OT: Help designing power supply and toroid transformer

[Daniel Reynolds-2]

This is slightly off topic - but since there is such a generous group of guys
that are very knowledgable in electronics (and since my brain is a little soft
in this area), I think I'll toss this request out and hope for the best.

First off - please reply off the list since this does not pertain directly to
Elecraft equipment (except that it might be fun to eventually take the same
design and build a larger p/s for the K2/QRP).

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
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[Don Brown-4]

Hi

The first thing I see as a problem is the toroid transformer. At 60 Hz none
of the common cores are going to work. You really need some steel in the
core. I have seen some transformers made for 60 Hz wound on a spring steel
toroid core. This is not something that is easy to build. I would use a
small transformer from radio shack or even better a door bell transformer as
they are very well shielded to meet class 2 UL requirements. On the input to
the transformer use 2 .01 uf 600 volt caps from each input wire to earth
ground. I would then come out to a bridge diode rectifier bypassed by .01uf
caps across each diode. Then into a large filter cap about 2000-4000 uf also
bypassed with a .01 uf disk across the filter cap for the RF bypass. If you
are using a 10 volt doorbell transformer you should have about 14 volts DC
at this point. From there I would use a three terminal regulator like a
LM117 or LM337 followed by a 10 uf filter and finally pass both the plus and
minus wires through a 1 inch ferrite toroid core several turns. If you would
like me to draw up a schematic for you I could email it to in PDF format.
There are many other ways to do this this is just one off the top of my
head. Someone may come up with a better idea but these simple power supplies
are fun and easy to build

Don Brown
KD5NDB




----- Original Message -----
From: "Daniel Reynolds" <[hidden email]>
To: <[hidden email]>
Sent: Tuesday, January 10, 2006 8:54 AM
Subject: [Elecraft] OT: Help designing power supply and toroid transformer

> This is slightly off topic - but since there is such a generous group of
> guys
> that are very knowledgable in electronics (and since my brain is a little
> soft
> in this area), I think I'll toss this request out and hope for the best.
>
> First off - please reply off the list since this does not pertain directly
> to
> Elecraft equipment (except that it might be fun to eventually take the
> same
> design and build a larger p/s for the K2/QRP).
>
> 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
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com
>

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[Don Wilhelm-3]

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
>

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[K3yt]

Hello Daniel,
 
As the Dons have spoken, let me add my $.02.
 
The 1991 Handbook has a project in it for a 5 amp
supply with variable voltage from 1.2 to 15 volts.  It has
a drawing (layout) for a board with the regulator on it.
 
It is a good article because it uses almost all Radio Shack parts.
It uses 2 transformers to get the 5 amps.  One would be fine for  you.
It uses the LM371K in a metal case for regulation.  Since it is a  5 amp
circuit
the design uses a pass transistor.  You could eliminate a lot of the  
circuitry
and just use it direct from the regulator since you only want 1.5  amps.
 
If you can't find another ham in your area that has the 1991 ARRL  handbook
I would say try a library. It is in Chapter 27 "Power Supply  Projects".  
 
The circuit board for the regulator circuit is very simple and easy to
make. I made one 14 years ago and it is still working.
 
BTW, your Dad's wall wart is probably a switcher and the source of the  noise.
 
73 de Bob K3YT
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