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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