-gap type surge absorber is one kind of discharge tube. The discharge ■ Characteristics where the part passes through pre-discharge, glow discharge and then arc discharge are shown in the illustration below.
This figure shows the V-I ■ Characteristics relation between voltage and current for the discharge tube. When the tube discharges, electric current flows and if moves to a glow discharge state and then an arc state all while the discharge voltage decreases. Conversely, as the discharge decreases, the voltage increases as it moves from an arc state to a glow state.
Pre-glow discharge
The voltage that is maintaining this discharge is approximately equal to the DC breakdown voltage of the part. A faint light can be seen from the part at this point.
Glow discharge
There is a constant voltage rate versus the changing current. The voltage maintaining this discharge will depend on the electrode material and the gas in the tube. The discharge light now covers one of the electrodes.
Arc discharge
With this discharge, a large current flows through the part and it puts out a bright light. The maintaining voltage at this point (voltage between the discharge tube terminals) is in the 10’s of volts range.
3. What is holdover?
When a discharge tube is used on a circuit that has a DC voltage component, there is a phenomenon where the discharge state in the tube continues being driven by the current from the power supply even after the surge voltage has subsided.
When holdover occurs, for example when it is occurs in the drive circuit of a CRT, the screen darkens and discharge in the absorber continues, which can lead to the glass tube melting, smoking or burning.
Holdover can occur when the current can be supplied to the discharge tube due to varying conditions of output voltage and output resistance of the DC power supply. What then are the conditions that allow current to continue to flow to the discharge tube?
The relation between the power supply voltage (V0), serial resistance (R), discharge current (I) and the terminal voltage are shown in the linear relation below:
If voltage V0 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 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 the 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, with a high resistance in the power supply, the output characteristic line (pink) and the discharge tubes V-I characteristic curve (red) 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 (pink) intersects with the V-I curve of the discharge tube (red), it is possible for 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 drive the where it intersects in the glow or arc discharge region. This is called holdover, and is the condition where the power supply continues to supply current to the discharge tube at the intersection on its characteristic line and the discharge tubes V-I curve.
The figure below shows where the power supply can continue supplying current to the discharge tube when it’s 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.
4. Follow-on-current from AC sources?
When using the discharge tube for AC sources, when follow-on-current occurs as per the case with DC it is easy to understand.
That is, as can be seen in the figure below, 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 ■ Characteristics are displayed as follows:
With v being the voltage at the power out terminal, and I the current of the circuit…
V0(t) will vary with time, so when displaying the above equation on a graph, it will appear as in the figure on the left below. Then when V0(t) is shown as:
When the power supply voltage becomes 0 (zero cross), there is a short time around this crossing where the voltage range 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, more than holdover it is easier to stop the discharge. In the vicinity of the zero crossing, it is impossible to maintain the discharge since the current to the discharge is cut off. The discharge is then halted by the fact that the gas molecules, which were ionized during this time, return to their normal state.
Because the terminal voltage does not exceed the direct current break down voltage, if the discharge is halted it will not be able to 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 follow-on-current occurs, the tube stays in a discharge mode and the glass of the tube will begin to smoke, melt and possibly ignite.
It is important to insert a resistance in series that is sufficiently large to prevent follow-on-current from occurring according to the conditions of the alternating current.
With 1Ω and 3Ω resistance, results are the same as those in picture 2, follow-on-current is disrupted and discharge stops.
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, however for safety a value of 3Ω (for 100V) or greater is recommended.
In addition there is also a method to use a varistor in series that acts as a resistor. In this case the varistor 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 wave.
Recommended varistor values:
For 100VAC lines Varistor voltage of 220V minimum
For 200VAC lines Varistor voltage of 470V minimum
Our DSANR and DSAZR series are made for power supplies and are designed to prevent follow-on-current.
5. Applications with a high risk of follow-on-current
1) Holdover
CRT circuits and circuits using DC power supplies
2) Follow-on-current
Circuits using AC power sources
|
