Electronic – Transformer fuses for Primary and Secondary

Imagine you wound an inductor but instead of one thicker wire you used two thinner wires (this is commonplace by the way). An AC voltage is applied on both wires - lets call the AC voltage AC live and AC neutral and lets say neutral is connected to 0V for reference.

No problem so far?

Now, imagine you only applied AC live to one of those wires and left the other one open circuit (AC neutrals both at 0V still). What would be the induced voltage in the unconnected wire? Would it be: -

• In phase
• Out of phase by 180degrees
• Same amplitude
• Different amplitude

If the induced voltage on the unconnected wire were antiphase then joining it up to the AC live would create a seriously big problem and there would be sparks and breakers tripping. If the induced voltage on the unconnected wire were significantly different to the AC live and you joined them up then same problem as before - sparks and breaker trips.

This means if you wind a transformer with two wires in the same way there will be no phase shift but you can always swap the primary wires or secondary wires over and get 180 degrees.

Inherently there is no 180 degrees phase shift - you only get 180 degrees when you get into a muddle with the wires.

with a 1 Amp slow blow fuse on the primary

Given that your secondary fuse rating implies a load of 120 VA and your primary fuse rating implies an input of 240 VA it could be argued that the magnetization current into the primary is in the order of 0.5 amps\$^1\$ leaving 0.5 amps for real load current seen at the primary (0.5 amps x 240 volts = 120 VA).

This implies to me that your output fuse should be significantly smaller than 1 amp i.e. more like 0.5 amps because if you were to take 1 amp from the output winding of the dual-connected transformer, you would inevitably be taking 1.5 amps into the primary of the transformer connected to raw mains and this could cause a fire.

is it recommended to install a current limiting circuit (or component) to limit the output current to below 1 Amp or is this overkill?

Given the info you have provided I have to be cautious and say that the output fuse should be no more than 0.5 amps. You will still need to use a 1 A input fuse too.....

But, there is a further consideration. The transformer that is receiving the main AC input is not only taking magnetization current for its own core but supplying magnetization current for the other transformer's core.

It logically follows that in the dual connected scenario, the 1st transformer might be taking up to 1 amp just to magnetize both cores leaving you with no headroom for load current.

There is insufficient information to safely conclude anything else so, I would recommend you measure the primary current (nothing on the secondary) and, if you find it to be (say) 0.25 amps you can also assume that it will be 0.5 amps when both transformers are cascaded. This would mean that your output fuse should be 0.5 amps.

\$^1\$ This is a bit of an approximation and underlies the basic problem of cascading transformers. If the full rated primary current is 1 amp and this current contains 0.5 amps of contribution from a 120 VA load, magnetization current could be as high as \$\sqrt{1^2 - 0.5^2}\$ = 0.866 amps RMS. The square root calculation takes into account that magnetization current is due to primary inductance and this will normally be 90 degrees shifted to resistive load current. However with two cascaded transformers the total magnetization current could, theoretically be as high as 2 x 0.866 amps. It's very problematic cascading transformers like this.