Odd application of neon glow lamps
The dimmer
Neon glow lamps can also be used only for their U/I characteristic and not for
producing light. These lamps are electrically very close to diacs: no current can
flow through them since the voltage is lower than a trigger value (about 70 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 50 V for neon glow lamps and
about 30 V for diacs).
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 40 V to about 70 V. Some adjustments to R and C values may
be required to match specific glow lamps and triacs.
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 reaches the trigger value of the glow lamp the
SCR discharges this capacitor through a trigger transformer into 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.
The energy in the electrolytic capacitors can be lethal even when the circuit is
switched off. Always discharge these capacitors before touching the circuit.
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 these 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.
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
required) and the bulb will light in proximity of the antenna.
Connecting a small dipole to the lamps terminal dramatically increases its
sensitivity.
Since this circuit is very simple is usually necessary to readjust often the trimmer
to compensate the battery that is discharging itself.
