Electronics4you

Hi there

I need a 18V DC power supply from a 28V/7.5A AC toroidal transformer. The current output should be so close to the 7.5A as possible. What should I use to step down the power - transformers, lin. regulators...?

Sceadwian

Depends on how much that 28volts will sag when the output is at the full 7.5 amps. Have you measured 28 volts at the full load? If so what's the no load voltage?

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Electronics4you

I haven't measured anything yet. I have just found a toroidal transformer with a 28V output. The transformer says just 28V/7.5A. Isn't that the max current at 28V?

cadstarsucks

That would be the RMS voltage when supplying 7.5A to a resistive load. It would be horrendously inefficient to use a linear as you would be seeing to 40V peak plus line variation or most likely 35V average.

(35-18V)*7A=120W that you be dissipating and would have to supply a heat sink for. You need a switcher.

Dan

Sceadwian

You could always re-wire the transformer.

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RODALCO

Or reduce the primary voltage by adding a suitable inductor in series with the incoming mains.
This can be calculated or done on a trial and error basis,

Of course be safe when working with mains voltages.

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ericgibbs

hi,

The best thing to do would be to take off a few secondary turns. [28v winding]

A rule of thumb is 6 turns/volt per square inch of core. So a core area of 2 square inch would be 3 turns/volt.

As its 28Vrms at 7.5Arms and you want 18Vdc at 7.5Adc, I would give the linear regulator transistors at least
2 to 3 Volts overhead.[ V c to e]

Assuming fullwave rectification use a 10A FWB, rated about 120V.

Ripple capacitor of 10,000uF at 35Vdc.

If you reduced your winding turns to give say 20Vrms, this would give you a Vpk of 28V at no load and about 21V on load.

Connect up the transformer, measure the secondary Vac off load.
Disconnect the transformer from the supply, remove 10 turns from the secondary, reconnect and remeasure the secondary Vac.

This will give you a turns/volt figure, so calc how many more turns need to be taken off.

Do you mind if I ask what you are going to use the transformer for ?

EricG

Hero999

Why are you using a 28V transformer.

An 18V transformer should suffice, assuming a 3V loss in the regulator, 1.8V in the rectifiers and 1V ripple.

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ericgibbs

hi Hero,

I think the OP has a 28V transformer on 'the shelf' that he wants to use for the 18Vdc psu?

Hope he lets us know what he going to use it for, we may be able to make
an alternative suggestion on how to use it.

Eric

eblc1388

The OP requires 18V DC at 7.5A. A 18V transformer will not do.

To be picky, a 7.5A transformer secondary will not do either. It needs to be 10.6A or more.

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L.Chung

Hero999

Why would you want any more than 18V?

I suppose it depends on the size of the capacitor.

I agree a 7.5A transformer won't be enough, 10.6A sounds about right but that too depends on the amount of ripple which will depend on the capacitor size.

I find questions like this interesting, is it cheaper to use a larger transformer and therefore a larger heat sink or a larger capacitor and smaller transformer and heat sink?

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eblc1388

Why?

Because a 18V transformer cannot provide a regulated DC 18V output at 7.5A. You have mentioned correctly all the voltage drops but I don't understand why you would then went on to suggest a 18V transformer instead of 22~24V.

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L.Chung

Hero999

If the transformer has 5% regulation (which isn't unreasonable for a torroidal transformer of this size) then the output will be 24.18V which is still higher than the minimum for acceptable regulation. Also, voltage regulators tend to increase their dropout voltage with temperature and therefore a lower average input voltage will reduce the 3V worst case prediction in our favour. Anyway isn't the voltage rating transformers normally specified at the maximum load current?

The only time I can really see this becoming a problem is if the mains voltage is on the low side (which doesn't normally happen here in the UK).

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eblc1388

I see that you have added 5% in your calculation of final voltage 24.18V. Why?

The transformer voltage regulation does not mean that the voltage is 5% higher when at full load.

At best the transformer secondary is 18V at full load, with 5% voltage rise without load. In some cases, it could be even a bit lower than 18V at full load because of losses.

Remember also that now the winding ac current is 10.6A and there are more I2R drop too.

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L.Chung

Hero999

Sorry perhaps I didn't explain it well enough, I subtracted not added, the pea

I know, it means the voltage will be 5% higher off load. I make this mistake of assuming it would be 5% lower at full load.

I would hope the manfucturer has factored in the losses into the regulation which is why it's 5% higher without a load.

I suppose the peak voltage will be slightly lower as it's drawn in pulses rather than a continious waveform.

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