Can I connect transformer secondaries with different voltage in paralel?
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Can I ...
We have no way of knowing your capabilities, so there is no way to answer this question.
Connecting multiple transformer secondaries can yield useful results in some cases. In series, the voltages add, and the current capability is that of the lowest current winding. In parallel, the voltages must be the same, and the current capability is that of all windings combined.
The above assumes that phase was properly considered. When polarity of one winding is important relative to other windinds, the schematic shows a dot by one end of each winding. All the dot ends will be in phase, and 180° out of phase with the not-dot ends.
To usefully parallel two secondaries, both these conditions must be met:
The two windings have the same open circuit output voltage.
The two dot ends are connected, and the two not-dot ends are connected.
When you do this right, you get the combined current capability of both windings, and the voltage of each winding.
Connecting two windings of different voltage is a bad idea. This effectively shorts part of the transformer against itself.
A high impedance meter in most cases will give more accurate readings because it does not draw much power from the circuit under observation, but it can give misleading readings when the circuit produces high voltages without a load. What you are observing is the result of an overly sensitive meter combined with inductive and capasitive coupling. If you put a load on your transformer, you won't notice this.
Leave the center taps disconnected, since with floating center taps there'll be no chance of imbalance in the 12.6 volt windings causing damage anywhere.
Also, be sure that your variac drives the transformer array instead of the array driving the variac, since that way the array won't have to supply the variac's losses.
Best Answer
We have no way of knowing your capabilities, so there is no way to answer this question.
Connecting multiple transformer secondaries can yield useful results in some cases. In series, the voltages add, and the current capability is that of the lowest current winding. In parallel, the voltages must be the same, and the current capability is that of all windings combined.
The above assumes that phase was properly considered. When polarity of one winding is important relative to other windinds, the schematic shows a dot by one end of each winding. All the dot ends will be in phase, and 180° out of phase with the not-dot ends.
To usefully parallel two secondaries, both these conditions must be met:
When you do this right, you get the combined current capability of both windings, and the voltage of each winding.
Connecting two windings of different voltage is a bad idea. This effectively shorts part of the transformer against itself.