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Neon glow bulb (ne-2) arcs/flashes when powered

JBPisgah

Mar 11, 2016
2
Joined
Mar 11, 2016
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Ne-2 glow bulbs are flashing bright purple when I first plug in, frying the bulb and sometimes the resistor, too. I've tried three resistors (47, 100, 150), and and two different types of glow bulb with the same results. So , too much current I assume, but I don't understand why this is happening. Any ideas? Thanks!

Facts: Tried 150, 100, 47 all with same results. Resistors from Radio Shack, 1/2 watt. To test I've just twisted the wires to a lamp cord and plugged in to standard household outlet. The wiring in the original equipment (old clocks) just connects straight to the mains. Bulb is ne-2, with mfg. recommended 150 ohm resistor. http://www.allspectrum.com/store/neon-lamp-ne-6x20-30mm-leads-p-145.html
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
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you probably want a 150,000 ohm resistor (150kΩ)

330kΩ is appropriate for 240VAC, and I assume you have 110VAC mains.
 

Alec_t

Jul 7, 2015
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Agree with steve. That 'k' is very important ;).
 

Minder

Apr 24, 2015
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Doesn't the spec sheet show 3ma?
Assume the Neon to be zero ohms.
M.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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I don't read Chinese, but the at the end of the top line of the two columns labeled SERIES RESISTOR, indicating appropriate resistance values to wire in series with NE-2 type lamps, probably is a clue.

@Minder is right: assume small neon lamps have zero ohms resistance (they actually exhibit negative resistance, but we won't go there). They also have (roughly) sixty-five volts drop across the neon plasma inside when lit, so take away sixty-five from the rms line voltage (nominally 115 VAC in the USA) leaving fifty volts to "drop" across the series resistor. The rated current is 0.3 mA = 0.0003 A. Using Ohm's Law (ancient but venerable tradition in electrical work) calculate the value for the series resistor as R = V / I = 50 / 0.0003 = 166666.66666666666666666666666667 Ω. You can safely round this down to 150000 Ω = 150 kΩ. Power dissipated in this resistor will be I² R = (0.0003)² (150000) = 0.0135 W. A teeny tiny 1/10 watt resistor will work just fine but larger wattage ratings (like 1/4 watt or even 1/2 watt) will be okay. It's the resistance value that counts and you need substantially more than 47, 100, or 150 ohms to limit the current drawn from your power line.

Many manufacturers soldered these physically small resistors to one lead of the NE-2 lamp and then covered both leads with insulation, hiding the resistor. Many novices have simply replaced a darkened NE-2 without knowing the resistor was even there, the result being a spectacular flash (maybe accompanied by exploding glass) when line power was subsequently applied after the "repair" was made. Don't ask me how I know about this or when I found out about it.

Good luck with your clock restorations... and welcome to Electronics Point!
 

JBPisgah

Mar 11, 2016
2
Joined
Mar 11, 2016
Messages
2
I don't read Chinese, but the at the end of the top line of the two columns labeled SERIES RESISTOR, indicating appropriate resistance values to wire in series with NE-2 type lamps, probably is a clue.

@Minder is right: assume small neon lamps have zero ohms resistance (they actually exhibit negative resistance, but we won't go there). They also have (roughly) sixty-five volts drop across the neon plasma inside when lit, so take away sixty-five from the rms line voltage (nominally 115 VAC in the USA) leaving fifty volts to "drop" across the series resistor. The rated current is 0.3 mA = 0.0003 A. Using Ohm's Law (ancient but venerable tradition in electrical work) calculate the value for the series resistor as R = V / I = 50 / 0.0003 = 166666.66666666666666666666666667 Ω. You can safely round this down to 150000 Ω = 150 kΩ. Power dissipated in this resistor will be I² R = (0.0003)² (150000) = 0.0135 W. A teeny tiny 1/10 watt resistor will work just fine but larger wattage ratings (like 1/4 watt or even 1/2 watt) will be okay. It's the resistance value that counts and you need substantially more than 47, 100, or 150 ohms to limit the current drawn from your power line.

Many manufacturers soldered these physically small resistors to one lead of the NE-2 lamp and then covered both leads with insulation, hiding the resistor. Many novices have simply replaced a darkened NE-2 without knowing the resistor was even there, the result being a spectacular flash (maybe accompanied by exploding glass) when line power was subsequently applied after the "repair" was made. Don't ask me how I know about this or when I found out about it.

Good luck with your clock restorations... and welcome to Electronics Point!
 
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