On Tue, 26 Nov 2013 23:01:57 -0500, tm wrote:

That's normal. The secondary winding is floating.

Whew! I had expected a center-tapped transformer or something.

Thanks for setting my mind at ease on the odd voltages to the
ground lead.

The 20Volts I measured is probably because the RMS voltage must
be the 16.5Volts, so *that* part at least makes sense.

What voltage do you see across the aux terminals (DC)
with the transformer connected to the alarm board?

I measured 13.61 volts across the DC terminals with the AC
transformer in place.

That seems good, to me, if a bit low for charging a 12V battery.
http://farm8.staticflickr.com/7440/1...deb1394f_o.gif

Get a fuse in line with the secondary to save the transformer
in case something is shorted.

The transformer has a PTC fuse in the secondary.
But, I *do* agree with you.
I'm shocked there is no fuse.

I even asked at the intrusion alarm houses I visited today.
They said the board itself is protected from overloads, so,
nobody bothers with a fuse.

This reminds me of the situation with garage door torsion
springs where the "industry" puts in the cheapest spring
saving *them* ten bucks, but if you put in your own spring,
you *always* opt for the better (longer life) spring!

For me, I agree with you. I'm heading off to RadioShack
or Frys to get an inline fuse holder.

You don't want to damage the new transformer. I would
use a fuse equal to the secondary current rating marked
on the transformer.

It's a 16.5VAC (RMS) output, with a power of 45VA, so,
given I=P/V, I get I=45/16.5, where I is 2.7 Amps.

So, how does double that sound for a fuse, e.g.,
roughly around 5 Amps for the inline fuse for the
transformer secondary?