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I am new to neon lamps:

enter image description here

I just discovered its electronic symbol:

enter image description here

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:

enter image description here

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

JRE
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Dau
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    According to [this answer](https://electronics.stackexchange.com/questions/347930/anyone-recognise-this-capacitor-and-its-symbol-circle-with-dot), 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 Answers3

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

enter image description here

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.

enter image description here

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.

Transistor
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    _"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
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    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
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    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
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    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 think hearth's comment is worth including in the answer, as it explains why the dot is there (and indirectly, why its location isn't particularly important) – Doktor J Mar 23 '20 at 13:44
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    I did already, yesterday. See the last line of my answer. – Transistor Mar 23 '20 at 13:46
  • @Transistor Nixi-tubes however are polarised, as only their numbers should glow. How would they be drawn? just wondering? great answer btw. – Teck-freak Mar 24 '20 at 09:14
  • @Tech: [Lots of ways](https://www.google.com/search?tbm=isch&sxsrf=ALeKk03FtTTX1s61DLYrAU9uIAMNUclORQ%3A1585042263933&source=hp&biw=1827&bih=960&ei=V9N5XqWNNorikgXgz7HoBg&q=nixie+schematic&oq=nixie+schematic&gs_l=img.3..0i8i30l4j0i24.354.8605..8754...9.0..0.115.1826.19j4......0....1..gws-wiz-img.......35i39j0j0i5i30._FS8jU5uad4&ved=0ahUKEwjllN_I5rLoAhUKsaQKHeBnDG0Q4dUDCAY&uact=5) apparently. I imagine that the "anode" would glow if connected backwards. – Transistor Mar 24 '20 at 09:32
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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)?

Dave Tweed
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  • Neon bulbs may typically be designed symmetrically, but that doesn't necessarily imply that they're not polarized. Video https://www.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
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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.

vu2nan
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