On 7/18/11 9:22 PM, Luc Small wrote:
Hi everyone,
I'd like to use the SN75468 as a nixie driver in my next clock.
According to earlier posts, the key thing to ensure is that the common
pin (pin 9) is clamped to a voltage below the ic's darlington
transistor rating of 100V.
Looking at some earlier designs (some of which are based on the
similar but lower voltage rated part, the ULN2003), this clamping has
been achieved with a zener in one of two ways. I've illustrated these,
which I call options A and B, pictorially here:
http://lucsmall.com/2011/07/19/using-the-sn75468-as-a-nixie-tube-driver/
Can you tell me what the right approach to take is or, if things
aren't so cut-and-dry, the relative merits of each approach?
Here are the earlier designs to which I refer:
http://www.cathodecorner.com/nixiewatch/theory/nwrf-schem.pdf
http://web.jfet.org/nixie-1/
http://joule.bu.edu/~hazen/Nixie/
Many thanks,
Luc
Luc,
Circuit B guarantees that the other cathodes are pulled up to 91V by the
clamp diodes in the driver chip.
Circuit A depends on stray current flowing from the anode to the unlit
cathodes to energize the Zener diode. This current may be very small, in
the microampere region, since these cathodes are unlit. The Zener diode
therefore may be operating at a much lower voltage than 91V, resulting
in white dots or other unwanted artifacts. Depends on the Zener diode,
of course.
Short answer: B is guaranteed to work, A is try-it-and-see.
--
David Forbes, Tucson AZ
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