Hi folks,
while cleaning out I found an old Numitron clock I once built and never
finished.
It uses six IW-9 Russian Numitrons who are driven by one ULN2803A each
who have their own CMOS 4094 shift register. The STROBE of the shift
registers is PWM modulated to adjust the brightness of the tubes.
I was trying to include a DCF77 module to turn it into a radio clock,
but it never worked, so I put the board away. Just for fun, I checked
the board again today with my 25MHz scope, and I think discovered the
culprit:
PWMing the Numitrons causes about 1V ripple on the supply. The DCF board
(designed for operation between 3.3V..5V) - probably can't manage the
supply voltage of unstable 4 to 5V. The noise on U_supply has the same
frequency as the PWM, and when turning PWM on 1 or 0 continuously, the
problem vanishes, and the DCF module works fine.
A long text, but the main question is: How do I resolve that issue?
I have already connected 100nF blocking caps near the uC and near the
DFC module, plus a 1uF elco next to the DCF module as well. The problems
stated above still occur.
I was thinking, maybe the Numitrons work like some inductance in the
circuit and do not like being PWMed, i.e. producing negative voltage
spikes? Then I could just add some freewheeling diodes which are already
included in the ULN2803A ics.
Any ideas?
Thanks,
Jens
--
You received this message because you are subscribed to the Google Groups
"neonixie-l" group.
To post to this group, send an email to neonixie-l@googlegroups.com.
To unsubscribe from this group, send email to
neonixie-l+unsubscr...@googlegroups.com.
For more options, visit this group at
http://groups.google.com/group/neonixie-l?hl=en-GB.
