Anode driver

In this post, we will see how to build the anode driver:

- principle,
- basic components,
- how that works,
- how to calculate the proper values

The anodes of the Nixie gas tubes need a driver when multiplexed in order to switch the appropriate digit.

This driver is built using two transistors (1 NPN and 1 PNP) able to withstand high voltage (the Nixie tubes need 170VDC to switch on).
MPSA42 (NPN) and MPSA92 (PNP) are commonly used, as they are available and cheap components.

The circuit for the anode driver should be as follow:

The resistor R4 is there to limit the current that flows to the Nixie.

The tricky part, at least for me was to properly choose the resistors R1 to R4.
Let's start with R4, obviously the easiest one.
You need to know your Nixie tube characteristics:
- voltage to ignite the gas in the tube,
- the maximal current the gas tube needs to glow properly.
and also how many digits you would multiplex with one anode driver.

I have chosen classic IN-14 tubes, as they are available and not too expensive.
The datasheet (from tubehobby.com) gives:
- supply voltage Ub = 170V,
- maintaining voltage Ua = 145V,
- current of Ik = 2.5mA max per gas tube.

As we have two tubes in parallel:
- R4 will have to let pass 5mA (Ik +Ik = IR4),
- the voltage for the Nixie will still be Ub = 170V.

Once fired up, the Nixie tube will require "only" around 145V to maintain the glow (Ua = 145V)

We can then determine UR4, voltage drop at the resistor:
Ub = Ua + UR4 => UR4 = Ub - Ua = 170V - 145V = 25V.

And from that, resistance value of R4, that we will call... R4 (...erm):
UR4 = R4 x IR4 => R4 = UR4 / IR4 = 25V / 5mA = 25V / 0.005A = 5000 ohms

5k resistors do not exist, 5.6k should be used, and as the tubes will still require 145V to function, then I4 will slightly change:
I4 = UR4 / R4 = 25V / 5600 ohms = 0.0044A = 4.4mA => each tube will receive 2.2mA, which is good value.

And the power used for this resistor is then:
PR4 = U4 x IR4 = 25V x 0.0044A = 0.11 W

It is now time to give values for R2 and R3.

So we know the resistor R4 will see 4.4mA passing through.
We need to know the min current in order to saturate the PNP transistor and "feed" the Nixie gas tubes connected to R4.

MPSA92 has a transistor effect (beta coefficient) of at least 25 according to the datasheet.
The min current Ib1 through the base needs to be at least 1/25 of the current going through the collector, which is I4:
Ib1 > I4 / 25 = 0.0044 / 25 = 0.176 mA

And from there, we can determine R2 and R3, using the characteristics of the saturated MPSA92:

Yes, you have seen that for this try, I was using Ub = 175V instead of 170V.
Using this 170V, then:
UR2 = 169V
R2 = 169 / 0.0046 = 36.7 kohms => so we would still choose a 39k resistor.

And finally:
PR2 = UR2 x IR2 = 169 x 0.0046 = 0.78 W
PR3 = UR3 x IR3 = 0.9 x 0.0044 = 0.004 W