On 3/11/12 4:44 PM, Cobra007 wrote:
Then don't turn on the other tube at that time! I turn on only one tube
at a time, which removes that problem.
That is not completely true, it's not that I turn on both tubes at the
same time, it is because the current starts to flow through the wrong
tube because IT CAN. With a 170V anode voltage and a 50V clamping
zener, the minimum voltage on either of the tubes (on or off) is 120V.
A tube that has 120V, is switched off but was switched on just a
fraction earlier (due to multiplexing) will leak current from anode to
ground through the zener clamping diode. You can't stop that unless
you disconnect the anode from the HV power supply, through an extra
transistor or something else.
I agree that adding complexity will in most cases reduce the
reliability. In my case, it is not so much that the circuit becomes
more complex (replacing 8 transistors in 1 package by 8 discrete
transistors) it is more an increase in PCB complexity. So I am not
worried that it will decrease the reliability.
Michel
I am confused. If you are turning off one tube then turning on the other
tube, and operating at a low duty cycle (I assume you're still
discussing the dimmed case), then there will be a long delay between
turning off the first tube and turning on the second tube. So the first
tube should have time to deionize. Is the leakage strong enough to make
a glow? For how long?
My Nixie watch displays an interesting result when one tube is broken -
the other tube will light all cathodes faintly when the broken tube is
selected. That's because all the cathodes are clamped to 50V, but the
power supply makes about 200V to an open circuit. (I connect both anodes
directly to HV without resistors.) In that case, the tube is lighting
when it shouldn't. But there is a broken tube, so there's no reason to
make the display function properly in that case.
I provide display blanking in my watch, which is accomplished by feeding
an MPU output to the HV supply feedback node through a carefully-chosen
resistor, which makes the HV drop to 110V when the bit is set to 1. This
allows me to turn off the display and not excite the case described
above in normal operation. It's a very low cost solution to blanking, as
I share the MPU pin with the accelerometer enable.
The long and short of it is that you ought to be able to use a 50V
driver chip in a two-tube design with a bit of effort.
--
David Forbes, Tucson AZ
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