The simplest application is the main voltage monitor that is simply a
lamp that glows when the main voltage is present.
To obtain such a monitor it's enough to connect a resistor in series with the bulb and connect at a main outlet. The resistance of that resistor may vary on the type of bulb and the main voltage but it's not critical: about 150 KΩ for 220..230 Vac and about 39 KΩ for 110..120 Vac.
It's very important to choose a resistor that can support the desired voltage or connect two or more usual resistors in series. The power of that resistor is quite low and a 0.25 W is usually enough.
To light a neon glow lamp a relatively high voltage is necessary (about 70 V) and this make the realization of a telephone ring monitor quite easy: the voltage of the telephone line is usually less than 50 V and it's not enough to start the lamp. When the phone rings an AC voltage of about 150 Vac is present on the line and the bulb will glow. The low current consumption of glow lamps is very useful in this case since it doesn't overload the phone line.
The circuit is very simple and can be installed into a telephone plug.
With only a neon glow lamp and two resistors it's possible to build a useful
tool that checks if the earth wire of an AC outlet is correctly connected.
In facts the neutral wire and the earth wire are usually connected together in the transformer and this connection short circuits the lamp that cannot glow. If the earth wire is not connected or if the live and neutral wires are inverted the full voltage would be present on the terminals of the lamp that would glow indicating the presence of a problem.
This circuit is very simple and can be mounted into an AC plug as shown in the
This circuit can be adapted to 110 Vac mains by reducing the two resistors to about 33 KΩ.
To improve the above circuit two more neon lamps are needed as well as some
zener diodes. The circuit is a little bit more complex.
Usually the neutral and the earth are at the same electrical potential and the two resistors of 150 KΩ are in parallel making all three lamps glow. If a connection is missing or is inverted the resistors will form a voltage divider that reduces the voltage and makes one or more lamps dark. The two anti-series zener diodes are there to avoid that the lamps glows with about the half of the main voltage.
Lowering the resistance of the resistors and the voltage of the zener diodes this circuit can be adapted to 110 Vac.
This circuit is a little to complex to fit in a plug but it can easily fit in a
small plastic box.
The meaning of the three laps follows:
|Lamp 1||Lamp 2||Lamp 3||Meaning|
|ON||off||off||Live - Neutral inverted|
|off||off||ON||Live - Earth inverted|
Important remark: This circuit only works correctly if the neutral is grounded at the transformer (TN-S or TN-CS systems).
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