This raises a question. Are you using a power transformer which is normally intended to step down? Whereas you have switched around the primary and secondary so that it steps up?
If this is the case then consider that the primary was designed to be connected to the electric company (which is very low resistance). To prevent frying, the primary needs to present a high impedance. It consists of many turns of thin wire.
The primary is not designed to be used as a secondary. That is, it is not designed to have a load in series with it.
So it seems to me that when you attach your load, you are combining two high impedances in series: (1) Your load, and (2) the primary which you are using for a secondary.
I believe your output winding needs to be thicker wire (possibly a different amount of turns), to reduce its resistance, so it can power your intended load.
I could be wrong.