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?

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.

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.

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.

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!

"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.

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.

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.

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

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.

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