Low wattage AC transformer US voltage to Japanese?

I have looked around a lot trying to find out if there is a way to use a small Japanese AC clock on a US plug without using a big bulky transformer rated for 100 watts or more.

I looked into a voltage divider circuit that uses two capacitors to lower the voltage from 120 to 100. I think it would need resistor 1 to be 10uf and the second 2uf. I, however, am not sure what rating or impedance the capacitors should be. I have only worked on and tinkered with DC circuitry for the most part. There are just so many different types of capacitors that I get lost trying to find one that will work for this project.

I also want to be careful while using mains AC power. I think the clock only uses between 1 and 2 watts, so I think there might be a small modification that would work. If anyone has experience with AC voltage divider circuits or can point me in the right direction on what kind of capacitors I would need, it would be a huge help. It just seems foolish to spend 20 to 30 dollars on a big transformer on such a small appliance. I know this should be possible, but I have gotten very lost and confused.

by whizzard
May 13, 2021

What is the rated voltage and ac frequency of the clock? There should be a nameplate or label somewhere on it with this information.

by Foxx
May 17, 2021

Thanks for answering!! 100volt AC 60hz. (Japanese power standard)

by whizzard
May 17, 2021

Also the device is rated at 4 watts.

by whizzard
May 18, 2021

It sounds like you want a simple cheap and dirty voltage divider so here's one; see https://www.circuitlab.com/circuit/ytcm3bdn46aw/volt-divider/ When looking at the numbers don't forget that circuitLab shows ac voltages as peak values, not rms values so 120 vrms shows up as 120X1.414 = 169.68 and 100 vrms shows up as 141.4 . For resistor ohms use the nearest standard value and for resistor watts use the nearest size above the values shown.

by Foxx
May 19, 2021

Thank you so much for your time and all the great info.

by whizzard
May 19, 2021

I think these resistors may work... the first is RES 225 OHM 5W 5% AXIAL, and the second is RES 2.5K OHM 5W 1% AXIAL. These seemed the closest and over the wattage as shown in the diagram. I also noticed that the clock itself shows up as a resistor in the diagram (if I am interpreting correctly). Interesting how the power lead going to the device is coming from between the two resistors. I am learning so any other ideas or tips will help and I am always being as safe as possible.

by whizzard
May 19, 2021

OK the resistors you've chosen should be fine; axial or other type of lead is irrelevant. The 2500 ohms I chose to model the clock is based on its watt and voltage rating which you gave; 4W 100v. Watts = volts squared / R(equivalent resistance). A bit of algebra gets R=volts squared / Watts so R=volts X volts / Watts so R=100X100/4=2500. Actually other values could be used for the voltage divider but the calculations become more involved. I would mount the components on a terminal block inside a box with a plug on a tail in one end and a receptacle on a tail on the other end. If the box is metal it should be a 3 prong plug with the ground wire connected to the box.

by Foxx
May 19, 2021

Can't thank you enough for taking the time to teach others.

The clock itself only uses two prongs, I was assuming that one is live and the other neutral and treat neutral as ground? Should I make the project box a 3 prong regardless? Again this is my first time working with AC other than hooking up a few power supplies. I was planning on just using the two prongs originally but am not sure that is standard. I ordered some terminal blocks and project boards. I was also thinking of putting a fuse in as well, would that go before resistor 1 on the schematic? Thanks again.

by whizzard
May 20, 2021

If the clock itself has only 2 prongs it's what is commonly called "double insulated" and safe to use without a separate ground. I called for a 3 prong plug and separate ground if you use a metal box because an accidental short circuit connecting the high side of the 120vac to the box would put 120 volts on the box where you would probably get a shock if you touched it and any grounded metal in the room. Of course you can get around all this by using a non conductive box which would safely allow a 2 prong plug. Many people dismiss a 120v hazard as not too dangerous but I had the job many years ago of investigating an accident in which a child was killed by 120v so I take some care with it.

by Foxx
May 21, 2021

Nothing wrong with your idea of using a fuse and it adds a bit of safety for fires if nothing else. re neutral grounding: the neutral is tied to ground back at the utility transformer supplying your house and it should not be grounded anywhere else. This neutral is intended to carry load current and adding grounds to it leads to an indeterminate current path to the transformer. The safety ground is normally tied to a ground rod at or near your house and not intended to carry load current but is intended to carry fault current in case of a short circuit from the live wire to ground which would quickly blow a fuse or trip a circuit breaker. In addition a "ground fault circuit interrupter" or gfci if used compares compares current in the live wire and neutral and if different trips its breaker. I hope this does not add to confusion

by Foxx
May 22, 2021

No, you didn't confuse me at all; your descriptions are quite clear, all things considered. You confirmed what I was thinking, that two-prong would be fine, but now I know what that third-ground prong is for. I will use a plastic project box anyhow, but that makes sense if you do use a metal one. I also take AC very seriously, both for shock power and fire-starting potential. Again, you set me on the right path, and I might have more questions for you on this little project later. Once again, thanks so much for your time and knowledge.

by whizzard
May 22, 2021
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