UnintegratedCircuit
Part 4 - The Nixie Cathode Driver
​
How Nixie Tubes Are Driven
As previously described in this project, Nixie tubes require a high potential in order to ionise the neon gas within and cause the cathode to illuminate. The way to do this involves using a low-side switch in an open-collector configuration, the switch in question could be an NPN transistor or an N-channel MOSFET with the emitter/source grounded, the collector/drain tied directly to the Nixie cathodes, and the base/gate connected to the digital, logic-level control signal.
​
The most common implementation of a Nixie cathode driver is an array of MPSA42 (or the surface mount equivalent MMBTA42) NPN transistors. This is one of the more cost efficient and definitely more reliable implementations due to how common these components are, and their voltage rating of 300V, which means they can easily withstand the full supply voltage across the collector and emitter terminals. The downside, however, is that each transistor requires an individual base resistor to limit the base current flowing through them - this in turn requires a respectable amount of 'PCB real estate', especially if using thru-hole components.
​
Exploiting Specifications
There is an alternate solution though using an incredibly 'jellybean' part: the ULN2803A octal darlington array. Essentially this part incorporates eight NPN transistors into a single DIP (or wide SOIC) package, along with the base resistors, and some flyback diodes to help with controlling inductive loads such as motors. An internal schematic is shown below in Figure 1 where the actual transistors are shown simply as inverters (the equivalent logical function of an open-collector transistor that is pulled up to the positive rail via some resistance. Figure 2 shows the actual equivalent scheamtic of a single channel of the ULN2803A.
​
On the face of it, this part may not seem suitable for the task since the rated breakdown voltage of the transistors is only 50V; however, this is where the specifications really come into play. The eagle-eyed viewer may have noticed that the sum of the minimum striking voltage (120V) and the minimum breakdown voltage of the transistors (50V) sum nicely to the supply voltage of 170V.
​
These numbers represent the worst-case scenario for the voltage stress placed upon the ULN2803A transistors - The tube that is easiest to strike (and hence drops the least amount of voltage across it) coupled with the weakest transistor (will conduct with the least amount of voltage across it). In the case of Nixie tubes, the main goal of the transistor switch is not to turn the cathode on and allow it to illuminate, but to hold it off and keep it fully extinguished. If the switch breaks down, current will be allowed to flow through that cathode of the Nixie tube and it will illuminate regardless of the state of the control signal.
​
This theory can be reinforced by observing the I-V curve of a neon lamp from the previous part: neon lamps are current-driven devices; whilst current is low, voltage is held at the striking voltage until the current becomes sufficient to cause visible illumination. At this point, the voltage drops to the sustain voltage since the gas has ionised.
​
If working with a high-tension supply voltage of say 165V this provides a decent margin for the transistor breakdown + minimum tube striking voltages and should be enough to illuminate the cathodes on the vast majority of tubes.
​
A final note to make with this driver is specifically the fact it has eight channels, a perfect match for the eight cathodes of the IN-19V (hence that was a reason I chose it) but not enough for a standard '0-9' numerical tube, unfortunately.
​
Setting The Tube Current
The final step is to set the current flowing through the tube. This is accomplished by a simple Ohm's law calculation:
​
A current of approximately 3mA was chosen to ensure good brightness since the flickering effect will often operate at low duty cycles and cathodes will alternate fairly frequently to maintain good visual variety. This results in an anode resistor value of:
​
An anode resistor value of 22kΩ will therefore suffice.
​
Now that all the Nixie tube related circuitry is accounted for, attention can turn to the logic circuitry behind the scenes; this will be explained in the next section.
​
​
​