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Re: A couple of B7971 board questions

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  • Chris
    ... yesterday - :) ... be to ... dont ... an ... Hello, If the datasheet for the B7971 s is to believed then the different lengths of the cathodes lend
    Message 1 of 7 , Apr 29, 2008
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      --- In smartsockets@yahoogroups.com, Brett Paulin <yahoogroups@...>
      wrote:
      >
      > I just finished building my Smartsockets and got them fired up
      yesterday - :)
      > Wow, B7971's are *big*..
      >
      > I just used a link for R31, since I'm running at 170v.
      >
      > If your power supply is not adjustable, then the accurate way would
      be to
      > meter the current and choose a resistor accordingly, although I
      dont
      > quite "get it" as to how a single anode resistor can be accurate if
      an
      > unknown number of cathodes are illuminating.



      Hello,
      If the datasheet for the B7971's is to believed then the different
      lengths of the cathodes lend themselves very well to approporiately
      chosen resistors in the standard values series.

      Not sure if that was the intention or if that's how it worked out,
      but to clarify , you should choose the resistor value for the cathode
      resistors to get you as close as possible to the desired current
      values for your device , ZM1350 or B7971. However you may be slightly
      out even then , and the voltage from the supply a little high still,
      in which case adding a small amount of anode resistance, used just to
      trim the final voltage, can be used. It was probably overkill on my
      part to include it, but I felt that for all the time and effort it
      takes to be able to quickly add a bit of resitance or possibly even a
      fuse it was worth including.

      In a test I did with a 200V supply I found that when comparing one
      tube with the the resistors for 170V plus a small anode resitor,
      against a tube with reistors for 200V and no anode resitor, there was
      only a very slight difference in the apparent brightness of the
      cathodes regardless of the number which were being actived. Flat
      orange is a difficult colour to judge brightness of and radiant
      orange is very difficult to judge in close relative terms.

      Basically what I wanted to say was that the anode reistor is moreof a
      luxury and for added protection. A small reduction in cathode current
      can create a much longer tube lifetime in the right circumstances and
      so I opted to have one, just in case anyone needed to set the
      voltages from a non-adjustable supply which demand odd valued
      resistors.

      As long as the bulk of the current limiting is being done with the
      cathode resistors any anode resistance you need to choose to further
      limit the current for one cathode will not produce a very noticeable
      effect when all cathodes are activated


      Chris
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