I've been working through this schematic for a Nixie Clock and I'm having a hard time understanding how the power supply (for the nixie tubes, not the digital logic) works. Afaik, the power supply is a boost converter that uses a Mosfet and Inductor to raise the voltage to ~180V. The increase in voltage is dependent on the duty cycle of the input to the mosfet (this info found here). The duty cycle is produced by an astable 555 timer. The duty cycle is required to be around 95%.
The ratio between R1 and R2 on the 555 does not create a 95% duty cycle. I'm assuming that the resistor values are chosen to create a specific frequency for the mosfet, and the duty cycle is altered via the control input. Is this correct? And, if so, how is the duty cycle affected by the control input?
R24, the 1kTrim Potentiometer, is used to vary the voltage around the 180V mark to make minor adjustments to the power flow to the nixie tubes. Does it do this by altering the voltage to the control pin? Or by sinking some portion of the 180V output power to ground?
In either case for the previous answer, why is a pin on the potentiometer connected to the control pin via a transistor? Wouldn't that sink the control pin to ground once the threshold voltage for the transistor is reach? If so, why?
Is there a reason why the schematic doesn't specify an input current?