Their function is very simple: if you send to input PINs a number (0 to 9) in binary form, the corresponding output PIN is set high. These ICs are called bcd-to-decimal decoder to drive nixie tubes and have 4 input PINs (A/B/C/D) and 10 output ones (0.9). In the past, some integrated circuits were produced to drive Nixie tubes the most widespread ones are 7441, 74141 and their russian “clones” K155ID1, KM155ID1 – you can still find them on eBay. 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. Not every transistor is able to sustain the high collector-base voltage (Vcbo): the most widely used one is MPSA42, which – as you can read on its datasheet – sustains a Vcbo of 300V. The simplest solution is to use a transistor and connect its base to a microcontroller PIN. You can’t directly drive a Nixie with microcontroller’s output PINs: each Nixie cathode not connected to ground is at a voltage near to Vm. In my clock I won’t use the decimal point, so I choose a 12Khom 1/4W resistor.
(for decimal point) R = 60Kohm and P = 0,015W.(for numerals) R = 12Kohm and P = 0,075W.If I apply previous formulas to my Nixie values I get: The formula for calculating resistor R value is: You can find all these values on Nixie’s datasheet: average/peak decimal point current (Ikdp).Current limiting resistorĮvery Nixie tube is characterized by two voltage values:
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In this second post about my Nixie clock, I’m going to show you how to drive those tubes with a microcontroller.
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