How To Combine Dissimilar Transformer?
This is a tricky one. The transformers won't have identical open-circuit voltages and will differ in internal resistance and other subtleties. The danger is that one will supply the majority of the current and overheat. 

When paralleling coils on dual-winding transformers we have the assurance that the magnetic circuits, wire gauge and number of turns will be close enough to identical that there will be no surprises.

Even with your separate rectifier idea you can't be sure that things will balance out. The transformer with the higher voltage will supply the pulsed current drawn by its rectifier.

If you need to do this as a temporary fix or for very light duty cycling then I suggest:







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Test circuits.


Connect one of each of the secondaries together as shown in Figure 1a.
VM1 should read close to 0 V AC. If not (you'll probably read 24 V AC) then swap one of the secondaries.
Next put a decent load on each secondary. Measure the no-load voltages and then the load voltage and currents. From these readings you can calculate the internal resistance of each coil.


With the internal resistances and voltages measured you can now model the parallel coils (or use a simulator) and see what will happen when you try to draw 8 A from the combination. You can use a DC voltage source for the simulation if it is easier to visualise.

You might find that with the addition of a low value series resistor on one of the transformers that you can balance things out correctly.