How to drive a Nixie Tubes

How to drive a Nixie Tube

If you want to build a Nixie display or a Nixie Clock you’ll have to know how to use and drive Nixie Tubes. We will discuss here basic and multiplexed use of the Tubes. There are a number of schematics to illustrate various usages.

1. Direct Control of Nixie Tubes

Most Nixie tubes share the same internal structure: a common anode (positive terminal) and ten individual cathodes (negative terminals), each responsible for displaying one digit from 0 to 9.
Nixie tubes operate at high voltage, typically between 170V and 190V.
⚠️ Caution: Such voltage can be dangerous.

As an example, let’s consider one of the most popular Nixie tubes — IN-14.

To light up a Nixie tube, you will need: a 180V high-voltage power supply, a current-limiting resistor, a switch, and wiring.
Using a current-limiting resistor is strictly mandatory — without it, you risk damaging either your power supply or, more likely, the tube itself.

Each type of Nixie tube requires its own value of anode resistor.
The table below lists typical resistor values for the most common Nixie tubes:

 

	IN-1	IN-4	IN-8(2)	IN-12	IN-14	IN-16	IN-17	IN-18	Z573M	Z5660M	Z568M
Ra	24k	24k	24k	27k	24k	33k	27k	10k	27k	12k	8.2k
Ia	2.5 – 3mA	2.5 – 3mA	2.5 – 3.5mA	1 – 2.5mA	2..4mA	1 – 2mA	1.5 – 2.5mA	4 – 8mA	1.5 – 2.5mA	3 – 6mA	4 – 7mA

For Nixie tubes not listed in the table, the anode resistor can be calculated using the following formula:

Ra = (Vhv – Vs) / Ia, where:
  Vhv — supply voltage applied to the anode (from the high-voltage power supply),
  Vs — Sustain (Maintaining) voltage,
  Ia — allowable anode current.

Values for VS and IA can be found in the datasheet of your Nixie tube.
Unfortunately, not all datasheets specify the sustain (maintaining) voltage, but that is not a problem — it can be easily measured with a voltmeter.

To measure VS, connect the tube to a high-voltage supply through any resistor between 4.7 kΩ and 47 kΩ, and measure the voltage between the anode and cathode when the tube is lit.
You can safely perform a short test even with a non-optimal resistor value — a brief operation in this resistance range will not harm the tube.

As a general guideline:

• for small tubes, use a resistor from 12 kΩ to 47 kΩ,
• for large tubes (with digit height above 0.6 inch), use a resistor from 4.7 kΩ to 12 kΩ.

For larger tubes, use a power resistor with a sufficient power rating; 1 W is usually enough.
For SMD resistors, the appropriate size is 2512.

Example:
For an IN-14 tube powered by a 180V supply,
Ra = (180V-130V)/0.002A = 25 000 Ω (25 kiloohms)

The nearest standard resistor value is 24 kΩ.

The diagram shows a single push-button, but you can connect as many buttons as you need and light up different digits by pressing them sequentially.
This simple direct control method can be used, for example, in escape rooms or other applications where the tube is operated manually by an operator.

2. Electronic Control of a Nixie Tube

The mechanical switches from the previous example could be replaced with relays, but such a design would be bulky and outdated.
A better approach is to use electronic switches — transistors.

You cannot connect the tube’s cathodes directly to the microcontroller pins, because the tube is driven by a very high voltage (typically 180V), whereas microcontroller pins are rated for a maximum of 5V or 3.3V, depending on the model.

There are transistors, such as PMBTA42 (SMD package) or MPSA42 (Through-hole), which can easily withstand the high voltages used in Nixie tube driving circuits. Any other transistor with a collector-emitter voltage rating (Vce) > 200V can also be used.
This allows you to control a Nixie tube using any microcontroller you prefer — Arduino, ESP32, Atmel AVR, STM32, or even Raspberry Pi.

Below is a typical schematic for driving one Nixie digit using an NPN transistor and a microcontroller:

A base resistor must be connected to the transistor’s base, with a value between 3.3 kΩ and 10 kΩ.

Now, by commanding the microcontroller to set a high or low voltage on the pin connected to the transistor’s base (through a resistor), we can turn on or off the specific Nixie tube digit whose cathode is connected to that transistor.

This is a very simple way to drive Nixie tubes, but it also has drawbacks.
If you control only one tube, you need:

• 10 transistors (one per cathode),
• 10 base resistors,
• and 10 GPIO pins on the microcontroller.

However, if you want to control, for example, six tubes (to build a Nixie clock), you will need:

• 60 transistors,
• 60 resistors,
• and 60 GPIO pins.

GPIO (General Purpose Input/Output) pins are microcontroller terminals that can be controlled at our discretion. Typically, this means we can instruct the microcontroller to set the pin either to 0 volts or to the supply voltage (usually 5V or 3.3V). Most available microcontrollers don’t have that many free GPIO pins, and the GPIOs are also needed for scanning the clock control buttons, controlling the separator dots, and so on.
Now that we understand how to control Nixie tubes electronically, let’s move on to control methods that help reduce the number of GPIO pins required.

