Calculator Comparisons

From: Dave Dameron <ddameron_at_earthlink.net>
Date: Sun Oct 24 21:13:27 1999

Hi Tony and all,
At 07:26 PM 10/24/99 +0100, Tony wrote:

>> >And yes, VF displays are almost always multiplexed The multi-digit ones
>> >always are, but there were some single-digit VF display tubes. Never seen
>> >the latter in use, though.
>>
>> I have a calculator board with 12 single digit VF 7 segment displays. By
>> looking at the transistor placement, they appear to be multiplexed. Have
>> not traced the circuit to figure out how they biased each display (all the
>> filaments are in series). It also has a few early NEC ic's such as uPD10,
>
>Well, if you think about it the 'filaments' for each digit of a
>multi-digit VF display are in series, in that there there's normally a long
>filament that runs from one end of the tube to the other, across the
>front of all the digits.
>
>Normally, the filament is run off an AC supply centre-tapped to ground
>(or to a bias voltage) so that the average DC voltage on each point of
>the filamnet is constant (either 0 or said bias voltage). The result is
>that the digits seem to be all about the same brightness.

Yes, but normally the AC is 2 volts or so. I guess the AC voltage for this
board is higher, which might be harder to ignore circuit wise.
>
>The AC supply comes from a winding on the little transformer in the DC-DC
>converter used to get the HV (about 30V-ish) for the anodes of the
>display.

I have no power supply, but would guess its the vintage that all voltages
come from windings on the mains power supply, no converter.
>

>One thing... I have a portable calcuator where the display is individual
>7-segment neon displays for each digit (I am pretty sure they're
>multiplexed BTW). These display tubes look a lot like the single-digit VF
>ones, but of course light up orange (VF displays are normally
>blue-green). Do you have a blue-green display with an orange '-' sign, or
>what?
Yes, I think so, though I haven't powered it yet. (Quess at least 4
voltages are required: filament, logic, VF, and neon) There is also a neon
'-' sign on the right hand of the display as well.
The 2 neon lamps could be argon, but these are much less common.
-Dave
Received on Sun Oct 24 1999 - 21:13:27 BST