Re: Alarm System Transformer

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

On Dec 19, 9:48=A0pm, Jim <alarmi...@xxxxxxx> wrote:
> On Dec 19, 6:36=A0pm, Robert Macy <robert.a.m...@xxxxxxxxx> wrote:
>
>
>
>
>
> > 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=
 unit say
> > > > > > 16.5 VAC, 20VA. =A0I found a replacement unit that has 16.5VAC,=
 40VA. =A0I
> > > > > > think the 40 VA is 40 watts output which I believe should work =
since
> > > > > > my previous transformer only put out 20 watts. =A0Correct?
>
> > > > > It doesn't matter in this case .... because you are using a large=
r
> > > > > 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 D=
C
> > > circuit which is still measured in watts. So .... forgetting all of
> > > that, for our purposes in the alarm trade, =A0if you just consider th=
at
> > > 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, bu=
t
> > > 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 determi=
ne
> > > how much battery power (DC watts) was necessary to provide enough lin=
e
> > > 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 a=
n
> > > 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. =A0However=
,
> > 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.-
>
> I think it still all boils down to =A0..... if you don't know exactly
> what kind of load you're powering, cut the VA in half. And as long as
> you're not designing or engineering products, the worst thing that can
> happen in the alarm trade is that =A0you pay a few bucks more for a
> transformer that is twice the actual rating that =A0you need. For my
> purposes, that's all I need to know. It's the same formula that I use
> for specing infrared distance capability in CCTV cameras. What ever
> distance you want to see at night, buy a camera with twice that
> distance capability. =A0 =A0 Just like the VA rating .... the worst that
> could happen is, you have something better than you actually need
> under normal conditions ..... at a minimal cost.
>
> and ......... anyway ....... I not a injunear.

Two to one, interesting. For reliable design that is the same derating
we used to apply to resistors. If the resistor was 1/4 W, you only ran
it at 1/8W.  ALL, based upon some prediction of MTBF published by IBM,
and drawing conclusions as to maximizing the component cost versus its
lifetime [from memory]

2:1, seems reasonable to me. That would take care of the products that
don't quite meet the full spec, and a lot of companies ship those.

But, it means, if you're in a bind, you could probably safely assume
you can power 30 or even 32W with a 40VA and not compromise it.

That's applying the 80% rule. The same IBM publication suggested
operating caps at 80%, or less, of their rated voltage.

Regards,