Hi

I build something similar for an musicinstrument with 30 dc-motors and each
rated to 12V/1,25A which results in a lot of powerwhen "flashing" all
motors.....

I used a normal 15V/40A transformer with an dimmer in front (the basic can
be found on www.sts.com, AN 308, I replaced P1 in figure 11  with an
optocoupler)

The dimmer was controled from an opamp-circuit so that the outputvoltage is
held on 13.8V, which is enough to hold a lead acid-battery in the 95% full
state.

I also included a simple current masurement between transformer and battery
so I can see the actual load of the transformer, which is a good visible
feedback.

Also I added a softstart-circuit, which looks for 230V and if primary power
is connected, it starts the transformer with a low current to avoid high
inrushcurrents when connecting transformers with this power to 230V.

The battery I used was 12V 30AH, I took it from a car-mechanican, who gave
it to me without cost.....

When the peak current occurs, the battery supports the transformer and when
it comes back to normal currents, the transformer supports the battery. I
know that the high currents loading the battery are not the optimum for
this kind of battery, but this was not such a problem, because the battery
is a carbattery and it can never become uncharged, the system had to be
connected to 230V after the show at least 45 minutes.... (which was no
problem at all, after the show we had to answer questions, eat and drink
something.....)

This system worked for several years without problems (well, I had to learn
how to work with lead acid-batterys....)

It is very simple to adapt this for much higher loads. Maybe you have to
connect a loadresistance direct on the secondary side (15V) to hold the
"inductive part" of the transformer low (important when the battery is full
and no more current from the transformer required)

One of the great things is, that most parts are not very sensitive
electronic parts, even the dimmers triac, rated with 100A is a very robust
one, so usualy the fuse (F6,3A) blows earlier then the triac. And if 1 lead
acid battery isn`t enough, use a bigger one....


but it is heavy (30kg), and you need some kind of (slow) cooling, the
rectifier and the transformer get warm when running with 80% load.....


Michael Wieser
m.k.w@nextra.at

Service and Audiodesign