9
\$\begingroup\$

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?

\$\endgroup\$
1
  • 5
    \$\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
    Commented Mar 22, 2020 at 11:41

3 Answers 3

Reset to default
22
\$\begingroup\$

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.

\$\endgroup\$
8
  • 1
    \$\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
    Commented Mar 22, 2020 at 11:36
  • 5
    \$\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
    Commented Mar 22, 2020 at 11:41
  • 3
    \$\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
    Commented Mar 22, 2020 at 11:46
  • 2
    \$\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
    Commented Mar 22, 2020 at 11:46
  • 1
    \$\begingroup\$ I did already, yesterday. See the last line of my answer. \$\endgroup\$
    – Transistor
    Commented Mar 23, 2020 at 13:46
14
\$\begingroup\$

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)?

\$\endgroup\$
1
  • \$\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
    Commented Mar 23, 2020 at 23:16
3
\$\begingroup\$

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

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

\$\endgroup\$
0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.