[Boatanchors] Unused transformer windings.

[Brian Clarke]

Hi Robert,

What you say is technically correct.

However, there is a wee problem.
Transformer designers lay up the windings and the inter-winding insulation to achieve 
as gentle an electric field gradient as possible - gives more copper in the 'window' 
and raises the transformer's efficiency. If the now unemployed 5 V rectifier heater 
winding is connected in series with the primary - for either buck or boost purposes, 
it will be connected in either the phase or the neutral line, no matter what country 
you are in. The 5V winding is now at a much lower Voltage with respect to the 
transformer's frame than if it were feeding the heater of, say a 5Z3 or similar - which 
may give pause for thought. In other words, the insulation is being stressed in a 
way the original designer had not intended. Try to find out where in the winding 
lay-up the particular winding is before using it in this buck or boost way. If it's right 
next to the HV winding, I'ld think twice - perhaps even carry out a HiPot test.

In fact, if you use any external transformer to achieve a buck or boost for the mains, 
check where the windings are with respect to the frame.

The fear that unterminated windings will have high Voltages on them is unfounded - 
the only extra Voltage that should appear there is due to that winding's IR drop, ie, 
its load regulation.

73 de Brian, VK2GCE.
  Robert said:

  Ken,
  Your explanation of why it works (it adds turns to the primary and therefore 
  reduces the primary to secondary turns ratio) is of course correct. You 
  connect the primary Start to one side of the AC line, the primary Finish to the 
  filament winding Start and the filament winding Finish to the other side of the AC 
  line (or vice versa). So the two windings are in series aiding just as if 
  they were one winding.
  <snip>
  And incidentally, there are no significient efficiency penalties incurred in 
  doing this (using a filament winding to reduce the secondary voltages). With 
  more turns in the primary, R sub Primary is slightly higher. But both core (H &
  E) losses and I Square R losses are less than they would be if the filament 
  winding were being used normally (less current in the filament winding).