This Tech Note details how to supply the necessary voltages to a nixie tube – for the purposes of a real example, the specifications of the IN-14 tube will be used. So firstly some background…<--break->

<--break->The Background

Nixie tubes are gas discharge tubes, or also known as a cold cathode display. The tube is filled with a gas at low pressure, typically neon.

Specifications for a nixie tube quote a supply / strike voltage and an operating / sustain voltage. The difference relates to once the tube is “lit”, i.e., displaying one of its digits. The impedance of the tube once it is lit is very low and thus a resistor is required to be connected to the anode of the tube. This resistor limits the current through the tube – due to its low impedance – and is selected to match the specified operating current.

A theoretical example

The specifications for an IN-14 nixie tube are as follows (only the values of interest from the datasheet are included…):

Supply (strike) voltage 170 V
Operating (sustain) voltage 145 V
Cathode current, typical
(Decimal points, typical)
2.5 mA
(0.3 mA)

Therefore – via Ohm’s Law – the anode resistor for digits should be as follows:

Ranode = ( Vsupply – Voperating ) / Icathode, typical

= ( 170 – 145 ) / 2.5 mA

= 10 kOhm

The nixie tubes are just like regular valves / tubes in that if one operates them at a lower operating voltage/current, then they’ll last longer. So in this case no need to match exactly the cathode current, i.e., less is better! (And of course there can be a little variation in the actual operating characteristics…)

The practical example

Here is the circuit used in the IN-14 nixie driver kit:

The anode resistor is set at 15 kOhm to limit the current to approximately 1.7 mA – this being quite conservative, but also leading to a decent nixie tube life.

As for the driving circuitry, these are simply high voltage NPN transistors configured as switches to enable/disable the various digits and decimal points. Of course there needs to be suitable input signals to control each of the different digits / decimal points – thus the Nixie controller supply.

– Colin

References

For some further/different descriptions of how to drive a nixie tube review these following references: