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I have an electronic transformer which is meant to be used to drive a 12v halogen light. The specification on the electronic transformer box says 220-240VAC input, 12VAC output 20-60W.

When a 50W halogen bulb is connected to this electronic transformer the voltage across the bulb is approximately 10.5VAC.

Is there a way of using these low cost electronic transformers to supply power to a small single board computer?

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    \$\begingroup\$ Probably, but you'd be better off to use a wall wart or other packaged supply that provides regulated DC output at the required voltage (or a bit more and use a regulator). \$\endgroup\$ – Spehro Pefhany Nov 29 '14 at 2:53
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Listen to Spehro.

If you'd rather do things the hard way, take the transformer output and run it through a full-wave rectifier. Filter the output with a large (say, 10000 uF) capacitor, then drop the resulting 15 - 18 volts through a regulator (linear or switching).

schematic

simulate this circuit – Schematic created using CircuitLab

Note that the diodes must be power rectifiers, not signal diodes.

If you're planning on using the transformer with 110 VAC in, this may not work. If you draw several amps and load the transformer to 10.5 / 2 volts AC, the diode drops will probably keep the capacitor voltage too low, unless you are careful to use a LDO (Low DropOut) regulator.

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  • \$\begingroup\$ That's a full-wave regulator. \$\endgroup\$ – Leon Heller Nov 29 '14 at 4:16
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    \$\begingroup\$ @LeonHeller: As WhatRoughBeast correctly wrote, it's a full-wave bridge rectifier. I'm not aware that there's even such a thing as a "full wave regulator"; got a link, please? \$\endgroup\$ – EM Fields Nov 29 '14 at 11:39
  • \$\begingroup\$ I meant to say rectifier, of course. He originally called it a half-wave rectifier. \$\endgroup\$ – Leon Heller Nov 29 '14 at 11:41
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If halogen XFMRs are PSUs (not the heavy iron core ones), this schematic may be a better alternative:

schematic

simulate this circuit – Schematic created using CircuitLab

Cheers.

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