Is the following regulator valid, safe and efficient?

dual tapped 12v 5v linear regulator

The transformer is a dual 7V output toroidal. So B1 is supposed to rectify out 14v and B2 taps the "center" to produce 7V.

INAE and I've never used dual bridges in a tapped arrangement like this so I thought I should ask before playing around with mains like this.


The following is the circuit corrected as discussed but without the regulators:

transformer bridge filter

This is supposed to model the Amgis L01-6310 14V/7V transformer to see what sort of voltages might be supplied to the 12V/5V regulators (because I should have used a 12V/6V part since Vpeak of 14V is *1.414 = 19.8V. Secondaries have 7 ohms series resistance per datasheet. I have never modeled a transformer in LTSpice before but I reasoned the turns ratio is 120 / 14 = 8.57 squared = 73.5 but this seemed to be off by a factor of 2 so I figured the primary is wired in parallel for 120V mains so I doubled the ratio to 147 / 1.

Green is the 14V tap. Blue is the 7V tap. Red and light blue shows the load on the outputs.

So it seems I get 16V and 7.2V which isn't too bad. If it comes out to 16V thats only a 4V drop. But I don't think the 12V load will be loaded much. If I only load it with ~25 mA, I get 18V which is 6V * 0.025 = 150 mW.

INAE so this could all be wildly wrong. IIWAE it would probably still be wrong.

  • \$\begingroup\$ Dave Tweed's made a good call on B2 not being required (not that it's a safety issue just not needed). I can't see a general problem but what is the currennt draw? You'll probably have in the order of 19V going into the 7812 for example so may need a pretty decent heat sink depending on the current. \$\endgroup\$
    – PeterJ
    Commented May 29, 2013 at 14:44
  • \$\begingroup\$ @PeterJ Yeah, I kinda order the wrong part. I see there is a 12VCT series 6V parallel that would have been better. The 12V load is very light - maybe 20mA. The 5V load is driving a microcontroller and about 10 indicator LEDs but they won't be on all at the same time so the load is going to be well below the 100mA+ limit of the 3.2VA part I ordered. Not sure if the light load helps or hurts in this case. Is there anything I can do to work-around or should I just spring for a new part? This is just a 1-off so it doesn't have to be perfect. \$\endgroup\$
    – squarewav
    Commented May 29, 2013 at 21:48

1 Answer 1


The second bridge rectifier (the one connected to the center tap) is actually completely redundant. You can just connect the input of the regulator directly to the center tap, and it will get half the voltage, full-wave rectified by the two left-hand diodes of the other bridge.

  • \$\begingroup\$ Interesting! So is the filter capacitor on the center tap also redundant (assuming I doubled the size of the other)? \$\endgroup\$
    – squarewav
    Commented May 29, 2013 at 16:08
  • \$\begingroup\$ No, the capacitor is still necessary. The current flow through the transformer windings is still intermittent. \$\endgroup\$
    – Dave Tweed
    Commented May 29, 2013 at 16:34

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