3. Reducing the Number of GPIO Pins for Nixie Tube Control

(to be continued…)

 

How it works

A Nixie Tube is a gas discharge device. Their colour and operating voltage are determined by the properties of the neon gas within. Tubes ignite at 140-170 V, slightly varying by type. Once ignited, their resistance is very low so a series of resistors is necessary to limit the current, typically to 1-5 mA. The operational voltage of the Tube is 90-130 V depending on type. If the voltage drops below the turn-off voltage, the Nixie will go out. For a ZM1000, for example, the turn-off voltage is specified at 118 V at room temperature, but practice shows that japanese Nixies still work at 100 V.Because of Supply voltages of 170-300 V are used some people think they need switching devices that can handle these High voltages. In practice, however, there are two circumstances that reduce the voltage you have to switch:

• First is that the voltage across the switch only rises to the Supply voltage minus the turn-off voltage of the Tube. When it drops below this, the Tube will become an isolator.
• Second, when there is one of the cathodes switched on, the anode voltage will be significantly lower than the Supply voltage. So if your driver circuit always turns the next cathode on before switching off the previous one, your cathode voltages will stay below 65 volts no matter what your Supply voltage is. You could connect a zener diode to one of the cathodes to prevent a situation where all cathodes are off. The once-popular SN74141 decoder – Nixie driver had built-in zener diodes of 55 volts on all outputs that could handle 1 mA. It’s predecessor, the 7441, did not have these, so you would have to take care to keep it’s output voltage under 70 V under all circumstances.

 

 

Basic cathode driver circuits

The 7441 and its successor, the 74141 were very common Nixie driver IC’s from the TTL era. If you can find a few 7441’s or 74141’s, you can use these vintage IC’s in your project. But they are hard to get and being TTL devices, they use more power and have larger input currents than modern IC’s. Many older MOS IC’s are not able to drive TTL inputs. In those cases, you will need a buffer to drive your 74141! So what are the other options:

 

 

• Use high-voltage transistors after a CMOS BCD-to-decimal decoder. As mentioned above, an Uceo rating of 70V is sufficient if you keep the Supply voltage below 200V or clamp all cathodes to some clamping voltage of 60V. You could try and connect only one cathode to a zener diode of 45V, so this cathode will light if no other is switched on, but you would probably need to use a 120 V transistor. Anyway, 250V transistors like BF422 or MPSA42 are quite affordable (~ € 0,45), so you can decide to just use these.

 

 

• Use high-voltage driving ICs. For example SN75468, it works fine. It contains 7 darlington transistors intended to switch inductive loads up to 500 mA and has a maximum output voltage of 100 V. Unfortunately, it has been declared obsolete by TI and it is becoming rare. The ULN2003 is pin-compatible with the 75468 but it can only handle 50V. You will have to protect it by connecting its pin 9 to a clamping voltage of 45 V. It shows good results with a zener diode of 44 V connected to pin 9.

 

 

 

 

Multiplexing Nixie Tubes

In a multiplexed display, all the corresponding cathodes of the Nixies are connected together in a bus structure. The anodes of the Nixies are switched on one by one and the right cathode is activated for every digit. If you do this fast enough, you get the illusion that all digits are on simultaneously.

The advantage of multiplexing is that you need fewer decoders and less wiring.

Multiplexing is nothing new. Multiplexed LED displays are used on alarm clocks, digital room thermostats, digital voltmeters, etc. A lot of voltmeter chips, calculator chips, alarm clock chips etc. already have multiplexed outputs to save IC pins. If you use a microcontroller such as a PIC for your project, you’ll have to program a display multiplexing routine.

So how do you multiplex Nixie Tubes? Isn’t that complicated and expensive?

To multiplex Nixies, you need to include anode driver circuits in design. These turn on and off the anodes of the Nixies. The anode drivers “float” at the Supply voltage of 170-250 V, and they are driven by the main circuit (your Clock, Meter, Calculator) that is at ground potential. In order to bridge this voltage gap, either a high-voltage transistor or a capacitor is used. Depending on the technology used in the main circuit, the anode drivers are driven with a 2-5 voltage swing from TTL or CMOS powered by 5 V, or maybe 24 V for older PMOS or NMOS circuits.

 

Many examples of multiplexed circuits for Nixies can be found on the web. For example, an anode-scanning display by Philips Elcoma which even has a dimming control, was in a bulk post on the sci.electronics.schematics news group. In the Game Archive web site, which contains a lot of information for pinball machine and video game colectors can be found a schematic of a Bally 7-digit pinball machine display that uses 7-segment flat-panel neon displays. National used to have some anode driver circuits for Panaplex^® displays, like the DM8880, but they are in short supply today.

 

Nixie Clock: the Driver

Calculating current limiting resistor. Every Nixie Tube is characterized by two voltage values:

• ignition voltage (Vign)
• maintaining voltage (Vm)

and by one or more current values:

• average/peak numeral current (Ik)
• average/peak decimal point current (Ikdp)

These values can be found in Nixie’s datasheets:

 

The typical schematic for connecting the Tube is following:

 

 

And the formula for calculating resistor R value is this:

 

 

And the power dissipated on the Tube is this:

 

 

 

Discrete transistor driver

You can’t directly drive a Nixie Tube with microcontroller’s output pins: each Nixie cathode not connected to ground is at a voltage near to Vm.

The simplest solution is to use a transistor and connect its base to a microcontroller pin.
Not every transistor is able to sustain the high collector-base voltage (Vcbo): the most widely used one is MPSA42, which, as can be read on its datasheet – sustains a Vcbo of 300V.

The schematic for that drive is very simple:

 

 

The disadvantage of this approach is that you need a transistor for each cathode, so if your Nixie clock project uses 4 Nixies you need about 40 transistors!

 

Integrated circuit driver

 

 

 

 

 

Gra & Afch Nixie Clocks

We here at Gra & Afch offer a variety of parts for Nixie Clocks.

As well as Ready-to-Use assembled Clocks, Cases and DIY KITs that are presented in a separate sections in our shop.

 

And you can come visit it and look for yourself anytime: Spare parts, Nixie Tubes, Nixie Clocks without Cases, Nixie Clocks in Cases, DIY KITs for Nixie Clocks, Cases for Nixie Clocks.