typical PSU switching on
ON, wait 10sec -
wait 90sec - HT ON - Start
transformers used are obtained from
domestic microwave cookers. They will
handle 600W continuously and provide
about 2kV AC under full load. They do
need a slight mod - easily
with a heavy hammer and suitable
full-wave rectification two HV
are used with their primaries wired
in anti-phase. This is due to
one connection to the secondary
winding being physically close to
Although this adds weight it does
mean the PSU should run cool even at
side of the transformer secondary
is normally connected to ground.
Disconnect from ground and use as
240v 8A Variac is used to control
the primary of the HV transformers
to give from 0 to 3kV max out.
2.4kV on load)
the transformers from Microwave
ovens have a problem. They
appear to have a very high series
- great for constant current
supplies to magnetrons - but not too
for our purpose.
The PSU as
designed gave 3kV off-load and only
1.9kV under a 300mA load. Just
about enough for the baby 400W
- but not good enough for the next
'kW' HF project.
(10/03/03)- from Juha OH1MLZ.
He suggests a
simple mod to the transformers
which should make all the
difference. Apparently there
are two 'magnetic
shunts' built into the transformer
which raises the series reactance.
they are easy to remove.
Full details are available
but note -
this is quite a big file (2.5MB)!!
are two small sections of laminated
core wedged on either side of the
centre core (and between the two
Simply take a suitable punch + large
hammer and tap them out, taking care
not to damage the windings. (I used
a large nail, ground the sides flat
to fit the gap and flat at the end -
two minutes later the job was done)
can be seen here partially
RESULTS - What
a difference.... 2.85kV 50mA load
- 2.4kV 250mA - 2.35kV 300mA.
about 2.2kV @ 500mA continuous
which should be ideal for a linear
with a pair of Gi-7B
ran cool and no hum at 300mA.
suggestions ? - E-Mail RADARS@radars.me.uk
news. Bryan, G3RLE happens
to have a possible transformer in a
at about 1amp. If it is
working OK then a slight redesign to
bridge across the 1120-1120 taps
should give us an even bigger
Also - instead
of using a string of diodes, Rs
& Cs, for the rectifier
stack use a 16kV 1.2Amp stick,
(again for Microwave Ovens, from CPC
Ltd) for only about £6ea. brand
RADARS will be
ordering some so if anyone wants
any just let us know (RADARS@radars.me.uk)