VFDs

A VFD is a type of display that uses tungsten filaments to emit photons through a system of grids and to phosphor-coated segment plates to display information. You will find such displays in some kitchen appliances, automotive radios, and other devices.

A low voltage, typically AC but sometimes DC, and usually around 2 to 5 volts, is applied to the filaments. Then, in a multiplexed setup, which is common on VFDs, the grids are scanned with a 40 to 70 volt positive DC voltage. The selected segments to light are also referenced to this voltage.

In a static drive, the grid and segments are referenced to between 12 and 20 volts, as such a high voltage is not needed to drive a single digit.

Do not short the higher voltage to the filament connection — you will burn out the filament!

To get a number to appear in a single-digit VFD tube, we will need to set up the tube to get it to glow. For this we will use the DG10Q1 VFD tube.

Apply 0.8 volt to the filament, and 15 volts to the grid, with 15-volt ground tied to filament ground. Then route the segments to a UDN6118A or similar source driver, which will boost 5-volt signals to 15-volt signals for the VFD.

Set up a 74LS48 or 74LS248 as normal, but connect the outputs to the source driver. Once all connections are made, you should see a number in the tube.