``Cheap'' AC Power Conversion from
120 to 100 Volts AC

-An anonymous, conservative, engineer

Someone mentioned wiring a 24vac transformer (such as a doorbell transformer, or electronics store power supply transformer) as an autotransformer to drop USA voltage (117V nominal) to near Japanese voltage (100V nominal). After I saw this sentence I realized how easily somebody could endanger human life doing this carelessly and how wordy it would be to give complete directions. These are the most obvious dangers:

  1. Lifting the ground: USA power has a neutral (grounded, sort of) lead for one wire. If If the autotransformer is wired improperly, neutral will rise many volts above true ground, and any ground loop problems will become "line powered," meaning they can make smoke and fire as well as melted MD devices.
  2. Boosting rather than dropping the voltage: possible smoke and fire, probable melted MD devices.
  3. Painful or lethal electric shock because the connections are inadequately or improperly insulated from human or animal contact.

This being said, the project can be started on a breadboard, hence the term "breadboard circuit". You will be working with a high-energy source -- the power line -- which can electrocute you almost as easily as not, and doesn't care one whit that you are too nice a person to die so young.


Parts List

  1. 117 VAC -> 24 VAC doorbell or power supply transformer
  2. Fuse holder with 250 VAC, 1 Amp slow blow fuse
  3. AC line cord, with AC plug attached
  4. AC receptacle
  5. Breadboard (Do not use a wood breadboard, this can conduct electricity and kill you. Some ceramic breadboards are also conductive, and some plastic breadboards are flammable. Formica or Micarta are good electronic "breadboard" surfaces.)

Assembly Instructions

  1. Be certain that you are working in an appropriate work space with adequate light, heat, ventilation, and an electrical power source suitable for developing power-line devices, including GFCI, proper polarity and grounding. Comply with OSHA and obtain all necessary permits &c to do the work.
  2. Connect the 117vac neutral (grounded, sort of) lead to one 117vac lead. The neutral is on the wide spade of the plug, on the striped side of the zip cord, as long as they are not miswired (altogether too common).
  3. Connect the 117vac hot wire to a 1amp 250vac slow blow fuse.
  4. Connect one 24vac lead to the other side of the fuse.
  5. Connect the neutral of a receptacle (the thing you plug a wall wart in to) to the neutral connection at the 117vac transformer input. (This is the connection made in #1.)
  6. Connect the hot of the receptacle to the remaining 24vac lead and the remaining 117vac transformer lead.
  7. If the 24vac winding is center-tapped (not common in doorbell transformers, but quite common in power supply transformers), make no connection to the center-tap lead.
  8. Make certain all the connections are mechanically solid, electrically conductive, and insulated from human contact.
  9. Plug in the 117vac and read the output at the receptacle with a voltmeter. It will be either less than the (nominal 117vac) input voltage, about 100vac, or more than the (nominal 117vac) input voltage, about 140vac. (Note that US nominal 117vac can be as low as 105vac or up to 125vac for 90%+ of customers, worse during brown-outs and surges).
  10. If it reads about 140vac, reverse the 24vac wires and try again; it should read about 100vac this time.
  11. Perform electrical leakage testing, heating factor, maximum permissible current measurements and calculations; prepare standard engineering worksheets to document compliance with all the standards.
  12. Package it neatly in a proper enclosure, comply with all the provisions of the National Electrical Code, and submit it to Underwriters Laboratories for certification.
  13. After you have all your government approvals you can use it to your heart's content.
  14. Obtain product liability insurance, design proper shipping containers, write a Users Manual to protect the innocent, sell a million of them, move in next door to Bill Gates, and write articles about "Barriers to Entry."

Perhaps this is too much for the technical ability of the average MDer, although it is not much of a trick for any electrically oriented techie. What's missing here is a discussion of checking the wall socket for proper wiring and a discussion of the comparative benefits of a three- wire grounded implementation vs a doubly insulated two-wire version; maybe somebody else will contribute them.

There are many additional concerns that all we who have been designing have worked around forever, but which may be entirely novel concepts to newbies. I remain anonymous in this article because I want nothing to do with putting a loaded gun in the hands of an MD fancier. At the same time, this can be the answer to 100vac for about the cost of a cheap MiniDisc, and may make a big difference. There have been times in my life when knowing how to get by cheaply has made the difference, and I don't want to hold back information that might help others any more than I want to lead them recklessly into danger.

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