Neon bulbs, overcurrent failure mode?

Thread Starter

Hamlet

Joined Jun 10, 2015
398
I've now blown a few neon bulbs, overcurrent I suppose (100v DC, 330 ohms).

I expected them to rapidly darken, but not blow. Since they're not opaque yet,
and there is no filiment to break, why are they no workie-workie?
 

MisterBill2

Joined Jan 23, 2018
7,998
Consider that the only material inside the bulb is the neon gas and the two electrodes, it must be that the darkening is due to metal vapor condensing on the glass, which is a bit cooler. And also the metal vapor condenses between the electrodes so that no longer can enough voltage to fire the bulbs be developed. THAT is most of the reason why they no longer light. Check the voltage across the terminals and it will not be high enough to light .
 

crutschow

Joined Mar 14, 2008
26,417
Small neon bulbs typically take only a few mA.
You put way too much current through them with a 330 ohm resistor.
The resistor should probably be at least 30k ohm.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
398
YEs, I know that. I was attempting a sub for a diac. Neons trigger at 100v, and I wanted enough current to switch a triac.
 

MisterBill2

Joined Jan 23, 2018
7,998
It used to be that there were two classes of devices, sensitive gate and non-sensitive gate. I am not sure just where the division was, but certainly some are much less sensitive. That is where data sheets come in handy.
 

crutschow

Joined Mar 14, 2008
26,417
Typically, in a dimmer circuit, the voltage to the DIAC is removed as soon as the TRIAC fires, so you likely could get by with a smaller resistor in the actual circuit using a neon bulb, since it would be seeing just a short current pulse.

I assume you were applying a steady 100v through the 330Ω to the neon directly for your test(?).
 

Thread Starter

Hamlet

Joined Jun 10, 2015
398
Typically, in a dimmer circuit, the voltage to the DIAC is removed as soon as the TRIAC fires, so you likely could get by with a smaller resistor in the actual circuit using a neon bulb, since it would be seeing just a short current pulse.

I assume you were applying a steady 100v through the 330Ω to the neon directly for your test(?).
Yes, steady current, distructive testing prior to applicaton. The actual switching is a very brief 10ms pulse, maybe much less, every 1-2 seconds, which I also tried, but without a resistor, 'cause it such a short pulse. Worked for a little while, then the neon bulb started requiring a greater voltage to fire, became unreliable.

I suppose my next test to renew the neon bulb and test with 47k ohms, and see if it is enough, decreasing resistance as necessary.
 

crutschow

Joined Mar 14, 2008
26,417
The actual switching is a very brief 10ms pulse, maybe much less
It should only be as long as it takes the TIRAC to fire.
a very brief 10ms pulse, maybe much less, every 1-2 seconds, which I also tried, but without a resistor, 'cause it such a short pulse
Without a resistor, you could be putting amps through the neon bulb.
It has a very low resistance when on.
Use a value of resistance that will give you the TRIAC trigger current you need.

Once as a kid, I was playing with a small neon lamp and decided to connect it directly across the 120Vac (why not?).
There was a bright flash of light and then nothing, rather like the old camera flashbulb.
I ended up with a blackened bulb with the two electrodes rattling around loose inside the bulb.
 

schmitt trigger

Joined Jul 12, 2010
390
Did the same in my childhood Cruts, but in my case the bulb did not explode, but shattered in my fingers.

I remember my mother pulling glass shards from my bloody fingers.
 

crutschow

Joined Mar 14, 2008
26,417
Since they're not opaque yet,
and there is no filiment to break, why are they no workie-workie?
I don't believe your question was answered.
It likely stopped working because the high current vaporized some of electrode metal which contaminated the neon, making it no longer conducting.
 

MaxHeadRoom

Joined Jul 18, 2013
22,376
There were more that a few devices that operated with neon gas technology, the Nixie tube, Decatron, voltage regulator tube, etc.
If you can find details on these devices, it should offer some more clues on Neon operation. ;)
Max.
 

Bernard

Joined Aug 7, 2008
5,743
In one case a capacitor, .5 uF , is charged from 150 V DC via a 1M R, & dumped thru a NE2H directly into the gate of a C37B SCR in the process of flashing a 1kW lamp.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
398
5k ohm current limiting resistor in series with the neon bulb does not supply enough current to trigger the triac.
It will light an LED however. I am going to try using a neon in series with an opto-triac, to trigger the main current handling triac.
 

MisterBill2

Joined Jan 23, 2018
7,998
Note that the Neon lamp firing voltage can display odd sensitivity to ambient light- expect the unexpected.
This is certainly true! That is because the external light supplies additional energy towards ionizing the neon gas. The classic example is the old neao night light that goes off in the dark, but lights when you shine a flashlight on it.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
398
Note that the Neon lamp firing voltage can display odd sensitivity to ambient light- expect the unexpected.
Dang! As I was doing an extended test, I turned my back, flipped-off the shop lights, and took dinner. Came back, peered-in on how things were going while the main overhead shop lights were still off, and was disappointed by the erratic operation.

I need to paint the neon bulb black, re-calibrate, and test again...
 
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