I'm building an Arduino based coffee roaster, and I'm looking to power it by mains voltage in the U.S. (~120VAC, 60Hz).

What would be ideal would be a single transformer with a total of 18-20V AC secondary with a non-center split, something more like a 8V / 12V split. Is this asymmetric splitting even possible electrically? If so, is it sold at reasonable volume?

Single Output (6V - 8V): would power the Arduino, bluetooth module, thermocouple breakout board, solid state relay, 4x DC Relay coils and two ~6V vibration motors to the tune of ~2A maximum.

Both Outputs (18V - 20V): would power the fan motor to the tune of ~3A maximum.

Since I was unable to find anything asymmetrically split, I went with a 100VA Toroidal below, with two 9V outputs @ 5.5A maximum, but I'll have to drop ~4V to get to the Arduino's 5V logic level, and since that's so far I'll be using a switch mode power supply to drop it to ~6.5V before the Arduino for the current demands, which wouldn't be necessary if the transformer output was closer to 6V.

Toroidal Transformer

Are there transformers made with two different coil values, something closer to 6V/12V or 8V/12V for around these power requirements?


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  • \$\begingroup\$ Have you tried browsing e.g. digikey for what they have available? \$\endgroup\$ – PlasmaHH Oct 12 '16 at 8:24
  • \$\begingroup\$ Yes, and eBay, but for these power ranges all I'm finding are symmetric splits (2x6V, 2x7V, 2x9V, etc.) \$\endgroup\$ – Ehryk Oct 12 '16 at 8:25
  • 1
    \$\begingroup\$ Why not use a dual output switch mode power supply of about 100W. You can get these with a regulated 5V and 15V, 20V or 24V output. \$\endgroup\$ – Steve G Oct 12 '16 at 9:08

Yes, split-voltage transformers exist. The 12-0-12 and 5-0-5 transformers use a center tap, but there are other "unbalanced" transformers that are expressed differently. Instead of being called 5-0-12 transformers (or whatever), they're simply referred to as 0-5-12V ones.

I have used both of the following commercially-available transformers to do much what you describe - with a caveat. Although this is an Australian company (thus the input voltage is 240V), I can well imagine that their equivalent would be available in the US:

http://www.soanar.com/store/category/367/product/mm2015.aspx http://www.soanar.com/store/product/mm2011.aspx

The caveat is that the 15V device (powered from the 0-15V pins) didn't communicate with the 5V device (powered from the (12-18V pins). They don't share a common ground, so you'd have to do something extra to get them to communicate.


"I'm building an Arduino based coffee roaster, and I'm looking to power it by mains voltage in the U.S. (~120VAC, 60Hz)."

To properly design ANY project power requirements, you should define the tasks and available power needed then choose an off the shelf low cost reliable supply to power your needs.

A proper system design is not always MAKE but rather define the requirements so that more flexible options for make/buy decisions can be made easily.

  • This is a daily routine job for Engineers to make these decisions and write specs accordingly.

e.g. for each load

  • Max Power =
  • Input voltage range =
  • input current range =

Then try to use mass production PSU's such as ATX 350W $35 or $500W $50 with 12V, 5V etc. or 19.5Vdc universal laptop chargers with DC-DC converters or Printer Wall supply or any commercial PS for for ease of integration and no UL/TUV/CE certification costs required. R&D costs must always be weighed with all make/buy decisions EXPECIALLY when AC line safety certification costs are involved.

  • \$\begingroup\$ gotcha .. reworded to reflect my opinions and experience \$\endgroup\$ – Sunnyskyguy EE75 Oct 12 '16 at 13:31
  • \$\begingroup\$ I see; in my case I want to power a blower fan of up to 18V * 3A = 54W, and an Arduino with bluetooth module, and various relays and logic devices of up to 5V * 1A, but from the 9V output, so up to 9V * 1A = 9W in the case of a linear regulator. \$\endgroup\$ – Ehryk Oct 18 '16 at 18:15
  • \$\begingroup\$ (I actually want to build the power supply, for learning reasons, and have it all on display on a board with the roaster). I ended up going with this 50VA transformer, which means I'll need to relax the maximums a bit (2.7A max): digikey.com/product-detail/en/amgis-llc/L01-6381/TE2268-ND/… \$\endgroup\$ – Ehryk Oct 18 '16 at 18:17

The Arduino can switch 12V relays low-side, and also lowside switch your 6V motors to run at 50% PWM from 12V. So you can use a single 12V 5A supply for all the electromechanical components, leaving you with needing around 0.1A at 5V. You could regulate this down from the 12V, use a 12v to 5V DC/DC converter, or use a separate 5V 0.1A supply. Any of these is likely to be cheaper and easier than making a dual output linear power supply. For a one-off I'd tend to get a separate 5V power supply and not have to worry about any interference.

For a one-off mechatronic project it's probably better to use power supply modules rather than building a linear supply with a toroidal - usually that amount of effort is required for high quality audio rather than fan motors.


Since you are looking for reasonable volumes of the required transformer I would suggest the following.

1.Complete your prototype of the project with whatever transformer(s) you can find. The prototype only has to function properly and safe.

  1. Make an estimation of the required volume.
  2. Get quotations for the parts needed including a custom made transformer. When volumes are involved I am sure they end up not that expensive.
  • \$\begingroup\$ I'm not looking to purchase in volumes, I'm wondering if there are existing 'close enough' values that are already produced in volume, or perhaps some close-enough alternatives like 3x6V or 3x7V secondaries. \$\endgroup\$ – Ehryk Oct 12 '16 at 8:37

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