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Universal Neon Voltage Indicator AC DC High Low Cold Cathode Glow Tube Application

OddMix.com - Technical Note - TN090306 - by Karl Nagy

Figure 1. High Voltage Universal Neon Glow Lamp Voltage Detector Circuit [3 KB]
Figure 1. High Voltage Universal Neon Glow Lamp Voltage Detector Circuit

A simple but often used application for a small neon tube is a voltage detector circuit. A simple resistor and neon glow tube in series forms the general-purpose arrangement on Figure 1. Resistor R1 is always required in almost any cold-cathode glow tube circuit to prevent the tube from destruction due to excessive current. The value of R1 is selected to limit the tube current to a few mA [miliamperes] while allowing useable brightness. Since voltage detectors made to work on a wide range of voltages, the changing brightness is a good, simple indicator.

Figure 2. Commercial Neon Glow Lamp Voltage Detector [5 KB]
Figure 2. Commercial Neon Glow Lamp Voltage Detector

This basic voltage indicator circuit is useable on both AC (Alternating Current) and DC (Direct Current) as long as the available voltage exceeds the tube's ionization voltage. The only difference that is useful to indicate the type of potential difference, is that if the voltage is AC than both electrodes will glow, with DC only one.

Figure 3. Neon Tube of Commercial Voltage Detector [4 KB]
Figure 3. Neon Tube of Commercial Voltage Detector

On Figure 2, there is a commercially available voltage detector probe made to operate within 80 to 250 volt. The device is looks like and made to work also as a regular screwdriver. The screwdriver body has a hollow, transparent handle, housing the glow tube and the current limiting resistor. The resistor is sufficiently high value to limit the current to so small amounts that it would be undetectable by the operator. The conduction path for the indicator's current is via the operator, whose body resistance is added to the bulbs current limiting resistance. When the tip of the screwdriver contact points that has a higher potential that is required for the neon bulb to ionize - usually 65 Volts and above - the lamp glows, indicating the presence of high voltage.

Figure 4. Low Voltage Universal Neon Glow Lamp Voltage Detector Circuit [3 KB]
Figure 4. Low Voltage Universal Neon Glow Lamp Voltage Detector Circuit

The neon tube for this indicator is very similar to the well known often used fuse in which the wire is enclosed in a glass tube. Instead of a wire, there enclosed two wire electrodes with a parallel arrangement on Figure 3. The ISO symbol is used for the neon tube in the two circuits on Figure 1 and 4. The two wire electrodes shown like a capacitor within a circle. The slanted lines represent the gas filler within the glass lamp device.

As described, when the voltage is below the tube's ionization potential, that is usually under 65 volt, detection with the circuit on Figure 1 is not possible. The circuit on Figure 4, small DC and AC voltage detection is possible in a special way.

The inductor L1 is parallel with the neon lamp NE1. When the wire terminals connected to the battery, or some other voltage source current will flow. The current flow builds up a magnetic field in the inductor L1. When the current is interrupted, the coil's self inductance would try to maintain the current flow and the voltage would increase sharply. The more abrupt is the interruption of the current flow the higher and narrower voltage pulse induced. When the induced voltage exceeds that of the ionization voltage of the neon lamp NE1, the tube will light for a short while. The indication is NOT given at the beginning when the probe touches the low voltage, but when it is removed from it.


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