[neonixie-l] Re: How much voltage do cathode transistors need to be able to handle?

['marta_kson' via neonixie-l]

If You concider blanking the tubes, that includes PWM for intensity 
control, by the cathode drivers then use something that can take the whole 
anode voltage. I.e. MPSA42. Period.



If not You can clamp the voltage to a lower level and use lower rated 
transistors. The clamp must be there, transistors degrade from breakdown 
even if they don't may fail immediately when the current is limited. The 
needed clamp voltage minimum level can sometimes be found in the tubes 
datasheet. Look for "selection voltage". 

The antique 74141/7441/K155ID1 are such devices. The western parts have a 
60-something clamping voltage, the soviet versions about 100V. Those are 
good for any tubes. Unfortunately they are power hungry at about 25mA, 
otherwise they are a good choise. The K155ID1 use to be readily available 
at eBay.

Some tubes have very low selection voltages. I have some recollection of 
ZM1000 to be such a device. 

I also remember having seen some clock built with 40?? CMOS at 15V driving 
nixies directly. The protection diodes to Vdd was used for clamping. That's 
a bad pracice that could possibly damage the devices even if Vdd is safely 
held down by some means to ensure it won't rise.

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[neonixie-l] Re: How much voltage do cathode transistors need to be able to handle?

[gregebert]

>
>
> I also remember having seen some clock built with 40?? CMOS at 15V driving 
> nixies directly. The protection diodes to Vdd was used for clamping. That's 
> a bad pracice that could possibly damage the devices even if Vdd is safely 
> held down by some means to ensure it won't rise.
>

Actually, the ESD clamp-diodes on the 4000X CMOS part would likely turn-on 
*all* of the cathodes, because they clamp to the VCC of the CMOS device. 
You would be surprised how durable ESD structures are. About 10 years ago I 
did the ESD design for an ethernet PHY and all of the I/O's had to handle 
zaps around 1 amp, some even more. A few mA for a nixie tube wouldn't do 
any harm to the IC; definitely a bad design practice, though, as you 
mentioned.

My first clock was built with discrete 4000 series CMOS, but I used 
450V/10Amp NMOS devices (yep, overkill but I got tons of them almost for 
free..) to drive the cathodes. I also run it directly off the AC mains (no 
transformer). The 3 I have now have been running flawlessly for several 
years now.

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[neonixie-l] Re: How much voltage do cathode transistors need to be able to handle?

[Dekatron42]

Ronald Dekker has a lot of good things to say about driving Nixies with a 
lot of facts: http://www.dos4ever.com/QandA/QandA.html

He also shows how to drive a Nixie from a CMOS 
4017: http://www.dos4ever.com/ring/ring.html#HEF

Do check the whole ring counter article he has written (which the CMOS 4017 
is part of) and also other stuff that he has written to get a lot of good 
answers on things related to Nixies and Dekatrons, plus the E1T!

/Martin

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