Alarm System Transformer

[H Brown]

My home alarm system transformer is dead. Parameters off the unit say
16.5 VAC, 20VA. I found a replacement unit that has 16.5VAC, 40VA. I
think the 40 VA is 40 watts output which I believe should work since
my previous transformer only put out 20 watts. Correct?

[Bob La Londe]

"H Brown" <hehb...@gmail.com> wrote in message
news:c3332c00-f2fd-4c48...@z12g2000yqm.googlegroups.com...

It should work just fine.

[Jim]

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.

[Just Looking]

Well here is the rub. A 40 VA transformer is going to have a fusible link
inside nearly always. A 20 VA most likely not. If you popped a 20 VA because
of a problem then you might smoke a 40 VA in an instant.
For a few extra bucks you can get a self restoring transformer. The rub
there is that certain alarm panels (like Bosch G Series) go nuts with a self
restoring transformer.

"H Brown" <hehb...@gmail.com> wrote in message
news:c3332c00-f2fd-4c48...@z12g2000yqm.googlegroups.com...

[Robert Macy]

Never heard that 40VA into a resistive load is only 20W

Elaborate?

[Jim]

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, if 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 UPS 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.

[Robert Macy]

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!

[Robert Macy]

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.

[Jim]

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

I think it still all boils down to ..... 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 you pay a few bucks more for a
transformer that is twice the actual rating that you 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. 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.

[Robert Macy]

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,

[RockyTSquirrel]

using Kirchhoff's law of AC I'd go with the 70.7 % to be on the power side
of the sin.. wave power curve..
best guessing of course which side of the transformer they were using for
their power calculations and the Q factor of the transformer in general
for it's transfer of energy, primary to secondary ...

Merry Christmas and Happy New Year to all..
RTS




"Robert Macy" wrote in message
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