Building a dual-rail, regulated DC power supply: 1 diode bridge & center-tapped secondaries, or 2 diode bridges and separate secondaries?

Question

I'm looking to build a dual-rail, regulated, 18VDC power supply based on the 78xx and 79xx regulators. The circuit is inspired by AMB Audio's sigma25 (amb.org/audio/sigma25/) and sigma26 (amb.org/audio/sigma26/) regulated power supplies. I have also Googled around and came across these:

• eleccircuit.com/wp-content/uploads/2008/10/dual-variable-regulator-power-supply-5-25v-by-lm7805lm7905.jpg
• eidusa.com/Interface_Boards_PN15_PS.htm

However, I am torn between using a 2x9V transformer and connecting the secondaries as such, using only 1 bridge rectifier to supply both regulators: ...and using a 2x18V transformer, each secondary feeding its own bridge and regulator, as such: What are the pros and cons between the two, apart from (slightly) more parts count in the latter schematic?

Many thanks for any enlightenment given.

Answer 1

As drawn, the 2x9v transformer gives +/- 13 ish v pre-regulator. The 2x18v gives +/- 26v. Once you've sorted out like for like with the transformers (your use of a factor of 2 between the voltages clearly shows that was what was intended), then you can read the rest of the answer that I wrote before I thought about your transformer configuration.

The first diagram with the single bridge rectifier has a slightly higher pre-regulator voltage, as there is only a single diode drop rather than 2 in series.

This means as the mains fluctuates lower, or as the temperature of the transformer (and so its resistance) increases, or as the load current increases, there will be more headroom in hand on the regulators, so they will stay in regulation for a bit longer.

OTOH, the regulators will have to deal with a slightly higher voltage drop, and so get hotter, which is a disadvantage in hot conditions or high mains voltages.

For the same loading conditions, the first design has more dissipation at the regulators, the second has more dissipation at the diodes.

Neither of these advantages or disadvantages are fundamental, and can be designed with, they are only relevant if you have already got the transformer and the output voltage.

There are theoretical cross-regulation issues, but their effect is vanishingly small, and completely irrelevant (unless you are hyping your £3000+ magic HiFi)

I'd go for the lower parts count.

Answer 2

Watch out for that treacherous 79XX pinout!!!

It's wrong on your schematic.