On 3/13/11 12:31 PM, ggtnuminix wrote:
I have been reading Jason Harper's article on driving nixies with discrete transistors. I understand the portion of using a MPSA42 to control the cathode side of a nixie; however, I dont understand why you couldn't use the same approach to control the tube anode. Most of the designs seem to use a MPSA42 and MPSA92.
The Nixie tube has two voltage levels to which your logic needs to exert control. The cathodes are connected by transistors to the low voltage (ground), and the anodes are at the high voltage of 180V.
A transistor's base, its logic input, must be at the same voltage (within one volt) as the emitter, which is connected to either ground for the cathode or to 180V for the anodes. The reason is that the base and emitter are connected internally by a diode which conducts current and has that 0.7V voltage drop that you read about.
The logic circuits all run near ground, so they can be connected directly to the transistors which control the cathodes. But if you connected the logic signals directly to the transistors at the anodes, then there would be 180V passing through the transistor's base into the logic circuits. This would destroy the logic circuits immediately, as they can only handle about 5V.
The MPSA42 transistor is needed to provide a path for each anode control signal to be translated from the 0V level to the 180V level that the MPSA92 anode transistor bases require.
-- David Forbes, Tucson AZ http://www.cathodecorner.com/ -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
