Driving VFD Tubes

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I never worked with voltages over 12 V and got lost after I recieved some old russian VFD tubes I found on ebay.

The first problem I got was the datasheet which is completely written in russian. However, I found someone to translate it so here are the specifications of the tube:

Filament voltage: 1 V @ 50 mA

Anode voltage: 25 V (Works with 24 V)

Element voltage: 25 V (Works with 24 V)

It took a while until I got the first one working and in the end I had no idea how to drive the voltage for the elements (25 V) with a normal 5 V microcontroller.

But have a look at this schematic:


As you can see there are 4 wires coming from the microcontroller (in the upper left) to control 4 elements of the tube (high = on/ low = off).

The chip in the middle is a SN754410 motordriver. It has two voltage supplies, one for the internal logic (5V) and one for the target circuit (24 V).

Please have a look at the datasheet of this chip (SN754410NE Datasheet). As you can see the pins EN1 and EN2 are tied to a high level which means that all parts of the chip are in a state where they can be permanently controlled. The outputs M1A, M1B and so on will have a voltage of 24V if the corresponding pin is set to a high level by the microcontroller (5V).

You can see the voltage for the filament and the grid of the tube are wired on the right side of the picture (24 V/1 V).

With one of these SN754410 chips you can drive 4 of the 8 elements of the tube. But I think this solution is better than using many many transistors.

Edit:

One thing I found out after testing is that you need 10kΩ pull-down resistors (to GND) for each line going from the SN754410N to your microcontroller if you don’t want your Atmel or whatever baked.

Have fun.

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