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Odd application of neon glow lamps

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The dimmer

Neon glow lamps can also be used only for their U/I characteristic and not for producing light. This lamps are electrically very close to a diac: no current can flow through them since the voltage is lower than a trigger value (about 80 V for neon glow lamps and about 40 V for diacs). If the voltage is high enough to trigger the device a current can flow through it and the voltage drop is a little lower than the trigger voltage (about 60 V for neon glow lamps and about 30 V for diacs).

Diagram of the dimmer

In this circuit of a dimmer the diac is replaced with a neon glow lamp, but it works exactly in the same way than a classic one: an RC network delays the trigger pulses on the gate of the triac. The glow lamp rises the trigger voltage from about 1 V to about 80 V. Some adjustments to R and C values can be required to match the specific glow lamp and the specific triac.

Picture of the dimmer

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The stroboscope

A stroboscope is another application that requires a diac (or a neon glow lamp) for triggering the xenon tube: this circuit uses a neon glow lamp but it works in the same way as a classic diac one. The main voltage is rectified by a diode and charges two electrolytic high voltage capacitors (in parallel) that are used as an energy tank for the xenon lamp. A current also flows though a resistor and slowly charges a trigger capacitor: when the voltages reach the trigger value of the glow lamp the SCR discharges this capacitor through a trigger transformer to the xenon lamp that flashes. Once the energy in the tank is over this cycle repeats; the frequency is adjusted by varying the resistance that charges the trigger capacitor.

Diagram of the stroboscope

The energy in the electrolytic capacitors can be lethal even when the circuit is switched off. Always discharge these capacitors before touching the circuit.

Picture of the stroboscope (internal view) Picture of the stroboscope (external view)

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An RF field monitor

This is the oddest application of a neon glow lamp, that is used as a electromagnetic field detector. In fact the trigger voltage of this lamps is a little bit lower in presence of a strong field.
A UJT transistor and a transformer (a common low power AC transformer) are used to produce a high voltage of about 200 Vac. This voltage is reduced by the trimmer to a value just below the trigger voltage of the lamp. In presence of a strong field the trigger voltage drops and the lamp lights.

Diagram of the RF monitor

To use this monitor just set the trimmer to 0 V, power the circuit on (when no field is present) and slowly turn the trimmer to rise the voltage until the glow lamp lights. Turn now the trimmer in the other direction and stop as soon as the lamp turns off: now the monitor is ready and a strong RF field will light the bulb. For testing you can try a cellular phone or a transmitter (some Watts of power are needed) and the bulb will light in proximity of the antenna.
Since this circuit is very simple is usually necessary to readjust often the trimmer to compensate the battery that is discharging itself.

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