# Neon lamp symbol

I am new to neon lamps:

I just discovered its electronic symbol:

I am a little bit confused, What does the dot in the symbol mean? It looks like the neon lamp is polarized. How do I know what lead of the neon lamp corresponds to the dot?

I have seen other circuit schematics where they place the dot at the center:

Where do I place the dot, then? At the center or at one extreme?

• According to this answer, the dot doesn't indicate polarity, but indicates that the bulb is filled with a special gas. Does that answer your question? – marcelm Mar 22 '20 at 11:41

## 3 Answers

Neon lamps are not polarised but only the more negative electrode will light up.

Figure 1. Only the cathode lights up. +DC (left), -DC (center), AC (right) supplied to NE-2 type neon lamps. Source: Wikipedia Neon lamp.

For an AC supply the anode and cathode swap on each half-cycle of the AC supply so connection polarity doesn't matter.

Figure 2. NE-2 type neon lamp powered by alternating current (AC). Source: Wikipedia.

The linked article may explain more.

Where do I place the dot, then? At the center or at one extreme?

Either location of the dot should convey the meaning that it is a gas-filled lamp.

• "Yes, neon lamps are polarised ..." - I don't think that's true; it would mean the neon lights are asymmetrical, while as far as I know such indicator bulbs are symmetrical. If they were polarised, it should matter which way around they are connected to DC. Can you provide a reference that this is the case? – marcelm Mar 22 '20 at 11:36
• Maybe I can phrase it better. Figure 1 shows what happens when connected to DC. Only the cathode lights up and I provided the reference. How about, "Yes, neon lamps are polarised in that only the cathode lights up. Their construction is symmetrical however and either side can act as cathode."? – Transistor Mar 22 '20 at 11:41
• I see what you mean, but I still don't think the components themselves are polarised... Perhaps something like "Neon lamps themselves are not polarised, but only the negative-connected terminal will light up."? – marcelm Mar 22 '20 at 11:46
• Whether you consider it polarized or not (which I don't), the dot is not to indicate polarity anyway--it's the symbol for a gas-filled (as opposed to vacuum) tube. – Hearth Mar 22 '20 at 11:46
• I did already, yesterday. See the last line of my answer. – Transistor Mar 23 '20 at 13:46

The dot denotes a gas-filled tube, which distinguishes it from a normal vacuum tube. Neon bulbs and voltage regulator tubes1, as well as certain types of high-power tubes such as thyratrons and mercury-vapor rectifiers fall into this category.

The neon bulb is not polarized; it is symmetrical.

• Neon bulbs may typically be designed symmetrically, but that doesn't necessarily imply that they're not polarized. Video youtube.com/watch?v=vrmwql2msbU about a neon-opto-isolator-based meter demonstrates how some tubes can be sensitive to insertion direction in some circuits. – supercat Mar 23 '20 at 23:16

A neon lamp is not polarised. It may be connected either way.

The dot signifies that it's a gas-filled lamp.