NO,NO,NO!!
The rectified line- rectified and filtered , that is- does not go to
the chassis!!!!
Ever wonder why AC/DC radios were built in a Bakelite case? It was because
the radio chassis inside was FLOATING. Every ingoing and outgoing connection
was routed through a capacitor or a transformer. In our case all that's
connected to the mains is the rectifier board and the amplifier board.
Everything on insulating standoffs!
Even today, in a modern TV about a quarter of the electronics inside
are at line potential.
Since the input and output are isolated by transformers, which can
easily withstand 2500V- the standard withstanding voltages of Formvar wire
or Teflon insulated types, not even talking of the 3000-5000 Volts of coax-
everything else is enclosed within the enclosure and can be even doubly
enclosed in some protecting internal fencing.
RFI filtering has nothing to do with the transformer: the interwinding
capacitance is of the order of a few nanofarads. What do you think the RF
isolation is? That problem is taken care of by the RFI filter, which should
be installed as a matter of course in each and every amplifier, no matter
the topology.
As for safety: do you guys really think that a ham, self educated to survive
a 3000V/1A power supply will be unable to behave near such an amplifier? If
he can't, he shouldn't be a ham in the first place! I am one of the few
lucky ones who survived a 2400 V shock from a GRC-106 amplifier. I know how
it feels: believe me, 300v is unpleasant but definitely very much less that
the 3kV.
I can vouch that before long there will be 300V operating modules on the
market for industrial equipment (there are a few already). How long do you
think it will take for them to show up in homebrew equipment?
Alex 4Z5KS
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Leigh Turner
Sent: Monday, September 16, 2013 2:26 PM
To: amps@contesting.com
Subject: Re: [Amps] Direct rectification of AC mains to derive the amp VDD
supply
Well Alex an amateur / ham could consider doing such an abominable thing in
a home brew project, but directly rectifying the AC mains would never pass
regulatory approval for commercial equipment sale in most jurisdictions
which rules have tightened up in recent years...particularly in respect of a
DC and harmonic components going back into the mains, and power factor
mandated by CISPR, CENELEC, EN standards, IEC, etc.
That 2 x ARF1500 amp on the drawing board is an interesting project with
nice comfortable output impedance levels for facilitating easy broadband
matching. It should be easy enough for the RF transformers to isolate the AC
mains using the direct rectification system you propose. One must of course
take extra care to ensure the AC mains neutral is connected to the chassis!
Leigh
VK5KLT
-----Original Message-----
From: Alex Eban [mailto:alexeban@gmail.com]
Sent: Monday, 16 September 2013 7:43 PM
To: 'Leigh Turner'; amps@contesting.com
Subject: RE: [Amps] Direct rectification of AC mains to derive the amp VDD
supply
Why?
The TV industry and the American radio industry has been doing it for tens
of years! You don't have any qualms about using a transformer coupled power
supply, but you shrink when considering a transformer coupled amplifier. all
you have connected to the live mains is a piece of about 6 by 12 inches.
The argument of the existent high voltage is irrelevant, since you have in
any case about 150 volts on the amplifier while here in 4 Zulu land we end
up with about 300 VDC.
All you have to be careful about is to have the chassis connected to the
neutral side of the line during work on the open amplifier!
I'm just planning one of these right now: 2 ARF1501 in push pull. The nice
thing is that the output impedance at 300V is quite close to 50 ohms! So,
all you need there is a 1 to 1 balun.
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Leigh Turner
Sent: Sunday, September 15, 2013 2:37 PM
To: amps@contesting.com
Subject: Re: [Amps] Direct rectification of AC mains to derive the amp VDD
supply
That's right Alex.
However my primary concern expressed in my original email was truncated...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.
Leigh
VK5KLT
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Alex Eban
Sent: Sunday, 15 September 2013 8:25 PM
To: 'Bill Turner'; 'Amps'
Subject: Re: [Amps] Combiners for SS amps + cooling issues.
....it's quite a primary concern!!
The problem with filter capacitors is their relatively large series
inductance-ESL.
Since most diode killers are fast rising transients, of relatively short
duration, most of the transient voltage develops across the ESL and the cap
doesn't help.
In modern TV sets there is a special low ESL film capacitor- about
2uF- connected across the input line, ahead of everything except the fuse.
This is the best protection you can reasonably have. Tackling longer
duration transients with high energy content would require such massive
components that you couldn't afford them, or else, some kind of cutout
circuit to disconnect the power supply from the line during the transient.
This was usually done in military vehicular equipment.
Alex 4Z5KS
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Bill Turner
Sent: Sunday, September 15, 2013 5:49 AM
To: Amps
Subject: Re: [Amps] Combiners for SS amps + cooling issues.
ORIGINAL MESSAGE: (may be snipped)
On Sun, 15 Sep 2013 08:59:59 +0900, Leigh wrote:
>A secondary concern would be decoupling the direct rectification
>circuitry from transients and large amplitude spikes often riding on
>the incoming mains voltage. These spikes would need to be adequately
>suppressed from reaching the FET drains lest there be a risk of
>breakdown
damage.
>
>Leigh
>VK5KLT
REPLY:
If the bridge rectifier is feeding a large capacitor directly, shouldn't
that be enough protection against transients?
I'm no expert on power line transients so I'll defer to those who are.
73, Bill W6WRT
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