15 - 0- 15

[Puffin]

I am a newbie and have tried to find the answer to this without success. Can anyone tell me what the 0 means when a power supply is said to need 15-0-15. Is there a transforner or power supply lead that has to go to 0 and what does this mean. I assume that 0 is not the same as - (i.e as in + and -). Sorry to sound stupid. I ask because I want to upgrade a Dac power supply and I was intending to use a +/-15 volt supply.

[sangram]

15-0-15 is a spec for a transformer with a center-tapped secondary.

Basically you can get a split supply from such a transformer using just 4 diodes, with the center tap forming the ground and the two voltage ends connected to the bridge.

Check on this forum and also the 3875 assembly guide from chipamp.com, lots of explanations on power supply schemes.

[AndrewT]

Hi,
you can buy dual secondary and single secondary transformers.
Of the dual secondary type, you have the choice of two separate windings eg. 0 15, 0 15 (sometimes called up as 2*15), or the centre tapped version 15 0 15.

The output voltage is specified from a fixed AC voltage (in the UK 220, 230 or 240Vac) to feeding a resistive load that exactly draws the rated current from the transformer.

If the input voltage varies then the output voltage varies in proportion.
If the load is less than the maximum then the output voltage rises.
If the output load is zero (open circuit) then the output voltage is maximum and is usually specified by the regulation figure in the specification.

eg. 230Vac to 2*15Vac 60VA 12% regulation will produce 15Vac output when delivering 2Aac (2Arms) into two 7r5 loads.

Staying with 230Vac the output voltage will rise to a maximum of 16V8 when on zero load and your following circuit will see a supply voltage a little below this.
Now for input supply variation.
In the UK this is (I think?) +6% and -10% to make us apparently compatible with the 220Vac supply the EC wants us to use.
So our maximum and minimum supply voltages are 254.4Vac and 216Vac. The effect this has on your downstream side of the 15Vac transformer is considerable.

At 216Vac (-10%) the rated output falls to 14.1Vac (216/230*15) and at 254.4 (+6%) rises to a maximum of 18.6Vac (240/230*1.06*1.12*15).
For this example your hypothetical circuit has to work with an AC supply that operates correctly and safely within the normal range of 14.1V to 18.6V for an AC supply. The rectified voltages (after the diodes) are about 18.5Vdc to 25.3Vdc. Someone will need to decide if this is safe with 25Vdc working voltage capacitors or go with 35Vdc working voltage.

[cpemma]

Originally posted by Puffin
...I assume that 0 is not the same as - (i.e as in + and -).

It's the reason voltage is also termed potential difference. If two 9V batteries are connected in series, a multimeter across the pair will show 18V. Or swap the probes over and it shows -18V. Purely for convenience, we assume the negative probe is at 0V, all measured voltages relate to that reference level.

Putting the negative probe at the junction of the two batteries and applying the positive probe to the ends will show +9V at one end, -9V at the other.

With a dual-rail supply from a centre-tapped or dual-secondary transformer it's usual to connect the mid-point (or an appropriate end from each secondary) to ground, which becomes our 0V reference. The power rails are then one positive relative to ground and one negative relative to ground.

[Puffin]

Cpemma. Thanks for that I think I am understanding it better now. I have looked on the ESP web site and they have details of a schematic for a 15 -0-15 supply using a +/- ac transformer. The OV is available from the DC board and not the transformer. I assume this would work.

[Puffin]

See Attached

[cpemma]

The above circuit is only practical AFAIK at low currents, in effect it's a potential divider. Similar ideas are used to produce dual-rail from a single battery (eg portable headphone amps), and there are buffer ICs made to improve performance with audio loads. No real point with cheap dual-secondary transformers so easily available.

(edit: seems to be better than I thought, http://sound.westhost.com/project05.htm - but I'd still use a more conventional circuit.)

With the usual dual-secondary transformers you can do another trick, using identical LM7815 regulators on each positive rail and forming the power ground last. (I understand the positive regulators give better results than their negative counterparts.)

Schematic is simplified, needs caps adding.

[Puffin]

Thank you. Presumably 7815's for 15 volts ?

I don't know if you can answer this for me. I have a couple of good quality toroids (removed from old amps) They are measuring 48volts AC. Can I drop that to work at 12 - 15 volts DC ?

[richie00boy]

No. Even if they were 24-0-24 ones the voltage is still to high.

[cpemma]

Originally posted by Puffin
They are measuring 48volts AC. Can I drop that to work at 12 - 15 volts DC ?

I wouldn't bother trying. Look in Rapid's non-RoHS sale for one more suited.