I have just purchased a 45 W 12 V transformer with an integral photocell for landscape lights. I measured the voltage while it was plugged in and the photocell closed the circuit (off) and there was still about 3 volts across the terminals.
The question is, since the transformer is delivering 3 volts to the terminals, how much energy is it consuming just by being plugged in with no lights attached? Is this normal or bad design?
Input 120VAC 60Hz 52VA OUTPUT 12VAC 3.75A 45VA
Yes, it does use power, but only a small amount compared to its capacity. It's one of the issues that is called "standby load" and all who leave phone chargers plugged in when not charging their phones contribute to it. One solution is to switch it off - the extension leads with a supply switch work well - phone, laptop, tablet all off when not needed.
The loss in a transformer is due to a variety of real-world factors such as non-zero DC resistance in the wiring. Quoting from (what else!) the Wikipedia page,
For simplification or approximation purposes, it is very common to analyze the transformer as an ideal transformer model as presented in the two images.[5] An ideal transformer is a theoretical, linear transformer that is lossless and perfectly coupled; that is, there are no energy losses and flux is completely confined within the magnetic core. Perfect coupling implies infinitely high core magnetic permeability and winding inductances and zero net magnetomotive force.[6][c]
Deviations from ideal The ideal transformer model neglects the following basic linear aspects in real transformers:
a) Core losses, collectively called magnetizing current losses, consisting of[17]
Hysteresis losses due to nonlinear application of the voltage applied in the transformer core, and Eddy current losses due to joule heating in the core that are proportional to the square of the transformer's applied voltage. b) Whereas windings in the ideal model have no resistances and infinite inductances, the windings in a real transformer have finite non-zero resistances and inductances associated with:
Joule losses due to resistance in the primary and secondary windings[17] Leakage flux that escapes from the core and passes through one winding only resulting in primary and secondary reactive impedance.
