A large variable power supply using a VARIAC as the control element.
by ralph klimek VK3ZZC March 2011
project was created to permit me to experiment with MOSFET RF power
amplifiers. I wanted something that was continuously variable from near
nothing to as high as the main transformer , about 70 volts, would
permit. It also should be capable of at least 10 amperes output
at any voltage in its range.
There are any means of producing
this outcome however I did not want to have the trouble of designing a
boost-buck converter. A switchmode converter of this range of voltage
from say 12 to 70 volts is not trivial to design. I had in my
junkbox this very large transformer with a 35-0-35 output.
The large core would allow this to continiusly handle 500VA. The
output should be remotely switchable at any setting. It should also
safely shut down under a gross shorted condition.
I also biult
this to demonstrate that a classical full-wave-center tap
or classical bridge rectifier supply could be fed from a variac.
Everything I required for this project was found in the junkbox.
internal variac is only rated to 2A intermittant, 1.5 A full current.
However due to the magic that is a transformer the secondary output
current can be nearly ten times this without adversely impacting the
In operation, the output voltage varies very smoothly
and controllably with the variac. The regulation and ripple, as
you would expect, is not great without any true electronic feedback and
control, but for my purposes is adaquate for powering large amplifiers.
is a large contactor type relay for selecting the secondary tap,
allowance for an external variac to permit a full 20A at 70 volts, a
mercury relay on the
output is capable of switching the full load ,
at DC. This is impossible with a conventional relay as it would
just arc and melt. Another benefit of the mercury relay is that it is ,
by default, open circuit. It means that in the event of some
catastrophe, it can be relied upon to disconnect the load.
I have another use for mercury relays here.
internal use only auxilliary supply give operational voltage for the
two internal 24V relays. There is also a very substantial DC
circuit breaker on the output that will trip at 20A. DC rated
breakers are as rare as rocking horse dung. This one was a
relic of my days of computer mainframe service and lay in my junk box
for nearly twenty years before finding its second life here.
There is also a MOV across this circuit breaker. This kind
of breaker can induce a mighty inductive surge as they contain a
magnetic armature that releases the springs stored energy as an
inductive electrical spike. Just what your test projects want !
Use AC rated magnetic breakers with caution on DC. The DC current
will trip the magnetic circuit long before AC will. An AC arc
will extinguish itself if the path length is long enough. A DC
arc can maintain itself in air nearly 10 time longer than an AC arc.
tap changer interlock consists of 3 front panel switches that
must all be thrown to permit the tap change relay to activate.
This was done
to prevent accidental tap changes under load
which could be bad for the rectifiers, the load and may cause the relay
to arc over.
The output current monitoring shunt resistor is a
very rare 0.04ohm Dale resistor. Our old mainframe used to have
hundreds of these and I had but one left in the junkbox. It is
basically a short circuit within a mineralised plastic body.
I dont know what part of this resistor is 0.04 ohms. Surely the
leads, alone would have this order of resistance !
protection consists of a fuse of last resort, magnetic circuit breaker
and MOVs on both the primary and secondary. I believe in MOVs, you
MOVs work better than clamp diodes across relay coils as
they permit the magnetic field to decay faster than a diode will permit.
|50V 20A variac control variable PSU|
|front panel power switches control mains input, int/ext variac select and output control||at the rear are auxilliary mains power connectors and external variac input||the variac is visisble here under the heatsink. some cooling is desireable|
|the main transformer, meter calibration pots and just a hint of the seperate auxilliary control PSU||50A
amp stud diodes provide for reverse polarity protection and inductive
surge clamping. also visible is the output control mercury relay, the
little black tube upper left||front
panel controls permit remote control, hi/lo meter range and interlocked
selection of centre tap or full secondary via a large open contactor.|
page created Wed Mar 23 19:57:08 EST 2011