[Amps] power supply transformer

[Manfred Mornhinweg]

Hi all!

With so much discussion going on, I will add my half cent worth on two 
issues. First:

>>Paul, you have to have a line transformer for isolation - 
>>it is life and death - no discussion possible...
> 
> Not that he should do it (for other reasons), but there is a 
> great deal of equipment we use every day that runs power 
> supplies that are not isolated from the power line.

Exactly. It can be perfectly safe to run an amplifier or other device 
directly from the line, without any isolation. The requirements for this 
are that any person will be safely kept away from the dangerous 
voltages. The codes in fact require TWO steps of safety. These can be 
either double insulation, or single insulation plus grounding.

In practice this looks like the following: Assuming your 100V load is a 
solid state power amplifier stage, you can feed it from the rectified 
and filtered line, and design the input and ouput RF transformers in 
such a way that they provide safe insulation. The codes require 4kV 
insulation, and creepage distances of at least 4mm, at least in my part 
of the world. Of course, you must comply with this insulation and 
creepage distances in EVERYTHING that connects to the amplifier board: 
The PCB mount, the transistor-to-heatsink connection, bias lines, etc. 
This is usually quite simply to do, but you have to think a little 
before starting work. Then, the second step of safety is implemented by 
connecting the chassis to earth ground via the power plug's third pole, 
in a tamper-proof way. The differential protection switches in the house 
wiring close the safety circuit.

The above provides fully satisfying protection to the user. It does NOT 
provide protection to a careless technician putting his fingers inside! 
When working on this amplifier, you must keep in mind that there is 
dangerous voltage all ofer the amplifier board. On the other hand, this 
voltage is hugely less dangerous than the several kilovolts we are used 
to in tube amplifiers!

Now the other issue:

I cannot understand why so many hams building equipment despair about 
the transformers! Used, surplus transformers are available everywhere! 
In old equipment, on junk yards, etc. I get so many of them as gifts 
from friends, that I have a big (and very heavy!) box full of them. When 
I need a specific power transformer, I pick one that has a core 
reasonably sized for the power I need, take it apart, and rewind it for 
the voltages and currents I need. I keep a stock of enameled wire for 
this, and buy more as needed. And if I'm lucky, the old transformer I 
pick happens to have a good primary winding for the correct voltage, so 
I only need to redo the secondaries!

I wound my first power transformer at age 11, and the most recent 3 
weeks ago. In between there have been close to one hundred, and ALL of 
them worked well. So I see no reason why a normally abled hobby 
electronician shouldn't be able to wind his transformers.

Here is my condensed wisdom on designing them:

http://ludens.cl/Electron/Magnet.html

To build them, a winding machine with turns counter is ideal, but high 
power transformers can easily be hand wound, because they use rather few 
turns. Good electrical supply stores sell not only the wire in many 
different sizes, but also insulation material in several thicknesses, 
shiny new iron laminations, bobbins, and varnishes that can be used to 
soak the transformer to improve insulation, heat transfer, and prevent 
any noises.

So, don't loose time searching for exact transformers for a given 
project, nor do kludges like 28V to 100V triplers for 30A output! 
Instead, wind your transformer exactly as you need it!

Manfred.

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