[AMPS] AC Line side regulator..

[Ian White, G3SEK]

Skipp wrote:
>
>The article was "A line Side Regulator" for HV...   Mid to late 80's QST
>I believe. 
>
Well remembered! AA5C, QST May 1987, p.23-25.

>Not a good idea.  One SF Bay Area Old timer became room temperature
>building the circuit. Lots of extra line noise to deal with,
>advantage/tradeoffs were marginal, SSB response was pretty poor in large
>load situations. Part costs  were another large variable. A big lamp
>dimmer with feedback control. 

Also lots of transient-induced stress on the transformer core, windings
and insulation - every single cycle. 

The other article in VHF Comms / UKW Berichte (4/88), mentioned earlier,
was developed by DL8MX and DB1NV. They had tried the AA5C circuit and
found the same problems as everyone else. 

Their idea was significantly different. It started with a series
resistor in the mains supply, and a solid-state relay that  shorted the
resistor out - very much like a step start circuit. 

However, instead of leaving the SSR permanently energized, it was
switched on and off in long bursts of several mains cycles to keep the
voltage on the smoothing capacitor *roughly* constant. The aim was not
for precision regulation, only to keep the output voltage within about
50V - which is plenty good enough for a PA. 

I just found the copy of the article. The voltage control feedback is
via a comparator, comparing a divided-down sample of the HV output
against an adjustable reference of about 5V. There is some hysteresis
built in, corresponding to about 50V variation at the HV output, and
this means that switching occurs much less often than in any cycle-by-
cycle regulation scheme. A capacitor holds the comparator off at first
switch-on, so the series resistor gives an automatic soft start.

With no load (bleeders only) the solid-state relay is never closed, and
the series resistor holds the output voltage down to about the same
level as is wanted at full load. As the load current increases, the
comparator switches the SSR on for longer periods. Beyond a certain load
current, the SSR is permanently on, and the power supply behaves as
normal except for the small extra voltage drop through the SSR.

The obvious disadvantages are the power wasted in the series resistor
(50-100W) and in the SSR. Also the series resistor has to be selected on
test. However, this idea should be much kinder to the transformer than
phase-control switching that operates on every cycle: in this setup the
switching occurs only once in many mains cycles, and the transformer
core is always mostly magnetized so the switching transients are smaller
(and limited by the series resistor).

The circuit was developed for a Drake L7, so it seemed strange to
publish the idea in an obscure VHF journal. Maybe it never had a fair
trial in the big wide world.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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