I am building a 12 V dual power supply. Since this is the first time I will be using mains (230 V). I would like to make sure that I am not going to burn down my house.

The transformer I want to use is this one in the 2x15 V, 30 VA rated version: transformer wiring

For my power supply I would like to turn this into a 15-0-15 center-tapped transformer by:

  1. Connecting the primary windings together (grey + violet).
  2. Connecting the secondary windings together (yellow + black). This would be the center tap.
  3. Putting a 100 mA fuse before the primary winding (either blue or brown).
  4. Putting a 1 A fuse on each of the secondary windings (orange and red).
  5. Feeding these secondaries into a bridge rectifier, filter caps, and regulators.

Is there anything that I missed, or should be aware of before I get myself electrocuted?

  • 1
    \$\begingroup\$ Looks correct so far but please draw your fuses and bridge rectifiers too since the devil is usually in the details. Welcome to EE.SE too! \$\endgroup\$
    – winny
    Commented Sep 1, 2019 at 15:50
  • \$\begingroup\$ If you would like to make sure that you are not going to burn down your house, you should think about protection against overheating. Good ventilation and may be a overtemperature breaker. \$\endgroup\$
    – Uwe
    Commented Sep 1, 2019 at 16:20
  • 2
    \$\begingroup\$ Looks OK to me too - but I want to point out that 100mA fuse may be too small. Of course it depends on the load and inrush current, so just something to be aware of - but toroids tend to have larger inrush currents than normal transformers. Fuses would be of slow-blow type. If you add a mains switch, you should consider a snubber to prevent the switch from arcing. If you mount this inside a metal case, the metal case should of course be connected to earth/ground so you need a 3-prong plug or IEC power inlet. \$\endgroup\$
    – Justme
    Commented Sep 1, 2019 at 16:35
  • \$\begingroup\$ If by dual supply you mean plus and minus voltage with a common ground, you should carefully examine how you plan to rectify this. \$\endgroup\$ Commented Sep 1, 2019 at 18:09

2 Answers 2


A few thoughts come to mind:

  1. 15v secondary implies ~21v peak. So the differential across your regulator will be 9 volts. Quite large. If you are intending to use 30va at 12v, this implies 2.5Amp max. Which means 2.5 x 9 = 22 watts in your regulator. But with max < 1amp (based on your fuse) this is lower, but still high.

However if it's a inductive buck regulator, then less of an issue.

  1. Center-tap transformer does not require a bridge rectifier. Only 2 diodes. Look it up.

[CORRECTION] For a dual supply you WOULD need 2 bridges.

Otherwise seems ok.

  • 1
    \$\begingroup\$ Center-tap for a dual supply requires a bridge rectifier. Or two sets of two diodes, if you like. \$\endgroup\$
    – Janka
    Commented Sep 1, 2019 at 16:49
  • \$\begingroup\$ center-tap transformers do require a bridge rectifier for a dual +- output. Read the question and look it up. \$\endgroup\$
    – Uwe
    Commented Sep 1, 2019 at 16:50
  • \$\begingroup\$ Oh yes. I missed the 'dual'. My bad. \$\endgroup\$ Commented Sep 1, 2019 at 16:51

Dual (+/-) supply with center tapped transformer:


simulate this circuit – Schematic created using CircuitLab

Without a load, the capacitor voltage will be a bit higher than 20V.

The primary connection you suggest should be okay, based on the datasheet. 100mA fast blow fuse in the primary will probably blow the first or second time you power it- surge from the caps and because it's a toroidal type transformer.

You might want to ask the manufacturer for the recommended primary fusing, probably a slow-blo fuse of some kind (I would guess 1A or more, but that may not provided sufficient protection so best to ask).

Can't really help with the safety aspects, best to have someone local who knows what they are doing look it over before powering it up.


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