On Monday, March 5, 2018 at 7:14:56 AM UTC, gregebert wrote:
> The MAXIM device uses BiCMOS technology, so the I/O pins are
> probably NPN devices and should be more tolerant of voltages above
> 76 VDC.

I'd prefer not to allow the voltage to rise that far. 70 volts seems
to be enough to reliably blank all the cathodes on the couple of tubes
I tested.

The data sheet I have says the output stage consists of a push-pull
pair of N and P channel FETs. A switched-on FET will conduct current
in both directions; even a switched-off FET has a body diode that will
start to conduct 700mV or so above the rail. There may well be
protection diodes in there too.

So I expect the chip to be capable of sinking a few uA of current back
from its outputs into the positive rail, and testing seems to confirm
that this works.

> It's best to follow the datasheet for all possible operating
> conditions, but I think you could use a bleeder resistor across the
> MAX6921 pins and GND.

I'm honestly not convinced that this is necessary. I think the driver
can handle the leakage on its own while staying within its specified
operating range. 

> First, measure the leakage voltage on all of your nixie tubes with a
> DMM that has an input impedance of 10Megs. Basically, connect the
> anode of the nixie tube to the anode supply, and measure the voltage
> at all 10 cathodes to GND. If it's significantly lower than 76 volts
> and you get no noticeable glowing on the tube, you can use a 10Meg
> bleeder resistor.

A 10 meg meter reads ~60V (so it's drawing 6uA) and just starts to
light the tube ever so slightly. The leakage current at 70V will
presumably be significantly less, though I don't have the equipment to
measure it easily. I don't think this is going to stress the driver
too much.

(This does prove that I have to be careful using my multimeter to
measure these voltages, though!)

Mike

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