You may notice how the indicator light in your mains sockets and switches dims with time and wonder why. These lights are usually small neon bulbs which glow when a small alternating current of 1 or 2mA passes through them. These bulbs are filled with neon gas which glows red when a current passes through it. There is no filament and no heating element. Neon is an inert gas which glows with an orangey red colour in these bulbs. If the current is too high or the electrodes are made of an unsuitable metal the metal evaporates from the surface of the electrodes and condenses on the inside of the glass bulb, making it opaque, so the light output dims as the glass gets covered.
When this happens the only solution is to replace the bulb. They only cost 30p and can last many years, even when they are on all the time, unless, as above, too much current flows through them.
Right: Another use for these neons – as a night-light (cover removed). This is a light the size of a mains plug with two neons in it which have a translucent cover and shine on the floor area near my bathroom. They light the way at night when all other lights are off.
These particular ones were replaced some years ago and have not gone black again. The reason they haven’t gone black is because I changed the resistor (increasing it to 100kΩ). Sorry I can’t remember its previous value. Now the current is 1.15mA.
Now I’ll get back to my original problem: changing the bulb in a 45A 250V Isolating Switch.
Because only qualified persons are allowed to work on Mains Wiring I will not discuss the removal or replacement of a switch like this within the Mains Wiring of any premises in this post. All my discussion relates to work on a bench under workshop conditions. All individuals must make provision for their own safety and not do anything that may endanger the safety of others.
Isolation Switch Neon Indicator Replacement Process
Right: Here are the innards of the Mains Isolation Switch on the bench. On the left is the neon indicator under a red translucent cover and in the centre is the rocker switch. Observing the plastic cover will reveal how it clips in place. Note how it holds the neon in situ.
Right: The neon glowing dimly with the cover removed. Compare it with the brightness after the repair at The Testing Stage further down the page.
Right: I have turned the switch over so you can see how the wires from the neon are inserted into the back of the switch (the ones on the left covered in yellow sleeving). The bare wire just pushes into the holes adjacent to the screw terminals for the main wires. In the holes the wires are held by brass springs pressing them against the sides of the holes. Consequently the neon’s wires can be pulled out to release it.
Right: The blackened neon with the sleeving removed from the wires exposing the current limiting resistor. This is already a high enough value at 270kΩ limiting the current to around 0.7mA. (I tested the value of the resistor with a ohmeter.) The voltage drop across these neons is about 60V leaving 180V across the resistor. From Ohm’s Law (I=V/R) the current through the neon and resistor is 180/270,000. This equals 0.66mA.
Going back to my Introduction to this post I have to ask, “Why has this neon gone black?” Since the current is sufficiently low. Perhaps the electrodes were made from an unsuitable metal.
Right: The old neon is clamped in a small vise with rubber glued to the jaws to give a soft grip. This is ideal for holding the neon while I unsolder the resistor from one leg and the extension wire from the other. The extension wire is used to make the other leg as long as the one with the resistor.
Reassembly with the new neon was the reverse of this procedure.
Right: The Testing Stage. Compare the brightness with the previous picture of the neon glowing dimly.
Right: The Mains Isolation Switch Neon Indicator replaced and the switch back in use with a nice bright light for all to see.