# How much output is 7VA?

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I have a MRC transformer model 1300. It is marked 7VA output. Is this just another way to say .77 amp if it is 110 volt?

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No. Its output is 7 watts maximum. However this does not equate to

7/110 amps at the wall because of the power pack's internal losses. Transformers buzz, produce heat etc all of which takes power, as does the electronics.
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The math would be 7/110=.064 A, but it is actually a little more complicated than that. The power=current x voltage is precise for DC circuits but not for AC (transformers). See this site for a pretty good explanation

this one for more grueling detail
Are you concerned with the current draw on the line side (110 V circuit) or with the current output on the DC side?

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A unit of electric measurement equal to the product of a volt and an ampere. For direct current it is a measure of power and is the same as a watt (P=IE); for alternating current it is a measure of apparent power.

Also: a unit of electrical power in an AC circuit equal to the power dissipated when 1 volt produces a current of 1 ampere.

So a 7VA transformer or power supply for all practical purposes is roughly equivalent to 7 Watts.

Bob

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It's a measure of the DC output: VA = volts x amps. So, if it's 14 volts DC to the track, then it's 1/2 amp.

-- Bill McC.

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Bill McC. replied: It's a measure of the DC output: VA = volts x amps. So, if it's 14 volts DC to the track, then it's 1/2 amp.

--------------------------------------------------- Yep! That makes the answer easy to figure out. Thanks for simplifying things.

History of N Scale:
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"> Are you concerned with the current draw on the line side (110 V

I was trying to compare it to the new Kato power pack which is simply marked "1 amp".

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Probably then the Kato delivers 1 A at 18V (or 16 or 14, however it is rated) which would give 18VA on the DC side compared to 7VA for the other so the Kato is marked with a higher power rating.

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OK, you guys all sound like electrical engineering geniuses, but not much consensus. We got 2 votes for 1/2 amp. So, may I assume that it is about

1/2 as powerful as the new Kato n scale power pack that is labeled "1 amp"?
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Ok, so we want to "normalize" the ratings of various power packs so we can compare them. Reasonable thing to do, should we be into comparison shopping. The 7 V/A rating is an "input" rating, a measure of the amount of power the pack will take from the wall socket. The "1 amp" rating of the Kato is an output rating, the Kato will deliver 1 amp at 12 VDC to the track. Let's assume the power pack is 100% efficient, that all the power that goes into the input comes out the output. Lets use this assumption 'cause it makes it easier to figure, and it isn't too far off. Practical power packs probably are 80-90% at full throttle, so let's use

100% as a ball park figure. Power is volts times amps. To make a motor rotate or a lamp light up we have to run current thru it as well as maintaining the voltage across it. So lets convert the two power pack ratings into watts and see where we are.
1. The 7 V/A lets just call that 7 watts. The "volt/amps" thing is a complexity that takes power factor into account. Power factor is of interest to people who's day jobs deal with electricity and electronics. For model railroading we can ignore power factor and call a volt amp equal to a watt.
2. The Kato's one amp output is likely speced at full output voltage to the track, which is 12 volts. So the Kato can do 12V * 1` amp or 12 watts out to the track.

This makes the Kato nearly twice as powerful as the other pack. If you figure the 7 v/a pack is less than 100 % efficient then the Kato is twice as powerful as the other one.

The low cost little packs that come in train sets still use just a rheostat to control train speed, where as the better packs like MRC and Kato use some kind of transistor throttle which will give better slow speed running than a plain rheostat can. The pricier pack will probably have a nicer "feel" to it. On the other hand, the low cost pack will make the trains run, and the nicities of a better power pack will not matter much to children.

David Starr

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That's my assessment; 1 amp at 12 volts = 12 VA.

-- Bill McC.

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Dictionary sez this is an approximation.

BYW, 120volt is now standard.

Bruce

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wolfee wrote: I have a MRC transformer model 1300. It is marked 7VA output. Is this just another way to say .77 amp if it is 110 volt?

---------------------------------------------------- The output voltage has been reduced from the input voltage through the transformer. 7va at 14v is 1/2 amp.