| The typical PSU switching on sequence is:
Mains ON, wait 10sec - Heaters ON,
wait 90sec - HT ON - Start Tx
|
The HT transformers used are obtained from scrap
domestic microwave cookers. They will handle 600W continuously and provide
about 2kV AC under full load. They do need a slight mod - easily accomplished
with a heavy hammer and suitable punch!! (See
Problems !)
To get full-wave rectification two HV transformers
are used with their primaries wired in anti-phase. This is due to
one connection to the secondary winding being physically close to ground.
Although this adds weight it does mean the PSU should run cool even at
1KW.
[Note: One side of the transformer secondary
is normally connected to ground. Disconnect from ground and use as the
'centre-tap' near-ground connection]
An external 240v 8A Variac is used to control
the primary of the HV transformers to give from 0 to 3kV max out. (hopefully
2.4kV on load)
PROBLEMS ENCOUNTERED
ON TEST
Unfortunately the transformers from Microwave
ovens have a problem. They appear to have a very high series reactance
- great for constant current supplies to magnetrons - but not too good
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 linear
- but not good enough for the next 'kW' HF project.
LATEST NEWS (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. However
they are easy to remove. Full details are available on:
http://web.archive.org/web/20030627162157/http://www.vaxxine.com/phil/QEX_HV_Power_Supply.pdf
but note - this is quite a big file (2.5MB)!!
Basically there are two small sections of laminated
core wedged on either side of the centre core (and between the two windings).
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)
| The two magnetic shunts
can be seen here partially removed. |
 |
RESULTS - What a difference.... 2.85kV 50mA load
- 2.4kV 250mA - 2.35kV 300mA.
This suggests about 2.2kV @ 500mA continuous
which should be ideal for a linear with a pair of Gi-7B
The transformers ran cool and no hum at 300mA.
Any more suggestions ? - E-Mail RADARS@radars.me.uk
Latest RADARS news. Bryan, G3RLE happens
to have a possible transformer in a bottom drawer!. 1640-1120-857-0-857-1120-1640
at about 1amp. If it is working OK then a slight redesign to a full-wave
bridge across the 1120-1120 taps should give us an even bigger PSU.
Also - instead of using a string of diodes, Rs
& Cs, for the rectifier stack use a 16kV 1.2Amp stick, HD207,
(again for Microwave Ovens, from CPC Ltd) for only about £6ea. brand
new.
RADARS will be ordering some so if anyone wants
any just let us know (RADARS@radars.me.uk)
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