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Re: IV-17 / IV-4 Smartsockets. Group input requested !

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  • taylorjpt
    ... http://www.tayloredge.com/reference/Electronics/Displays/VFD.pdf http://www.tayloredge.com/reference/Electronics/Displays/Neon_NegativeResistance.GIF The
    Message 1 of 18 , Jun 22, 2009
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      > Adjusting brightness by adjusting voltage is known to cause problems with nixies,and I have also read about some problems with VFDs being underdriven. I'll be adjusting brightness through PWM of duty cycle of individual segments.

      > The filament voltage isn't causing me problems, I just need to narrow my options. AC seems to only be used to prevent a voltage gradient from causing uneven tube wear. On a large display with long filaments there could also be a visible difference in brightness. But, the AC frequency is unimportant, and if it is easier just to switch the filament polarity once per minute or once per second then I may just do that, because there is no noticeable brightness gradient with these tubes, which I think is related to the folded shape of the filament, which is 'n' shaped.


      http://www.tayloredge.com/reference/Electronics/Displays/VFD.pdf
      http://www.tayloredge.com/reference/Electronics/Displays/Neon_NegativeResistance.GIF

      The VFD does not have the negative resistance characteristic of the gas discharge tube; brightness for the VFD is still a function of current but more so because of the speed of the electrons rather than the number. The faster the electrons go, the more that get through the grid which is usually at the same voltage as the anode. This is why the current for a display with all segments on is essentially the same as when they are all off with the delta being higher as the cathode to anode voltage is increased. In general, most of the electrons make the loop through the grid which is why a gridless tube is more efficient than a multiplexed tube for the same light output.

      One of the problems when using a DC filament drive has to do with the fact that to fully turn off a grid you usually have to switch it to a voltage somewhat less than the filament to create an opposing electrostatic field else some electrons will leak through to the high voltage anodes. With one end of the filament at ground, this would indicate a negative voltage for off grids. You can bias the (-) end of the filament above ground, but this only worsens any kind of unevenness across the character and eats some power.

      ...but I run on!

      jt
    • Chris
      ... So, according to that datasheet, controlling brightness by controlling voltage is definitely a bad idea. SO I m on the right track by sticking with duty
      Message 2 of 18 , Jun 22, 2009
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        --- In smartsockets@yahoogroups.com, "taylorjpt" <jpt@...> wrote:
        >

        > http://www.tayloredge.com/reference/Electronics/Displays/VFD.pdf

        So, according to that datasheet, controlling brightness by controlling voltage is definitely a bad idea. SO I'm on the right track by sticking with duty cycle PWM.

        I understand the possibility of those problems described in the datasheet when running the filament from a DC supply, but I'm satisfied that there is no visible leakage. Presumably the tubes become less sensitive with age, and darken accordingly, which would seem to suggest that if there is no ghosting or visible leakage now then there is unlikely to be any in the future ?

        John, what was your idea regarding the power supply ? I want to keep the amount of magnetics down to a minimum. I've even considered specifying an 12V AC supply, which would make a lot of sense if AC is required.

        I seem to be on the right track at the moment but I know what sort of problems to watch out for now !

        Chris
      • John Rehwinkel
        ... I m probably not the John you meant, but I like to capacitively couple a square wave to the filament. You can adjust the current via capacitor size and
        Message 3 of 18 , Jun 22, 2009
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          > John, what was your idea regarding the power supply ? I want to keep
          > the amount of magnetics down to a minimum. I've even considered
          > specifying an 12V AC supply, which would make a lot of sense if AC
          > is required.

          I'm probably not the John you meant, but I like to capacitively couple
          a square wave to the filament. You can adjust the current via
          capacitor size and square wave frequency, or use a series resistor.
          Try to have your filament drive frequency not be a multiple of your
          mux frequency.

          – John
        • Chris
          ... Good tips, thanks. I ll be looking to run the SMPS at above 20kHz, so what would be the best way to couple the filament drive to the square wave signal if
          Message 4 of 18 , Jun 22, 2009
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            --- In smartsockets@yahoogroups.com, John Rehwinkel <jrehwin@...> wrote:
            >

            > I'm probably not the John you meant, but I like to capacitively couple
            > a square wave to the filament. You can adjust the current via
            > capacitor size and square wave frequency, or use a series resistor.
            > Try to have your filament drive frequency not be a multiple of your
            > mux frequency.
            >
            > – John
            >

            Good tips, thanks.

            I'll be looking to run the SMPS at above 20kHz, so what would be the best way to couple the filament drive to the square wave signal if the output signal from the onboard PWM module switches 5V at about 20mA ?

            So far I have a 40 pin pic with 64k memory driving a Supertex HV5812 which connects to 4 multiplexed tubes, two of which are IV-4 and two are IV-17 (for comparison). The pic will eventually be changed for another one with the least required memory,to keep costs down. Vbb is 40V at 25mA. It might be possible to get the size down to as little as 90mm long and 25mm wide for a 4 tube array

            I've just finished tweaking one 'segment dance' transition effect and confirmed that one tube can run one effect while it's neighbour can be running another type of effect (such as crossfading) starting at a completely different time. Each tube acts completely independently of it's neighbours, which was the hardest part of coding to get right

            Now its just a case of writing the dozen or so fonts, and about as many transition effects too. (There's still room for lots more too)

            I'll look at the power supply in due course, and will probably implement either P+I or FL control of the output (assuming 'differential gap' isn't good enough). The only problem with coupling the filament supply to the Vbb supply is that if the Vbb is required to change duty cycle then _both_ output voltages will change. But thinking about it some more there is most likely plenty of tolerance around the filament voltage so maybe that won't be too much of an issue ? I'll have to test it for myself.


            Chris

            Chris
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