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.

Your explanation helped me understand, thank you.

a PEAK sinusoidal of 40 volt-ampere yields power of 20 Watts, remember
that pesky square root of 2 floating around to convert the peak of a
sinusoidal into its DC power equivalent?  It comes into play here,
twice.

has to do with the rms calculation

Interesting way to make the 'spec' look better.

I always thought those ratings were average, or at least rms, NEVER
thought they were relating to the peak through the transformer. It
never donned on me that a 40VA transformer should only supply 20Watts
of power. Now, THAT is embarrassing!