Can I use a capacitor balast for neon indicator bulb? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: November 09, 2022, 05:23:09 am
content: Why is it always a high resistance resistor used as a ballast for neon bulbs? wouldn't a small value capacitor do the same thing and be cheaper, and more reliable?

author: jonpaul
author_rank: Super Contributor
timestamp: November 09, 2022, 05:39:22 am
content: not good practice, the cap will need a safety rating and pass line transients .The neon lamps usually fail long before the tiny 1/2 w 100k resistorj

author: chess
author_rank: Newbie
timestamp: November 09, 2022, 05:57:16 am
content: resistor is more cheaper

author: Circlotron
author_rank: Super Contributor
timestamp: November 09, 2022, 06:01:50 am
content: If your mains supply is a perfect sine wave with no noise spikes and you always managed to turn on your neon lamp at the voltage zero crossing a capacitor might be okay. If either of the foregoing occur your neon will likely blow up or fail some other way because noise spikes and turn-on transients are high frequencies that essentially pass straight through the capacitor. The neon will try and clamp the voltage across itself to about 60V or so, similar to a zener diode.

author: David Hess
author_rank: Super Contributor
timestamp: November 09, 2022, 08:59:28 am
content: When the lamp turns on during each half cycle, its negative differential resistance characteristic will draw a large current spike through the capacitor.

author: wraper
author_rank: Unknown
timestamp: November 09, 2022, 09:11:55 am
content: Film capacitors in capacitive droppers are way less reliable than resistors which are not overloaded.

author: Zero999
author_rank: Super Contributor
timestamp: November 09, 2022, 01:12:55 pm
content: Quote from: David Hess on November 09, 2022, 08:59:28 amYes, this will shorten the life of the lamp and is why inductive ballasts were always used for gas discharge lamps such as fluorescent tubes.

author: David Hess
author_rank: Super Contributor
timestamp: November 10, 2022, 12:17:00 am
content: Quote from: Zero999 on November 09, 2022, 01:12:55 pmAt high frequency using a capacitor as a ballast works because the lamp never shuts off, but at 50 or 60 Hz the lamp, or at least a neon lamp, has to restart on every cycle.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 10, 2022, 01:00:16 am
content: Quote from: David Hess on November 10, 2022, 12:17:00 amThe current draw of a neon indicator is so low, that a resistor doesn't need to have much of a power rating --capacitors will almost always be more bulky.That said, I have used a capacitive dropper with an indicator lamp, but a LED,not a neon.It wasn't off raw Mains, either, but a 65v AC control voltage from a transmitter.

author: Circlotron
author_rank: Super Contributor
timestamp: November 10, 2022, 01:41:48 am
content: Could you use a xenon flash lamp in glow discharge mode, same as a neon? Mild source of continuous light, not bright like a flash?

author: Zero999
author_rank: Super Contributor
timestamp: November 10, 2022, 08:16:20 am
content: Quote from: vk6zgo on November 10, 2022, 01:00:16 amI've found some neon lamps light at such low currents, I can hold both wires and touch the outer sheath of a live wire with the glass bulb and it will flicker and glow dimly.Quote from: Circlotron on November 10, 2022, 01:41:48 amThe starting voltage of a strobe is very high, so it would need a step-up transformer to run off the mains.

author: David Hess
author_rank: Super Contributor
timestamp: November 11, 2022, 02:18:02 pm
content: Quote from: vk6zgo on November 10, 2022, 01:00:16 amThe resistor ends up dissipating about 50 milliwatts, but I always seem to end up using a 1/2 watt resistor for the extra voltage margin.I think I tried using a capacitor once and the lamp just exploded.