Isolation transformers output voltage higher than wall

Hey,

Trannies often are wound so the input to output is a little more than 1 to 1 when no load is applied. This is to compensate for inefficiencies that happen under load.

That is, with no connected devices it’s 1:1.05 but when the load is applied the output voltage will drop to around 1:1.

Acceptable AC voltage range in the US is 110V to 126V.

I have been in an apartment in San Francisco with 130V and PG&E did rebalance their loads to bring that down in a couple of weeks.  They said it was safe but not ideal.

Personally I notice a daily variance of 15V and use a voltage regulator as the front end of my audio conditioner chain. This happens even without the stereo being on or the heat pumps engaged. The VR keeps my voltage consistently between 117 and 122, regardless of the appliances or stereo state.

https://amzn.to/3IrSpXX

PS - Here is a good resource on acceptable voltage ranges:

https://voltage-disturbance.com/voltage-quality/voltage-tolerance-standard-ansi-c84-1/

erik_squires

Acceptable AC voltage range in the US is 110V to 126V.

That is mistaken. Nominal US voltage is 120VAC, most states regulate that at ±5 percent, so the range is between about 114VAC to 126VAC. That's actually explained in the link you provided, which also has other useful info.

Thank you both as the information is helpful.

@erik_squires Does it make sense to place an isolation transformer between the AC receptacle and a Furman Pst-8 which has all my digital gear plugged into it?  Secondly, should digital that use LPS still be on the "digital side" with other switching supplies or separated from them? I'd be interested in your thoughts on this. Thanks!

Hi @cleeds - I think the difference between your numbers and mine is service (what the power company provides) vs. voltage at the outlets, no? The minimum acceptable voltage at the outlets I believe is therefore lower, as I interpret the data on the link site. Still, we are not in disagreement that the max is 126 VAC.

The difference is that the NEC allows for a certain amount of sag in the household.

@jc4659 Depends how fancy you want to get. Ideally the two are combined, so the isolation transformer and the AVR are the same transformer. Makes for less power loss and higher efficiency. The best place for a voltage regulator otherwise is after any isolation transformers.

I think it’s most important to isolate cheap wall warts and digital supplies. So put any Ethernet, Roku, gaming appliances outside your clean zone.

What are you measuring with? There could be as much as a five to six volt difference between true rms multimeters and averaging multimeters when measuring wall voltage. All multimeters are accurate with a clean sinewave power but not with wall voltage.

It's a small compact plug-in meter with a display. My wall voltage does not drift much. It typically is 121-122 volts.

I've seen those cheep plugins that were accurate and those that were not.  

Anyway, you've got nothing to worry about.  Other than your transformer may be doing more harm that good.  

Jerry

@carlsbad I always listen to music with which I am very familiar and if I sense the sound quality is compromised in any way then I won't use them.