Blue halo arownd nixie digits

[ppTRN]

Hi everyone, I have a bunch of NL848 nixie tubes that I savaged from a very old multimeter. I also savaged the transformer and i manage to light one up. I noticed that around the digit there was a blue glowing that I do not see in other images online. Why is that?

Supply is 280V dc, taken form a 200V ac transformer output, rectified and filtered with a 100nF capacitor. Anode resistor is 68k.


See image attached.

Thanks for your infos!

[Gyro]

I believe the blue glow comes from other gasses included in the Nixie - the filling isn't pure Neon. In particular, there are traces of Mercury vapor. There are a couple of specific former Soviet tube models that don't include this* (most do) and have very short operational life as their digit cathodes tend to disintegrate and short!

* One of them being the IN-1 end view tube.

[retiredfeline]

Apparently a blue glow is a sympton of overcurrent but your current (under 2 mA) seems ok. Double check the resistor. Another suggestion is that you may have wired up another cathode instead of the anode to the HV by mistake.

[ppTRN]

Thanks for you answers. I checked the resistor and the voltage and everything seems to be fine, even the pinout has been double ckecked.
I read online about this effect, I find it to be really cool actually but if it really shorten the lifespan of the tube that much then I do not think it is wort it.

[Gyro]

In test instruments, the blue glow was taken out by a red (some tubes actually came with a red lacquer coating) or amber filter to improve clarity and contrast. As long as you stay within recommended [Ed: not maximum] datasheet values you will be fine.


Edit: You can see the blue Halo pre and post filter my old Advance TC12 images here...  https://www.eevblog.com/forum/reviews/lets-see-your-nixie-tube-equipment/msg992615/#msg992615  (It's a thread well worth reading for anyone interested in cool pictures of Nixies!).

[CaptDon]

Gyro is right on the money with his answer!! All of the older nixie displays had a red, orange or amber filter. The 'blue glow caused by overcurrent' is totally off base and don't know where anyone ever heard that? The blue glow is indeed from mercury as a gas added to the 'Long Life' variety of later production nixies. 2 ma. of current is well within spec for that particular tube. I have a display that has 6 nixies in a 1-of-6 multiplexing layout. The peak current driving the lit nixie is 9 ma. with the average current being 1.5 ma. That is the upper limit for good life with multiplexed nixies, the preferred layout is 1-of-4. The scan rate for my mux is around 50Hz for a complete scan cycle or 300Hz for the individual step rate. My display does not have a blanking interval between steps so in a totally dark room with the orange filter removed you can see 'ghosting' in the unlit digits. Under normal viewing conditions the ghosting is undetectable. Cheers mate!!! Nixies are my favorite display!! Sad they have a limited lifespan due to cathode sputtering. Lifespan is heavily related to peak and / or average current!!