The attached image is a representation of a circuit I set up to evaluate the power output of a transformer. In the image, the transformer is represented by an AC signal (I used LTSpice for the illustration). R1 and R2 represent a pot rated at 3A. R3 is a load resistor. The circuit initialized with "R1" at 0 ohms (pot turned all the way to "off") and the source started at about 23V. The illustration represents the state where turning the pot to increase voltage across the load, R3, resulted in a drop at the source of approximately 1.5V. It is from this static state from which I did my calculations. Please let me know if there is any fault with my setup.

Solving for I:
First, R1 and R3 in parallel equals a combined resistance of 24.4Ω (1/R = 1/R1 + 1/R3)

R1 and R3 in parallel: 1.22V/24.4Ω = .05A

Voltage of R2 = Vsource - Vr1 => 21.5V - 1.22V = 20.28
I of r2 = 20.28V/500Ω = .04A

Itotal = .05A + .04A = .09A

Pot resistances were determined by measurement afterwards.

So, is that a reasonable approach to evaluating the suitability of an unknown transformer for use in a project?

 

Anybody? Bump.

 

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So, is that a reasonable approach to evaluating the suitability of an unknown transformer for use in a project?

I'll tackle your question laterally.

To determine if a (mains) transformer is suitable for a project, I would connect it to the mains and measure the open circuit voltage of the secondary widning using a multimeter set to "AC volts".

The DC voltage after rectification, for light loads, will be about 1.4 times the "AC volts" reading.

The DC voltage after rectification, for heavy loads, will be similar to the "AC volts" reading.

A heavy load is one where the load is close to the transformer rated wattage.

To determine the rated wattage you measure the size of the transformer and compare to others in catalogues and online shops, as the wattage is always based on size (and weight). So two transformers of the same basic type and size will be the same wattage.