[Elecraft] OT: Help designing power supply and toroid transformer

[Don Wilhelm]

Daniel,

Answering on the list since there may be other folks interested too.

Most toroid cores will not handle 60 Hz, so you will be better off not
attempting to build your transformer.
The power supply construction articles that show winding a toroid
transformer are switching supplies and the transformer operates at a much
higher frequency (the switching at high frequency is what causes the trash
that some produce) - a linear supply (what you are contemplating) does not
produce switching RFI.

I suggest that you obtain a 120 volt transformer to 25 volt Center Tapped
transformer already built - follow it with a full wave rectifier, filter it
and follow that with a 3 terminal voltage regulator.  The LM317 is rated for
1.5 amps and is adjustable, so you can easily obtain whatever voltage you
need up to 10 or 12 volts (get the LM317 data sheet - download it from
http://www.national.com/pf/LM/LM317.html - the calculations for setting the
output voltage are in the data sheet).  Your 1987 Handbook should have most
all the circuit diagrams and information that you need to complete it.

On the 120 volt AC side, use sensible precautions - fused input, switch
contacts and other connections protected from accidental contact and so
forth.

73,
Don W3FPR

> -----Original Message-----
> ...
> My goal is to build a non-switching, voltage regulated 9V/1.5A
> power supply
> with enough isolation and filtering to keep RFI out of a Grundig
> Satellite 800.
> My father has one of these radios, and so do I (unfortunately, he
> lives out of
> state - so I couldn't compare the two radios and p/s side by side). When I
> visited him over the holidays, I discovered that the OEM power
> supply my father
> has is either producing RFI or is conducting RFI, because he can tune in
> daytime AM/MW signals from over 100 miles away when running the thing on
> batteries, but he has a hard time with signals beyond 50 miles
> away as soon as
> he plugs the power supply into the outlet (this was AFTER we
> already turned off
> the worst RFI generating offenders in the house).
>
> The Grundig Sat 800 power requirements are spec'd at 7-10V DC up to 1 Amp
> (530mA with 1/4W output and lamp off, and 830mA with 1/4W output
> and lamp on),
> and it says in the manual that the supplied adapter is 9V/1.5A.
>
> I've got an old 1987 copy of the handbook which I've got up in
> the attic for
> reference, and I realize some of the basic things (from my license/testing
> days) that go into a power supply (transformer, diodes,
> capacitors, inductors,
> regulators). I want to actually build my own power supply from scratch -
> starting with the transformer. If I'm not mistaken, I should be
> able to use a
> medium-large toroid core and some heavy wire to do this.
>
> [I could go down to Radio Shack and buy a wall wart, but I want
> to actually
> make something that is going to work 'really well' in terms of
> reducing RFI
> passing through the AC power supply - something that a cheap power adapter
> probably won't be designed for.]
>
> So - here's where I'm hoping you can help me with...
>
> What core (size and material composition) should I use for
> transforming an AC
> signal?
>
> How many windings of what kind of wire would be the 'best' for the power
> requirements of this supply?
>
> Should I build a 120/12V transformer and voltage regulate it down
> to 9V, or
> would it be better to simply build a 120/9V transformer and
> rectify the AC and
> smooth it out with passive components?
>
> How can I best filter out RFI being carried on the AC line?
>
> Are there any other considerations I should be aware of?
>
> Do you know of any helpful websites I should visit (I haven't googled this
> topic yet)?
>
> I'm sorry to waste the bandwidth of others here.
>
> Any help supplied is greatly appreciated!
>
> - Daniel AA0NI
> Oklahoma City
>