isolation transformer - high voltage.

[atmars]

I have about 124v at the wall outlet in my area. I have a 1:1 isolation transformer between a amplifier and the wall. Even thought the transformer is 1:1 the voltage it is putting out is closer to 130. Any ideas why this is the case? The transformer is rated at 115v . Could that be the reason? I thought the the idea of 1:1 is that the windings are identical on both sides, so what goes in one side comes out the same one the other. If the windings are not identical for some reason, could I reverse the input side and output side to reverse the voltage imbalance (high at the wall, lower at the amp).

I need the transformer for safety, but it is making my already high outlet voltage even higher.

Thanks.

[Frank Berry]

As you load the secondary of the isolation transformer, the voltage will fall.

[Corax]

Isolation transformers are designed to take into account the copper and iron losses at the rated output power. Therefore the turns ratio is not exactly 1:1, in fact the secondary side has a slightly higher turns count than the primary side. In the data sheet of that transformer it should be stated which side supposed to be the primary (input) and which side supposed to be the secondary side (output). If the 130 volt output is too much for you (or your equipment) just connect it the other way around. It won't hurt the transformer, but consider an even lower output voltage which drops further with increasing load.

[Eli Duttman]

Atmars,

Frank nailed it. The trafo is wound with a tiny step up to compensate for the losses (Copper, eddy, etc.) that must occur, when a load is applied.

BTW, 124 VAC at the wall is not out of specification. For quite some time, the North American power standard has been 105 - 125 V. What has changed is the average line voltage, which has gotten higher.

[zigzagflux]

Originally posted by Eli Duttman
For quite some time, the North American power standard has been 105 - 125 V. What has changed is the average line voltage, which has gotten higher.

That's pretty close, Eli, but some history helps understand the reasoning. IEEE 141-3.1.2 lists two ranges. Range A is typical operating conditions, which is 110V minimum, 126V maximum for utilization levels. Range B extends this window to 127V - 106V for limited duration excursions. Those customers provided with single phase secondary service (just about all of us users) can expect these windows, as the Public Service Commission requires the utility provide these levels.

The original NEMA specification was 115V +/-10%. The specification was governed more by induction motor requirement than anything else. This was later amended on a 120V basis, and became the Range B above. Also added was Range A, which addresses more consistent building lighting, to 120V +5% -10%, with some additional provision for voltage drop in the facility wiring.

At the end of the day, 120V nominal, +5%, -10% is the best way to look at it. Make sure your equipment is designed for this range, and you'll be good.

If the voltage at your panel deviates from 110V-126V on a continuous basis, call your utility; they are outside of ANSI/IEEE specs.

The "Red Book" is a great read for those interested.

[atmars]

Thanks everyone.

If the 130 volt output is too much for you (or your equipment) just connect it the other way around. It won't hurt the transformer, but consider an even lower output voltage which drops further with increasing load.

I am using this isolation transformer to run a transformerless amp from the fifties. The schematic and the back of the chassis say the amp wants to see 117v, so I thought 130v was way high. I don't know how much the amp loads the transformer, but the voltages are higher with transformer than they are with power straight out of the socket. I'll flip it around and see how that goes.

[dave slagle]

turn the transformer around and see if you are closer to your desired 117V.

dave

[atmars]

Reversing inputs and outputs results in nearly 117v on the money. Thanks for the help.