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Transformer selection for dual output power supply
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dcbrown73:
Hello,

I would like to design and build a dual output power supply that can produce 30v at up to 5amps per output.  (though 3 amp is easily probably enough)

I was looking at the Hammond MFG toroidal transformers, but on their datasheet it says the following:

The transformers have dual primary and secondary windings.


--- Quote ---Note: Units are designed to have all windings engaged (either series or parallel
connected) or connected as an autotransformer. Connection sheet included with
transformer.
--- End quote ---

Does this mean I cannot use the two secondary windings independently as two voltage sources?  Or is it just talking about the primary windings should be connected in either parallel or in series and not only one winding connected?

If I can, then I have the following question.  If I want up to 30v on the output.  This would mean I would have to select a transformer that provided a bit more than that, but I see them show 30v output then the next step up is 60v output.    A LM338 takes a maximum of input of 40v.    How exactly could I go about lowering it to an acceptable voltage in the most efficient way?   

Or a better question would be, what is the best course of action here?

Thanks!
Bassman59:
Dual primaries are provided so you can use the transformer with 120 V or 240 V mains. For the former, you parallel the primary windings, and for the latter you put them in series. But yes, please follow the connection sheet, as Hammond suggests.

As for the dual secondary. Do you want a single +30 V supply or a +/- 15 V supply? A transformer with two secondary windings (or a single secondary winding with a center tap) will do both, you just have to wire it correct -- as per that connection sheet. But make sure you get a transformer with adequate output voltage.
Kleinstein:
The nominal power is for using both primaries and both secondaries. One can still use the secondaries independent. This only means that one can not get the full power from just one secondary. If the transformer is something like 2x30 V and 120 VA, one can get about 2 A (AC) from each primary. If one uses only one primary one can not get the full power (e.g. 2x2 A) form that one winding, but there is no problem drawing 2A (or even 2.5 A) from one and 0 from the other.


To get 30 V out after the regulator is needs some 33 V DC at the filter cap. To get this is takes about a 28-30 V transformer. The filter cap will increase the voltage by up to a factor of 1.4 as the peak to RMS ratio. However there are possible losses like the rectifier, main voltage lower than nominal, ripple. With no load the voltage would rise to around 40- 45 V. So this may be to much for the LM338 if there is a short.

While filter caps makes the voltage to go up, the RMS current is higher than the DC current by about a factor of about 1.6 ( it can be a little more for a rung core transformer with a large cap). So for  3 A DC out it takes around 5 A AC current.

The LM338 like many other integrated regulator has limited power handling capability. For the LM338 the practical limit is somewhere around 70 W, more is difficult and would need a really large heat sink / fan.

For a variable supply one should consider ways to reduce the heat loss, e.g. by using 2 or more transformer windings depending on the needed voltage.
floobydust:
It's complicated. Consider a full-wave bridge rectifier as the most efficient for transformer utilization.
Working backwards, for a 30V output the LM338 would like 3-5V headroom so 33-35Vin.
Example 1182M12 24VCT 3.33A 80VA transformer gives about 32VDC under full load and 35VDC under no-load, and good for 2A DC.

But note the IC's available output current drops Fig. 2 with high >30V input-output differentials, due to SOA protection, so at a 5V output you it would get only around 1.2A with >30V input. The IC has to drop over 25V and it can't do that at full current.

This is why (tap) switching to lower voltage transformer winding is done.
If you DPDT switched the transformer secondaries from 24V to 12V, you could get 14.6VDC under full load and 16.2VDC under no-load, so I would expect around 12VDC max. output voltage and good for 4.1A DC.

My numbers are just rough calculations, AC mains varies and can be 115V or 120V and I used 9.7% regulation spec for the 80VA transformer.
LaserTazerPhaser:
Instead of transformers a fused capacitor dropper with mov across the film cap can be fullwave rectified then filtered and fed into a buck converter. The output power is much greater and this arrangement will output much less heat and be more compact than a transformer.
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