On 3/11/12 3:09 PM, Cobra007 wrote:
Fair enough.
I did measure that I-V curve long before I started drafting the
circuit as I had to figure out how long it took before ionization
would start and how much dimming I could get out of these tubes. For
my concept watch I used ULN2803 drivers which work perfectly fine in
most cases, but not in low light situations. As I only drive them with
very low energy in low light, there is usually 1 tube that won't light
up for a very long time (could be a couple of seconds). For the tube
that doesn't light up I need a higher voltage, which I can't get to
with the ULN drivers as the current starts to flow through the tube
that was already on (because of the zener clamping diode).
Michel
A fine way to achieve the PWM dimming you want with a low-voltage Nixie
driver is to have a current measurement device in the power supply's
feedback path, which is conveniently provided from the common emitter
terminal of the TD62083 (ULN2083).
You can detect when the tube begins to ionize by sensing the current
using a comparator such as an LM339 working across a small resistor to
make <1V drop from emitter to B-, and use this time to start the on-time
countdown in the PWM dimming code. You then have to reduce the OFF time
after that pulse by the measured ionization delay.
The advantage of this method is that it will produce a more uniform
brightness in your dimmed display, since you are guaranteed that the
ionization time is taken into account. Plus, you can use the small,
low-cost driver chips.
--
David Forbes, Tucson AZ
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