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Isolated AC power supply



Because messing with mains-connected circuitry has too high chance of getting kicked and/or frying expensive parts. In the standard setting, the unit is a weak isolation transformer. Also, sometimes a higher-than-mains or lower-than-mains voltage is needed, e.g. for calibrating voltmeters (with or without additional resistive divider or regulated dummy load) or for playing with triacs and thyristors at lower voltage safer and more comfortable to mess with.


The unit consists of three transformers, with dual secondary windings. The windings can be connected in series or in parallel with attached switches. The combination of the windings selects the output voltage.

The third transformer was added as an afterthought, for boosting the output to about 500 volts. Its secondary is then connected to the series with the second transformer. As it was a circuitboard-mountable transformer, mounting assembly was made ad-hoc from nuts molded into epoxy putty.

The output voltage is VERY load-dependent ("soft"). For the purpose of this device, which is design and testing of low-power circuits and assistance in repairing of low-power power supplies, this is considered an advantage. The short-circuit current is less than 200 milliamps, and the output voltage drops to negligible.

For higher currents, the Lightbulb Limiter or hw_LightbulbLimiterHighPower can be used instead (will not increase operator safety shock-wise but will protect the device under test from overcurrent), or a commercial 1:1 isolation transformer can be employed.

The epoxy mount of the third transformer failed in a brittle way when the device was accidentally dropped. As a 3D printer was available by then, a replacement bracket was printed, with holes with inner diameter suitable for cutting M3 thread for the original long screws. Heavy stringing was encountered as a minor annoyance.


Front view

Back view

Voltmeter detail

Ammeter detail

Inside view, towards front side

Inside view, towards the front side, detail

Inside view, towards back side

Inside view, towards back side, with front panel

Transformer 1

Transformer 3

Transformer 2


Voltmeter - internal system

Voltmeter internal system, from behind

Voltmeter scale circuit

Voltmeter scale circuit, detail

Top view

Bottom view

Rendered printed bracket

Bracket, as printed

Printed bracket, installed

Printed bracket, installed

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