Driving a Vacuum Fluorescent Display

Discussion in 'The Projects Forum' started by jgessling, Sep 21, 2009.

  1. jgessling

    Thread Starter Active Member

    Jul 31, 2009
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    I'm working on making a clock (or something) out of some VFD tubes that I got from an old calculator. I've figured out how to make a tube work. The filament get 1.2v, the grid and segments get 12v. Now I want to use a decoder to drive the segments off of a counter. I've got a couple SN74LS47 chips that look like they can do the job. I can figure out how to get the inputs, my confusion is how to get the 12 volts.

    I've attached a little schematic, the chip gets 5v, then the 12v is from another supply and needs to get turned on to light the segment. I think that would be called a pull up resistor, but what value should it be?

    BTW, the rules of this project require that all (or most) parts come from my boxes of ttl chips, no micro controllers allowed. Thanks for your help.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    The 74LS47 IC's have open-collector outputs. That means they can sink current, but cannot source it. When a segment is supposed to be lit, the 74LS47 will sink current (supply a ground to) the corresponding segment output pin.

    It's been awhile since I've played around with one of those. Seems to me that the filaments work better if powered by AC instead of DC, otherwise you may get uneven lighting.

    Figure out if the segments can be lit by supplying ground to their connections.
     
  3. jgessling

    Thread Starter Active Member

    Jul 31, 2009
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    If the open collector output can only sink current then that's not going to work. What I need is to put +12v on the segment to light it. Maybe I need a different chip? Or some clever way to turn sinking current into output voltage.

    You are right about the filament, sometimes AC is better, especially in a single tube with lots of digits. These old tubes have one digit per tube.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    This should do it for you:

    [​IMG]

    Q1 can be a variety of general purpose small-signal PNP transistors, like a 2N3906, 2N4402, etc.

    Rload represents one of your segments that needs a +12 source to illuminate.
    The 7447 label is where you would connect one of the segment outputs from the SN74LS47.

    R1 limits the base current of Q1 to around 5.5mA. This would give enough saturation current for a load of up to around 55mA, which is probably plenty.

    R2 ensures that Q1 is turned off (not sourcing current) when the 74LS47 output is not sinking current. It may prove unnecessary.
     
  5. jgessling

    Thread Starter Active Member

    Jul 31, 2009
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    I'll put that on the breadboard and try it out. Thanks.
     
  6. k7elp60

    Senior Member

    Nov 4, 2008
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    Here is a schematic of a digital clock I built a number of years ago with a VFD display. I used a clock chip. My diagram shows the polarity of the pulses to the grids and the segments. The filiment was supplied by dc.
    This is the second one I have built. I built one for my mother in 1984 and it is still working todate, with the original clock chip and VFD. My mom's used transistor drivers insead of the ULN2004A. My displays are multiplexed.
     
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