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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

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  • $\begingroup$ it should be noted that what you are calling a "transformer" is a regulated power supply. There is a bunch of circuitry in that box that is consuming power even when off. Also measuring the terminal voltage gives no indication of how much ; you would need to put an amp meter on the input. (note you can not do that with a typical consumer multimeter ) $\endgroup$ – agentp May 5 '17 at 12:06
  • $\begingroup$ @agentp I have a Fluke amp meter, but the draw is too low to measure with no load attached. I also understand there are inefficiencies in the circuity that consume power. I was more interested in knowing how much energy this device is consuming in order to produce a constant 3 volts to the terminal when it's "off" and also if this seems rather inefficient compared to similar regulated power supplies? It's producing 1/3 of it's rated output when it's "off". I have a similar device for landscape lights that switches off based on a timer and there is no voltage at the terminals. $\endgroup$ – John May 5 '17 at 14:26
  • $\begingroup$ you need a meter that can read milli-amps AC on the input side. You should not assume zero output volts means a device is not consuming anything either. $\endgroup$ – agentp May 5 '17 at 15:04
  • $\begingroup$ Can you measure the output voltage with the lights attached? $\endgroup$ – agentp May 5 '17 at 15:08
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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.

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  • $\begingroup$ This does not address the difference between a transformer and a regulated power supply. $\endgroup$ – Carl Witthoft May 5 '17 at 12:27
  • $\begingroup$ @CarlWitthoft the OP did not ask for the difference between a transformer and a regulated power supply - you may wish to ask that question yourself... $\endgroup$ – Solar Mike May 5 '17 at 12:31
  • $\begingroup$ @Solar Mike It's for landscape lighting/security so it needs to be plugged in permanently. The photocell is in charge of switching the lights on and off. I understand there are inefficiencies in all transformers. This is sold as a transformer and not a "regulated power supply". $\endgroup$ – John May 5 '17 at 14:44
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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.

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  • $\begingroup$ Re your comment below : where is the comparison with a regulated power supply? $\endgroup$ – Solar Mike May 5 '17 at 12:32
  • $\begingroup$ @SolarMike I'm answering a question about transformers, not integrated circuits. $\endgroup$ – Carl Witthoft May 5 '17 at 15:37
  • $\begingroup$ But you yourself replied in your comment to my answer as follows : "This does not address the difference between a transformer and a regulated power supply." Which then begs the question where do you address the difference? And, of course, how relevant is your comment to my answer? $\endgroup$ – Solar Mike May 5 '17 at 16:24

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