TI-45: Rejuvenation of a faded VFD-Display

[Pjwum]

Nice Sunday wishes,

I thought I'd share some calculator rework on a rainy Sunday.





The sample on the left suffers from a considerable dim display. As far as I know, the TI-45 is the only TI pocket calc featuring a vacuum fluorescent display or VFD.

It basically works like a valve tube. A filament wire, the cathode, is heated by a low current and starts to emit electrons. The electrons accelerate to the anode with a potential difference of about 30 Volts, they hit the phosphorized anode coating and the kinetic energy of 30 eV is mostly transformed into photons. The segments of each number are separate anodes and can be switched on and off individually. A grid between cathode and anodes for each digit can be either positive (on state) or negative (off state). This way the driver electronics multiplex digits.







The emissive layer on a cathode filament consists of metal oxides which can degrade over time by disuse. But it is possible to bring it back to life by injecting high current pulses. Usually the electronics drive the filament at about 1.5 V AC. Here 6 V DC are directly connected to the filament pins of the display package and the filament wire starts to glow. This treatment is applied about 20 seconds and two times with 8 V. The result is clearly visible.





One advantage of VFDs over LEDs is very low current consumption. Only the filament draws some milliamps. Anodes and grid have extremely high impedances and do not contribute to the current consumption. Hence, total current draw does not depend on the number of lit digits.
The pictures below show current draw in off-state (28 uA), on-state one digit (15 mA) and on-state eight digits (16 mA).