On another forum, we were discussing ways to step down the voltage from 120 to 100 volts, this in order to make japanese radios work with USA voltages. Japan mains is 100volts. Somebody posted a suggestion. Somehow, I don't see how it will work. Since I trust you guys, please comment on whether this will work and why it will/won't work. Thanks.

Here it is:
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Heres a trick to reduce the voltage using a transformer.
Connecting the 24 volt out backwards to subtract voltage from the 120 volts AC.
Will give you close to 100 volt needed.

 

Yes it will work. You will have a 180 phase shift so the 24vac will buck the incoming voltage.
http://sound.westhost.com/articles/buck-xfmr.htm
http://www.acmepowerdist.com/pdf/Page_104-109.pdf

I worked for a Fujitsu fab for a few years, we had tons of 120 to 100 step down transformers for all the Japanese gear we used.

Like these. http://www.international-electrical-supplies.com/transformers12.html

 

Thank you. At first, I thought it would merely behave like 2 coils run in parallel, but then I was unsure how the inducted current would factor into the equation.

 

If you decide to build one, measure the output voltage before attaching to your 100V device. It is easy to get the secondary wires connected backwards. In that case, you get a boost to 144V. We used the 144V connection to boost response time of supply and take-up reels on punched tape readers. Original tape reader design was for paper tape and we used alumanized mylar, which was heavier.

 

Bill. Can you elaborate on what you mean when you say connecting the secondary backwards?

 

If the secondary winding leads are one way, the secondary voltage is subtracted from the line voltage. That is what you want. If, on the other hand, the leads are reversed, the voltages are additive. Since the primary and secondary leads are not usually marked for phase relationships between primary and secondary, testing is required. Sometimes, if you have a datasheet for a particular transformer, the information is available. Are you familiar with the dots on a transformer schematic and what they represent?

 

No. I always thought the dots simply represented tie in points or connections. They have special meaning? Also when you say reversed, you mean where the jumper (in the image of first post) that ties the primary to the secondary, right? So basically, if we flip the secondary coil of the displayed diagram upside down, that is what you mean by reversed?

 

Look at the attached drawing. If you looked at the transformer windings as batteries with the dots representing Positive, you can see the results.

No. I always thought the dots simply represented tie in points or connections. They have special meaning? Also when you say reversed, you mean where the jumper (in the image of first post) that ties the primary to the secondary, right? So basically, if we flip the secondary coil of the displayed diagram upside down, that is what you mean by reversed?

 

That make is much clearer, and what I was thinking. Thanks. One final question. What effect does these various add/subtract configurations have on the normal isolation mode VA rating of the transformer?

 

In that configuration, all isolation is lost. I should have pointed that out. The current out will out will be limited to the current rating of the secondary.

 

Ok. Thanks, you guys are the greatest.

Yes, I could see that isolation is lost. But since the radios themselves do have isolation transformers installed to further step down the voltage to 12 or 15 volts, that's not as big a concern.