On 8/22/2012 12:47 PM, Matthew wrote:
Finally getting back to this after more boring bits of life, and finishing
a couple of nixie clocks I was working on. Back to FLW = Back to asking
questions - this time about resistor values.
* As before, I'm experimenting with cathode drivers - and came across this
blog
post<http://lucsmall.com/2011/07/19/using-the-sn75468-as-a-nixie-tube-driver/>which
explains it (via some help from this group). I also tried it with the
50v ULN2003A and a 47v Zener diode and it works - this is good news since
they are cheaper and more readily available. I'm not quite sure how to
calculate the pullup resistor, but it doesn't seem too critical.
* However, given that I know from the spec sheets what current should be
going through each cathode, I can't quite get my head around what voltage
differential I should be using to calculate the cathode resistors...?
* Similarly, I just discovered that unlike LEDs, nixie circuits seem to use
an anode resistor as well as cathode resistors - why is this? The spec
sheets of the tubes show the anode current - but as above, I'm not sure
what voltage difference I should be using to calculate this.
* Also been experimenting with anode drivers; I was going to use an
optoisolator, but I'm trying to make a design which will drive both the
nixies and LEDs (with appropriate changes in supply voltage and resistors),
and it turns out I can't get one to handle the current of the LEDs. So I'm
probably back to the common MPSA92/42 combination. I see lots of schematics
including these, but I'm not quite getting the logic of how the resistors
are calculated...
As ever, any insight hugely appreciated...
First, remember that the B7971 is not a stacked-digit Nixie tube, rather it's a
planar segmented display. So it's more like an LED character display in terms of
its circuit topology.
You don't need an anode resistor if you use cathode resistors.
The resistor value is Rcath = (Vsupply - Vtube)/Icath
Where Vtube is the voltage across the tube from anode to cathode when operating
at the desired current. This number is in the data sheet as an approximate value.
I recommend that you hook up a variable resistor to the tube's cathode and
measure the voltage across the tube and across the resistor at different
resistor settings. You'll get a feel for how the tube works.
--
David Forbes, Tucson, AZ
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