[reaction]

Hello

I want to build a ± 15V regulated power supply but I have only an 11,2V single output toroidal transformer for electrical bulbs. Can I use this transformer with BUF634-based virtual ground? I’ll put the BUF634 after the rectifier bridge and then I’ll put a classic LM317/LM337 dual voltage scheme. What is your opinion about that? Will this PSU be an alternative of classic PSU with traf ground?

Thanks

 

[reaction]

I think 11,2 V won’t be enough power for ± 15 V. Maybe I should go for 30 V single output traf or something?

Or I should get two identical 11,2 V trafs and put them in parallel?

 

[tangent]

For a linear regulated supply, you need more input voltage than output voltage, by 1-3 volts per rail. Go look at Jan Meier's HA-1 power supply circuit for a good example. The transformer is a dual 18V one (the schematic calls it a dual 15, but I've built an HA-1 from a kit and he actually uses a dual 18). He rectifies and filters the output (giving 36V p-p, I believe), then runs it though a pair of resistors acting as a voltage splitter to get +/-18V. Finally the positive rail goes through a +15V regulator (LM7815) and the negative rail goes through a -15V regulator (LM7915). These regulators turn the excess voltage into heat, and put out a pure + or - 15V.

If you don't give a linear regulator extra voltage to work with, it can only provide a "copy" of its input, noise and all. It needs that extra voltage.

 

[Joobu]

The output of the dual transformer is supposed to be AC ±18V but when lightly loaded can easily be ±20V. There is no resistive divider.

 

[Gman]

You would get around 14 volts dc using a standard bridge with a cap filter. You should be able to get a solid 12v bipolar supply using your transformer with a BUF634 ground driver.

Gman

 

[tangent]

Quote:

There is no resistive divider.

In the HA-1 power supply? Then what are the two 10K resistors immediately in front of the linear regulators doing?

 

[paulb]

reaction,
Could you clarify what you're looking for? Do you mean +/-15: a +15V supply and a -15V supply? Or do you mean a split 15V supply, of +/- 7.5V?
Either way, you can't get a regulated supply unless you use a switching power supply of some type, or unless you can put up with a lower voltage.
To get the +/- 15, you could use two half-wave rectifiers: the ripple will be twice as much as a full-wave rectifier, and the DC voltage will only be 15V without any load.
To get +/- 7.5, you can use a full wave bridge and a BUF634 divider as you indicated. Regulate this and you could get about +/-6V, same as what Gman said.

 

[reaction]

I am looking for +/- 15 V supply. I understood, I should go for dual 18~20 V AC traf or single 30~34 V AC traf with BUF634 virtual ground. But the dial traf idea is more stabile, I think.

 

[paulb]

Yes, either a dual secondary (15-18V would be enough) or a single secondary would work. The dual secondary is more flexible (because you can connect the windings in series to get a single secondary) and you would not need a BUF634.
However, using a BUF634, actively driving the ground, may result in better performance than just using a dual secondary. Others here or at headwize will be better to comment on this than me.

 

[Joobu]

If you look at the transformer secondary, you'll see the middle is tapped and connected to ground. The bridge then rectifies it to positive and negative voltages.

I assume the resistors are there to bleed off current from the capacitors.