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'[EE] Are these circuit assumptions correct?'
2002\08\07@172359 by

Hello,

I was going to build an adjustable power supply with an output of 1.25 to 26
volts at 5 amps maximum. The transformers I ordered came in and the rectified
and filtered output is 39.5 volts and I had calculated for 28 volts.

I was going to go ahead and use the transformers as the are as I am only
building two power supplies. I was wondering if a preregulator with a 2N3055
transistor, a 27 volt 1 watt zener diode and 1N4750A (27 volt, 1 watt, Izt =
9.5 mA nominal and 34 mA maximum) and a 1200 ohm half watt resistor (between
the collector and base) would drop the extra 11 volts I have?

I calculated this from a web site tutorial on zener diodes and pass
transistors. Since the problem with the transformer I don't want to have the
same problem with this preregulator circuit.

Aside from the heatsink I will require are these assumptions correct? If
someone would double check this I would be grateful.

Thanks,
Ed

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Ed,

you may not need the pre-regulator - if you are going to need the full 5
amps...

the 39.5 V is the peak voltage = ( 1.414 X 28 - the rms voltage)

once you load the supply down, the internal resistance of the transformer
secondary will start to drop the unregulated voltage considerably

Ken

>I was going to build an adjustable power supply with an output of 1.25 to
26
>volts at 5 amps maximum. The transformers I ordered came in and the
rectified
>and filtered output is 39.5 volts and I had calculated for 28 volts.

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You should check the unregulated /loaded/ voltage first.  Draw five amps
from it and see what the voltage is.

Chances are good that the voltage will drop significantly when loaded
with 500mA or so, and your regulators will be able to handle the large
voltage drop (suitably heatsinked, of course).  As the current
consumption goes up, the transformer voltage will drop and the regulator
will not have to drop so much voltage (though it will still be
dissipating more power).

Ed Edmondson wrote:

{Quote hidden}

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> >I was going to build an adjustable power supply with an output of 1.25 to 26
> >volts at 5 amps maximum. The transformers I ordered came in and the rectified
> >and filtered output is 39.5 volts and I had calculated for 28 volts.

The ripple and sag will easily bring it close to 28 volts, so you really
are where you want to be with 39.5 unloaded.

> >building two power supplies. I was wondering if a preregulator with a 2N3055
> >transistor, a 27 volt 1 watt zener diode and 1N4750A (27 volt, 1 watt, Izt =
> >9.5 mA nominal and 34 mA maximum) and a 1200 ohm half watt resistor (between
> >the collector and base) would drop the extra 11 volts I have?

You probably need more current to drive the 2n3055, it will likely have
fairly low gain at 5 amps.

I think the amount of heat you have to deal with will far overshadow these
small problems, however.

By the way, are you using a three-terminal adjustable regulator? It won't
work at the lower range of voltage with 28 volts in and 5 amps out. Too
much dissipation.

Perhaps five devices in parallel would work better.

Cheerful regards,

Bob

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