I've got a device (microcontroller-based energy monitor, https://shelly.cloud/knowledge-base/devices/shelly-em/) which I'm using to measure energy consumption of 240V split-phase devices (motors). Since the device is powered from one of the phases, it believes the voltage used by the monitored device is 120V, when it's actually 240V.

The device can't be powered by using both phases, it requires a neutral, but it can handle supply voltages of 240V. I'd like to use a small step-up transformer to provide it a 240V supply, but most of the devices I can find are designed for much higher current loads and are thus both larger and more expensive than needed here.

How would I go about choosing a proper, plain, transformer to use in this application?

  • 7
    \$\begingroup\$ How does the device know the difference between a neutral or the other phase? \$\endgroup\$
    – Kartman
    Feb 21, 2022 at 12:17
  • \$\begingroup\$ Why just do not keep in mind that the measured value is 0.5 of real? And maximum voltage is 230V, do not exceed it. \$\endgroup\$
    – user263983
    Feb 21, 2022 at 12:49
  • \$\begingroup\$ Or is the same version that is sold in EU, where RMS is 230 V and peak 325 V? Which would make 110 2-phase 180 degree phase angle to be within limits? \$\endgroup\$
    – Ralph
    Feb 21, 2022 at 15:37
  • \$\begingroup\$ @Kartman: That's a good question; I'm assuming that it wouldn't be safe to connect to 240V across a split-phase system, but I don't have enough knowledge to know why. I've contacted the manufacturer for guidance and they may be able to answer that question. \$\endgroup\$ Feb 22, 2022 at 13:12
  • \$\begingroup\$ @user263983: It's integrated into Home Assistant along with many other energy-monitoring devices, so the data needs to be consistent with them. \$\endgroup\$ Feb 22, 2022 at 13:13

1 Answer 1


So, simple; go and buy a step-up transformer and make sure that the rating of the transformer is enough, or the output power you want is less than the rating of your transformer.

  • \$\begingroup\$ It appears that the only low-cost way to do that is to buy a 'travel' step-up transformer; searching Digi-Key for bare transformers the least expensive I can find is more than US$50, because it is rated for 60VA. \$\endgroup\$ Feb 22, 2022 at 13:14
  • \$\begingroup\$ @kevinp.fleming any transformer with selectable 120/240V can be used. You can connect it as autotransformer. If galvanic insulation is not required. \$\endgroup\$
    – user263983
    Feb 22, 2022 at 13:59

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