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

[Don Brown]

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" <aa0ni at yahoo.com>
To: <elecraft at mailman.qth.net>
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
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