On Thu, 17 Sep 2009 23:08:23 +0100, Andy GD1MIP wrote:

thanks for all the comments folks. I considered all and decided to take
the learning curve. I was lucky the transformer was only held togethert
by a non sticky clear plastic wrap.
Having unwrapped the windings I took the 55v secondary off completely.
Re the 20v secs, I worked out the value in volts of each turn, then
removed 8 turns from each of the 4 secs. I now have 16.48 volt (near
enough to 16.5 volt for me) I had to re-space the secondary windings (i
did it to look neat - there may be a sound physics reason too). Lets
hope it works. Before the 'surgery' the 20 volt secondaries were good
for 18amp in parallel (according to manufacturer). Tim stated that
lowering the volts to 16 will give 25% more current. so I should be good
for at least 20 amps. I only wanted 15 so the margin is ok.

No no. Tim stated that _after_ you _completely_ rewind the secondaries
with wire that's got 25% more cross-sectional area you can run 25% more
current. if you increased the secondary current to 20A the per-inch
dissipation in the secondary would go up by about 56%; you can probably
run more than 16A in the secondaries now because they're more spread out
and because the primary will be cooler, but I very much doubt that you
can go as high as 20A and maintain the same temperature.

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