I concur. Direct drive with PWM, as opposed to multiplexing. I suggest adding a light sensor to lower the brightness automatically
via PWM. This way, as the sun changes angles, or lights get turned off and on, you are not subjecting them to any more current then
they need and they remain bright enough for the ambient light.
PWM may also prevent metal whiskers from forming by denying a long lived and continuous magnetic field that they like to form in.
Tubes with mercury, such as the IN-18, have less of a problem with this. So even if you operate them in full brightness (which I
like to do because I like brightly lit rooms during the day), they should spend some amount of time in PWM. This is probably a
bigger risk for digits that rarely change.
I have seen a lot of clocks run their de-poisoning cycle at regular intervals throughout the day, such as 3 seconds every 15
minutes. I am not a huge fan of this for cosmetic reasons. I have thought about adding a motion sensor to hold the de-poisoning
cycle until no motion has been detected for an hour or so, and then after another hour of inactivity, shut off the display
entirely. A de-poisoning cycle can be lengthened to make up for the amount of time that it has been put on hold.
It makes no sense to run through digits that go through 0-9 all day, but cosmetically, the poison-prevention sequence looks better
if all of the digits are showing the same value. But it does make more sense to cycle though the ones that stay off the most, in
proportion to how long they stay off in relation to the other digits. And if you are changing the brightness/current throughout the
day, then it might make sense to account for that too in that ratio.
Even if the clocks only display 1 or 2 in the left hours digit, usually all of the other digits are wired up to support menu
functionality or make the poison-prevention sequence look better (by displaying the same digit on every tube). Your design may not
need this. One reason to keep all of the digits functional is if you want to re-purpose those tubes for another clock in the
future. It is possible that the tubes might outlive the clock or your desire for that clock design.
I agree about minimizing the current, but to use PWM on top of that. But I am not sure if the lowest usable strike(ionization)
and/or sustain voltage will be usable as the tubes age. I wonder if per tube and/or per cathode adjustment might be needed or
desirable. They use the same current regardless, so supply voltage and dropping resistance have to be modified in unison. It
sounds like it can get very complicated very quickly, and is probably why people just design to the spec and not add any feature to
adjust this. There is some discussion about constant current power supplies. One thing that I hate are clocks that change
brightness as the digits change. While more of a problem for multi-segment digits, a most significant digit turning on and off can
cause this in a single segment digit display.
The tubes might outlive the high voltage components. I wonder if there is any risk to the tubes from one of the switch mode power
supply's failure modes. This is slightly off topic, but this discussion is about keeping the tubes running for decades.
On 06/04/2015 07:38 PM, 'marta_kson' via neonixie-l wrote:
If You are not going to shuffle around Your tubes, then de-poisoning will not help prolong tube life as the heavily used numbers
still wear out first. If You shuffle them they will have some mechanical stress every time that is done and maybe cause a leak.
Having moveable tube mounts and switch their allocations in software when moved is one solution, but will it really be done over
time? People are lazy by nature...
The calendar tubes may benefit from de-poisoning as they shift a little bit too infrequent to keep their cathodes fresh. Just step
thru the used cathodes at some hour when You are likely to be asleep. There are no need for fancy drivers, just let the cathodes
float when not lit. The never used ones are just never used. No need to waste driver outputs on them. This is not a rocket science.
In my opinion the best way to prolong tube life is to use lowest recommended current and use PWM to reduce brightness. In the well
known Weston book he states that tube life is inverse proportional to I^2.5, that means a lot of wear for just a little bit more
current. He also states that the mean current when pulsed (don't remember dutu cycle) just had an impact of I^1.5. He had no
definite explanation for that, just empirical data from sputtering experiments.
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