Electronics > Beginners
Transformer voltage for dual power supply (+12V / -12v)
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drussell:

--- Quote from: IanB on August 25, 2018, 06:37:15 pm ---IMHO, a 12 V, 12 VA transformer cannot be made to produce a 12 V, 1 A linear power supply. More generally, you cannot reasonably design for the same DC output voltage as the AC input voltage. The AC input voltage needs to be higher than the DC output voltage to allow proper regulation and ripple rejection.
--- End quote ---

I would agree if it were a 12 VA transformer (well, 24 since we're talking about a 12/24 volt dual secondary unit), but it is a 100 VA transformer, and it is a torroid, which generally have pretty good load regulation compared to a typical EI core unit.  Since the OP is unlikely to draw more than about 50 VA absolute maximum with a couple of 1A class regulators, I have a funny feeling that it will be JUST enough in this particular case.

The fact that it is only putting out 12.9 volts AC unloaded instead of 14-15 or so leads me to suspect that it will be pretty stiff (load-regulation wise) since it is rated 3.5A at 12 volts AC per secondary winding.

Like others here, I will be interested to see the actual test results.  :)
Zero999:

--- Quote from: Ian.M on August 25, 2018, 08:07:37 pm ---Don't bother load testing with bridge rectifiers etc.  Just connect your 12V *INCANDESCENT* bulbs or power resistors direct to the secondaries.  You cant use LED bulbs for this.

N.B.  metal case power resistors *MUST* be heatsinked otherwise they are only good for typically less than 10% of their full rating.

Your 3.5A secondary will only be good for a bit over 2A DC.  See Hammond (transformer division) Design Guide for Rectifier Use - Full wave bridge, capacitor input load.

--- End quote ---
Yes the load must be linear, resistive: no rectifiers, or LEDs.

Using a fullwave bridge rectifier results in current surges being draw, rather than a continuous current, which will cause increased heating in the transformer, hence the de-rating, and will distort the voltage waveform, which is why a resistive load must be used, to determine the transformer's impedance.


--- Quote from: drussell on August 25, 2018, 08:21:42 pm ---
--- Quote from: IanB on August 25, 2018, 06:37:15 pm ---IMHO, a 12 V, 12 VA transformer cannot be made to produce a 12 V, 1 A linear power supply. More generally, you cannot reasonably design for the same DC output voltage as the AC input voltage. The AC input voltage needs to be higher than the DC output voltage to allow proper regulation and ripple rejection.
--- End quote ---

I would agree if it were a 12 VA transformer (well, 24 since we're talking about a 12/24 volt dual secondary unit), but it is a 100 VA transformer, and it is a torroid, which generally have pretty good load regulation compared to a typical EI core unit.  Since the OP is unlikely to draw more than about 50 VA absolute maximum with a couple of 1A class regulators, I have a funny feeling that it will be JUST enough in this particular case.

The fact that it is only putting out 12.9 volts AC unloaded instead of 14-15 or so leads me to suspect that it will be pretty stiff (load-regulation wise) since it is rated 3.5A at 12 volts AC per secondary winding.

Like others here, I will be interested to see the actual test results.  :)

--- End quote ---
Possible, but I wouldn't be too sure. Another issue is enough headroom should be provided to ensure it works, when the mains voltage is on the lower end of the tolerance band, which can ruin your day.
David Hess:
12 volts AC * 1.414 = 17 volts peak
17 volts peak - 2 Vdiode(silicon) = 15 volts

That leaves only 3 volts of headroom for the 317 and 337 regulators which is not enough.  So replace the standard diodes with schottky diodes.

17 volts peak - 2 Vdiode(schottky) = 16 volts

That might be acceptable but 1 volt of input ripple at 1.5 amps requires a lot of input capacitance.

8200 microfarads*volts/amp * 1.5 amps / 1 volt = 12,300 microfarads

15,000 or 22,000 microfarads of input capacitance is a lot for only 1.5 amps but acceptable if a 12 volt AC transformer is all that you have to work with.  The capacitance could be halved by using low dropout regulators allowing an increase of the input ripple to 2 volts.

Do not forget that the input rectifiers have to handle the surge current to charge the excessively large capacitors.  The transformer helps by saturating which limits the surge current but use big rectifiers anyway.  And big rectifiers will boost the voltage a little by having less voltage drop.

Oh, and if you have two separate AC windings, then you do not need a positive and negative regulator unless you want a bipolar supply with only one control.  Two positive regulators can be used and connected only at their outputs; just treat them as two separate floating regulators.
Zero999:

--- Quote from: David Hess on August 25, 2018, 09:24:12 pm ---Oh, and if you have two separate AC windings, then you do not need a positive and negative regulator unless you want a bipolar supply with only one control.  Two positive regulators can be used and connected only at their outputs; just treat them as two separate floating regulators.

--- End quote ---
If a bipolar power supply is required, it's better to configure the transformer as centre tapped and use one bridge rectifier, than a bridge rectifier on each secondary, because it will have one less diode drop.
David Hess:

--- Quote from: Hero999 on August 25, 2018, 10:44:38 pm ---
--- Quote from: David Hess on August 25, 2018, 09:24:12 pm ---Oh, and if you have two separate AC windings, then you do not need a positive and negative regulator unless you want a bipolar supply with only one control.  Two positive regulators can be used and connected only at their outputs; just treat them as two separate floating regulators.
--- End quote ---

If a bipolar power supply is required, it's better to configure the transformer as centre tapped and use one bridge rectifier, than a bridge rectifier on each secondary, because it will have one less diode drop.
--- End quote ---

That is a great point.  And still use schottky rectifiers to reduce the voltage drop further.
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