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.
----------------------------
Visit my hobby website!
http://ludens.cl
----------------------------
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