Dimming the Nixies

Posted on Feb 5, 2014

Fact is that the Nixies need a particular cathode current to entirely light up the cathode at all. If you just reduce the anode voltage the current through the anode resistor drops and so does the cathode current. Up to a certain point where the cathode is only partially covered with the discharge glow - or doesn`t come on at all. Both is not what

Dimming the Nixies
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we wanted, right So here is my approach that is a bit more complicated but seems to work fine. At least for my taste better than just reducing the anode voltage. Your individual mileage may vary however. The circuit shown above is a simple oscillator with a 555 chip. The only uncommon part of the layout is the 1M Ohm Potentiometer and the two diodes. This part of the circuit allows to vary the duty-cycle of the output signal between 5 and 95 percent. The transistor in the upper part is wired into the high voltage supply line between power supply and the nixie anode resistors. The 555 is driving another transistor that simply cuts off the voltage on the base of that "main" transistor. Result: the lower the duty cycle of the 555 output signal, the longer is the conductive period of the main transistor - during which the full current flows through the anode resistors. The trick is, that the high voltage is switched and not regulated. The component values in the schematic allow a frequency variation between 3 - 8. 5 ms (about 117 - 330 Hz) and the best results with a set of ZM1080s is at about 3. 5 ms (285 Hz). Here the regulation of the duty cycle works from 5 to 95 %, where 95% is pretty dim, but still readable and the cathodes fully covered. The transistors must be any of the high voltage types, since it is either attached to the HV-supply or has to sink the base voltage of the main transistor. For my prototype I picked a...

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