Hi Mike, it is done. the output is now 119.5V. Thanks! one question: how much of variation in voltage can an equipment tolerate? How to calculate it?

Most equipment is designed to tolerate at least +-10% variation in the input voltage. Look at the detailed specifications for the device in question.

 

Yes, but if you dont put a switch in the primary, the transformer will always be on (even if the stereo is off) and be wasting substantial power.

How much power are you considering substantial? My books say that a ~250 VA transformer would likely only be 5 - 15 watts at best.

 

15W * 24h/day * 365 day/yr *1kW/1000W * $0.14/kWh = $18.36/yr. That will pay for a switch in the first three months...

Electric rates in most other countries are higher than ours...

 

Most equipment is designed to tolerate at least +-10% variation in the input voltage. Look at the detailed specifications for the device in question.

Dear Mike, the pictures are the final product, ugly, but it costs nothing since I just grabbed whatever by hands. I have tested it and it works very well. Surprisingly, the "turning on noise" some customers complaint about commercial available transformer does not exist with this one. I have not been able to find a switch yet, but I will keep look around. A question: while I opened the box (after I have tested the voltage output), a friend aside was afraid I am electrocuted, since he believes the winding may still carries high voltage. I tested all the terminals and nothing was there. But is it possible to carry some voltage at certain circumstances? Some of the appliance would have lethal voltage warning even when you disconnect the power, how would the disconnected coil carries high voltage? It may be complicated to explain, what book shall I look? Thanks!

 

If the Line cord is plugged in, then you may have lethal voltage if you touch the transformer terminals. (not on the unconnected secondary).

If the Line cord is not plugged in, then you can touch anything with impunity.