The general idea of a separation transformer is very sound!
Especially when building the amp and for initial commissioning you can't do
better. And it doesn't have t be too expensive. You really don't have to
test it full power via the transformer: test it and make it work to bout
100-200 watts, then switch to caution and no transformer. Nothing can beat
caution!!!
Alex 4Z5KS
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Manfred
Mornhinweg
Sent: Monday, September 16, 2013 10:36 PM
To: amps@contesting.com
Subject: Re: [Amps] Direct rectification of AC mains to derive the amp VDD,
> supply
Leigh,
> I would not consider schemes that directly rectify the mains to
> provide the DC supply rail to a QRO MOSFET RF amplifier because it's a
> daft idea from the concern of safety issues relating to electrical
> isolation.
If you don't feel comfortable working in equipment with dangerous voltages
in them, surely you shouldn't use direct mains feeding. But then even more
you should never even consider building a tube type amplifier, with its
several kilovolts inside!
Personally I feel perfectly comfortable with powering an amplifier directly
off-line. There are just three things to be aware of:
1- Simplicity
2- Safety when using it
3- Safety when working on it.
In regard to simplicity, MOSFET HF power amplifiers couldn't be simpler:
Anyway there are always transformers at the input and output. These can be
easily designed to provide the required insulation. The bias is only a
voltage, with no signiifcant current, so it can be derived from the main
supply. That means that you need no separate bias input. All measuring
(power, SWR) is done at the output, ground-referenced. Only the amplifier
module proper is floating at line level. It's really VERY simple! The only
limitation is that it forces the designer to use conventional transformers,
instead of transmission line ones. The other option, using transmission
lines and make the insulation by coupling capacitors, that have low
impedance at RF but high at line frequencies, can be used, but is less
advisable.
Regarding safety in use: Safety regulations used in many countries say that
any electrical device needs to fulfill either one of these: Having all
user-accessible metal parts securely grounded, or having double insulation
between the line and any such metal parts. A ham amplifier will usually be
grounded, and so the insulation in the RF transformers, plus this grounding,
is already safe enough to meet the law. You can add a built-in ground fault
interruptor to exceed the legal safety requirements.
If you prefer, double insulation can also be implemented, by using
additional RF transformers in series with input and output of the amplifier
module, or even by using two separate layers of insulation on the
transformer wire!
I have used simple insulation, winding with thin stranded wire bundles
inside teflon tubing, and using safe grounding, along with the ground fault
interruptors existing in my home.
Regarding safety when working on the amp: It can be done using nothing more
than caution. If you want more, an insulation transformer is great, but
expensive, big and heavy. Buying one just for that one amp sort of defeats
the purpose of using a transformerless supply! For a commercial amp using
direct off-the-line power, an interlock at the cover can be used, so that
the amplifier can't be powered up when open, unless tampered with.
But any household machine has live line voltage inside, and that's
considered safe as long as it carries writing in the lines of "no user
serviceable parts inside - do not open", and the like.
Manfred
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http://ludens.cl
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