I appear to be going backward.

I am trying to use a transistor to turn on the decimal point in a nixie tube. I have a 170 V power source connected to the anode of the tube. The current required to light up a digit is much greater than the maximum current allowed for the decimal point. The power to the base of the transistor will be coming from an output pin of a raspberry pi.

Whats wrong with the trimmers? I realize that the actual trimmers will have three pins, but this is just a simple symbol from diptrace.

 

I am trying to use a transistor to turn on the decimal point in a nixie tube. I have a 170 V power source connected to the anode of the tube. The current required to light up a digit is much greater than the maximum current allowed for the decimal point.

Assuming the 56k resistor gives the correct current for the digits, and that current is greater than the 0.3mA (is this what you want?), you can use the transistor to sink 0.3mA.

\( \small R = \frac{V}{I} = \frac{3.3V - 0.7V}{0.3mA} = 8.6k\Omega\)

The closest 5% value will be 9.1k.

The circuit on the right would let you vary the current from about 0.15-0.3mA. The arrow on the pot indicates clockwise direction.

Whats wrong with the trimmers? I realize that the actual trimmers will have three pins, but this is just a simple symbol from diptrace.

You don't have the wiper connected, so the resistance can't be varied.

EDIT: Add schematics.

 

I get the trimmer piece.

I am still confused on transistor. What part does the 170 V play?

 

I am still confused on transistor.

What is unclear?

What part does the 170 V play?

The applied voltage needs to be higher than the strike voltage of the tube. The manufacturer gave you a voltage, and resistance, that will insure that strike conditions are met. What the manufacturer may not tell you is how far below the strike voltage you can go, after striking, before the element stops glowing.

As you stated, the current in the DP needs to be lower than the digits. That's what the transistor configured as a current sink is doing.

Note that the sink current depends on the voltage applied to the base. If that voltage varies, so will the DP current.

 

I tried to redraw the schematic as you suggested.


I removed the trimmers to avoid any potential confusion. I completely understand how you arrived at 8.6K Ohm for the resistor. I went ahead and drew the standard value of 9.1K Ohm and calculated the current to be ~.25 mA. What I am struggling with is the voltage and current on the other side of the transistor. What is the voltage/current actually going into the nixie tube's DP?

 

What I am struggling with is the voltage and current on the other side of the transistor.

The voltage on the collector of the transistor will be whatever it needs to be to allow 0.29mA to flow through it. The transistor will saturate if it can't sink the intended current.

What is the voltage/current actually going into the nixie tube's DP?

Dp current will be up to 0.29mA.