# How a Neon Light Bulb works?

#### HighVoltage!

Joined Apr 28, 2014
142
I do not want to get to scientific, but more on electric technical side. Does it conduct at certain voltage (short). Why do some have resistors (To limit current?)?

#### BobTPH

Joined Jun 5, 2013
2,353
A neon bulb does not conduct at all, up to a certain voltage. Then the gas in the tube ionizes and it starts conducting. At that point, it pretty much looks like a short circuit, so the current must be limited by a resistor or it will blow up. Once the gas is ionized, it will conduct at a lower voltage than was initially needed. This is called negative resistance, since an increase in current was accompanied by a decrease in voltage across the bulb.

Bob

Joined Jul 18, 2013
20,473
Does it conduct at certain voltage (short). Why do some have resistors (To limit current?)?
Does it conduct at certain voltage (short). Yes
Why do some have resistors (To limit current?)? yes.
Neon tubes were used for counter indicators, voltage regulators, and firing tubes for mercury pool switches, (Ignitrons).
Max.

#### crutschow

Joined Mar 14, 2008
24,754
Because of the negative resistance you can make a simple relaxation oscillator with just a neon bulb, a resistor, and a capacitor, that will cause the bulb to blink.

#### oz93666

Joined Sep 7, 2010
737
Can be run on AC or DC , start to glow at 90V

Big Neon signs are just long gas filled tubes with electrodes at each end which require many 10's of KV

#### HighVoltage!

Joined Apr 28, 2014
142
A neon bulb does not conduct at all, up to a certain voltage. Then the gas in the tube ionizes and it starts conducting. At that point, it pretty much looks like a short circuit, so the current must be limited by a resistor or it will blow up. Once the gas is ionized, it will conduct at a lower voltage than was initially needed. This is called negative resistance, since an increase in current was accompanied by a decrease in voltage across the bulb.

Bob
I have recently experimented with one and applied 120VAC only to one side without completing the "loop"; The bulb was "dim".

The bulb "glows" (shorts) when circuit completed.

Why was bulb "dim" when only one side was energized?

#### dl324

Joined Mar 30, 2015
10,541
I have recently experimented with one and applied 120VAC only to one side without completing the "loop"; The bulb was "dim".
What was the other side connected to?

#### Alec_t

Joined Sep 17, 2013
11,172
without completing the "loop"
The loop was completed, by leakage current through its surroundings.

#### HighVoltage!

Joined Apr 28, 2014
142

#### HighVoltage!

Joined Apr 28, 2014
142
The loop was completed, by leakage current through its surroundings.
This is what I figured.

#### dl324

Joined Mar 30, 2015
10,541
This is what I figured.
One year people in one of my electronics labs entertained themselves by holding a neon bulb and inserting the other lead in a wall socket and watching them glow.

Joined Jul 18, 2013
20,473
One year people in one of my electronics labs entertained themselves by holding a neon bulb and inserting the other lead in a wall socket and watching them glow.
Basically the way that Neon AC power detection screwdrivers work.
Operates on body 'leakage' to earth ground.
Max.

#### Alec_t

Joined Sep 17, 2013
11,172
One year people in one of my electronics labs entertained themselves by holding a neon bulb and inserting the other lead in a wall socket and watching them glow.
What colour did those lab people glow?