Re: Alarm system transformer + power supply (would both go bad at once?)

["tm" <No_one_home@xxxxxxxxxxxxxxx>]

"Danny D'Amico" <danny@xxxxxxxxxx> wrote in message
news:pan.2013.11.27.05.16.09@xxxxxxxxxxxxx
> 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/7440/11078202723_a6deb1394f_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?
>

You will measure 16.5 volts RMS AC when the load is 45 VA.

Use a 3 amp fuse.

Can you check the current draw with your meter on AC Amps? Just for
reference.