[Amps] Direct rectification of AC mains to drive the amp,

[Alex Eban]

Children:
Did anyone hear about the rubber mat we were talking about at the beginning
of this thread???
Alex

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Manfred
Mornhinweg
Sent: Sunday, October 06, 2013 3:45 AM
To: amps at contesting.com
Subject: Re: [Amps] Direct rectification of AC mains to drive the amp,

Bill,

you are missing the main point here!

> Another way to float the scope is with a sine wave type inverter 
> running from a 12 volt battery. Mount the inverter and battery in a 
> box and you have a portable power supply to run your scope or anything 
> else. Add a trickle charger for the battery and you're all set.

Yes, sure!  But if you then connect the scope to a direct-line-powered
circuit, the scope chassis and knobs STILL will have a dangerous voltage on
it!  That's exactly the same as using an isolation transformer to power the
scope, or to simply use the scope's internal transformer for insulation, and
lift its ground. In all three cases the scope's chassis will end up
connected to the line through the circuit being measured!

The Fluke Scopemeter is a good solution, because that scope is fully
insulated. Its enclosure and controls are all plastic, and its probes too
use externally insulated BNC connectors. That's why it's quite safe to use
on line-connected circuits. But most normal/older scopes have the probe
ground connected to their metal enclosures, and so they are NOT safe to use
them this way, and if someone does it anyway, he needs to be careful!

Hmm... I'm starting to see why some people here recommend not to incite
anyone to build directly line-connected electronics... ;-)

As Peter says, running the amplifier, and NOT the scope, through an
insulation transformer, is a safe thing to do during the test phase. The
same applies to Bill's battery/inverter solution - it's safe to power the
amp from teh battery/inverter, but NOT the scope! The downside is just that
legal limit amps draw at least 2.5 kilowatts, possibly up to 3, and if poor
power factor is involved, that could be 4kVA. Isolation transformers for
that power level are expensive, and inverters and batteries even more so!

Despite all this, I will STILL build my electronics directly powered from
the line, when it is convenient. And sometimes I will even float my scope to
CAREFULLY take some measurements. But please, all of you out there, don't do
this if you don't understand CLEARLY and COMPLETELY what sort of safety
issues are involved! We don't have too many hams left these days, let alone
any willing to even use a scope...

Manfred

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