Re: Alarm System Transformer

[Robert Macy <robert.a.macy@xxxxxxxxx>]

On Dec 19, 12:25=A0pm, Jim <alarmi...@xxxxxxx> wrote:
> On Dec 19, 9:47=A0am, Robert Macy <robert.a.m...@xxxxxxxxx> wrote:
>
>
>
>
>
> > On Dec 4, 6:59=A0pm, Jim <alarmi...@xxxxxxx> wrote:
>
> > > On Dec 4, 1:11=A0pm, H Brown <hehbr2...@xxxxxxxxx> wrote:
>
> > > > My home alarm system transformer is dead. =A0Parameters off the uni=
t say
> > > > 16.5 VAC, 20VA. =A0I found a replacement unit that has 16.5VAC, 40V=
A. =A0I
> > > > think the 40 VA is 40 watts output which I believe should work sinc=
e
> > > > my previous transformer only put out 20 watts. =A0Correct?
>
> > > It doesn't matter in this case .... because you are using a larger
> > > wattage transformer .... However ... keep in mind that VA is not
> > > equivalent to watts. A 40 VA transformer is only about 20 watts.
>
> > Never heard that 40VA into a resistive load is only 20W
>
> > Elaborate?
>
> It has to do with the difference between DC power and AC power.
>
> DC power is static. AC power is dynamic .... that is ....it is
> different at every stage of the 60 cycle/hertz sine wave. As I
> understand it ( and I don't entirely) you'd have to measure what the
> current is at every point along the sine wave and average it, to get
> the wattage of an AC device. So they do .... and they call that VA or
> Volt/Amp. However it really has nothing to do with the wattage of a DC
> circuit which is still measured in watts. So .... forgetting all of
> that, for our purposes in the alarm trade, =A0if you just consider that
> anything that is listed as VA ..... generally speaking, is equal to
> approximately 50 to 60 percent of the VA rating .... in watts.
>
> You can google it, but it all come down to the difference in the
> formulas when applying Ohms Law to AC and DC circuits. Most people
> don't even know that there's an AC Ohm's law or that it's different
> than DC.
>
> I never did find out why ... that after years of specing transformers
> in Watts, all of a sudden they decided to change from Watts to VA, but
> I think ,,,, or ,,,,, it may have had something to do with the growth
> in use of =A0UPS power supplies, because they needed a way to determine
> how much battery power (DC watts) was necessary to provide enough line
> voltage output (AC watts) so people could determine what size UPS to
> get. I say that because most of the explanations that you see on line
> mention UPS power supplies as examples. But .... could be it's just an
> easy way to show the difference.
>
> Google it if you need greater detail. It's all in the math.

Now THIS has gotten very confusing!

from URL:
<http://whatis.techtarget.com/definition/0,,sid9_gci539345,00.html>
"In an AC circuit, power and VA mean the same thing only when there is
no reactance."
which is what I always thought.

That means a transofrmer rated at 40VA can supply a 40W bulb with NO
problem, because the bulb is resistive and has no reactance.  However,
supply anytrhing with a 'strange' load, a motor or something and it is
likely the VA rating will be exceeded in order to supply the 40W to
this 'unknown' load.

So, my conclusion is that for good margins, use a 40VA to supply 20W
of 'unknown' type. There does not appear to be any other numbers
associated with the VA rating that I could find in order to 'derate'
it to lower power..


Doing the internet search resulted in finding some very confusing
wording of this information floating around on the internet.


Again, the VA rating of a transformer means that it can supply up to
the rated voltage in rms and up to the rated current in rms, but be
careful in thinking ONLY in terms of monitoring the load's power. It
is possible to exceed the VA rating of a transformer when tyring to
supply the VA rating's equivalent wattage *IF* there are any reactive
components to the load.

Whew, I hadn't been doing this wrong for all these years.