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?