[volt-nuts] Neon bulbs for HP 3420B or 419A chopper circuit

[Charles Steinmetz]

Pete wrote:

> I've seen 95V but nothing as high as 125V.
> The high brightness ones are usually 95V

Neon gas at the typical pressures used in small lamps ignites somewhere 
near 200vDC.  For this reason, commercial neon bulbs use a mixture of 
Neon and Argon, which lowers the strike voltage to the 70-100v region 
(again, depending on pressure).  By this means, the original NE-2 was 
designed to strike at around 70v (DC or peak AC), while the NE-2H was 
designed to strike at around 120v (DC or peak AC).

BUT:  You never know what the heck you might get from many sources 
today.  If you need a controlled strike voltage, the best bet is to buy 
NEW bulbs from a reputable manufacturer who provides specs. Most such 
bulbs have designations other than "NE-", although some are also 
identified with an original type on the datasheet.

Note also that common neon lamps (e.g., NE-2) depend on some ambient 
light to promote ionization due to the "dark effect."  This is 
particularly true for striking (many neons will not strike at all in 
total darkness), but affects continuing operation as well.  For this 
reason, some variants had radioactive material added inside the envelope 
to promote ionization.  These were often identified as "dark effect 
reduced" bulbs.  The NE-2U, NE-2J, and NE-2H5 were among these.

Originally, Thorium was used for this purpose, but before long Krypton 
85 became the standard additive.  Being a gas, it was much easier to 
incorporate.  NOTE that the half-life of 85Kr is less than 11 years. 
This explains most failures of neons in chopper and LDR applications today.

Radioactive enhancement is very important for chopper operation, since 
the chopper assembly necessarily keeps the lamps in total darkness.  It 
also means that you probably do not want to buy/use surplus neon bulbs 
to repair choppers and LDRs (for two reasons -- you never know if they 
really had radioactive enhancement to begin with, and if they are old 
enough that you can be sure, they were made at least two half-lives ago 
and probably more like four, so they are all pooped out before you even 
install them).

I managed to source some brand-new NE-2U equivalents in small quantities 
a few years ago, but I can't remember where I got them.  (No problem if 
you want 10k of them -- several manufacturers will be happy to oblige.)

Whatever you get, test them with a beta-sensitive detector to make sure 
they are reasonably active.  The indication should be unmistakably 
strong -- if it's a case of, "Well, I think there may be some 
indication," they will not do.

Best regards,

Charles