Lance Allen wrote:

> I would consider rectifying the 220v AC to DC and then switching in a
> matrix using IGBTs.

  I was going to suggest Thyristors, but bit my tongue.  Why?  Well, you
must multiplex so that each lamp is pulsed at least 50 times per second.
For the same reason as you always requite a minimum multiplex frequency
of this order.  This means that for five rows, each row is driven for
4 milliseconds.  Note that your rectified DC must also be smoothed so
all rows get the same voltage!

  IGBTs it is!

  Also note - one diode in series with each lamp - this is the secret
to multiplexing incandescent lamps!

> Note that the supply to the lamps will at best be muxed at 5:1 so
> brightness may be a problem.

  Indeed.  Let's look at this.  At full brightness, the lamp has a
certain resistance.  Using rectified DC however, we are driving at 1.4
times the voltage, thus twice the power.  However, it is on at a 1 in 5
duty cycle, so receiving overall 40% full power.  If this is acceptable,
(it may be, to extend lamp life) then go ahead.

  Otherwise consider - using 110V lamps on the 220V supply, using the
PWM control to limit their full brilliance (and along the way,
compensating for actual power line voltage by measuring same).

  Also, you can consider "pre-emptive" filament warming of lamps at
least 1 second *before* they are switched to full brilliance, using
about 10% of full PWM time.  In other words, perform your animation one
second in advance!

  In case it's not obvious, this is a full-fledged PWM application and
also, if the micro "hangs" in mid-multiplex there's going to be
considerable smoke released unless you put in place a few protective
measures.  The WDT at *no* prescale is an obvious one to start, as is
using pull-ups to default to lamp drivers *off*.

> There are some seriously dangerous voltages and high insantaneous
> currents present so beware. (Standard warning for newbies who might
> not fully understand the dangers involved).  Kids... dont try this at
> home.

  Yep.  That's pretty obvious too!  It's a pretty exciting design.
--
  Cheers,
        Paul B.