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