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I am new to neon lamps:

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I just discovered its electronic symbol:

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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:

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

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    \$\begingroup\$ 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? \$\endgroup\$ – marcelm Mar 22 at 11:41
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Neon lamps are not polarised but only the more negative electrode will light up.

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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.

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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.

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    \$\begingroup\$ "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? \$\endgroup\$ – marcelm Mar 22 at 11:36
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    \$\begingroup\$ 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."? \$\endgroup\$ – Transistor Mar 22 at 11:41
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    \$\begingroup\$ 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."? \$\endgroup\$ – marcelm Mar 22 at 11:46
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    \$\begingroup\$ 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. \$\endgroup\$ – Hearth Mar 22 at 11:46
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    \$\begingroup\$ I did already, yesterday. See the last line of my answer. \$\endgroup\$ – Transistor Mar 23 at 13:46
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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.


1 See also Anyone recognise this capacitor and its symbol (circle with dot)?

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  • \$\begingroup\$ 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. \$\endgroup\$ – supercat Mar 23 at 23:16
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A neon lamp is not polarised. It may be connected either way.

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

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