Transformer that doesn't take power when not used

Question

With Is this (working) 2-doorbell system dangerous? and Why is it impossible to connect these two doorbells in series or parallel?, I arrived to the conclusion I need a new 220 V AC => 10 V AC transformer.

The old one I have (photo below):

• eats 0 VA when unused (which is good, since a doorbell rings less than 1 minute per day)
• but is unable to deliver 12 VA for my two bells, and so the voltage drops and one bell doesn't work

I tried with a AC/AC wall adapter transformer (from a synth, photo below):

• it is able to deliver 12 VA for the two bells (both are now ringing)
• but it eats 7 VA constantly all day even if the bells are not ringing (i.e. 99.99% of the time!)

What kind of transformer should I look for? I'm not looking for a shopping product recommendation (I know it's out of topic), but rather the right keyword / the right transformer/adapter "family".

What is the right terminology? for a transformer that eats ~ 0 VA if unused, and still is able to deliver 12 VA?

Answer 1

it eats 7 VA constantly all day even if the bells are not ringing

Not all of this is loss. In fact most of it is not loss. It is what is called "reactive power". The transformer temporarily "borrows" some energy from the mains, and then later in a cycle, returns most of that energy. Whether you are charged for this energy depends upon the rate schedule for your electricity, and also on what other devices or appliances you may have running in your house. In my area, commercial and industrial customers pay for reactive power, but residential customers do not.

What kind of transformer should I look for?

To minimize the power lost in a transformer when no current is drawn from the secondary, you will want the primary inductance of the transformer to be high. A high primary inductance will mean a low primary current when the secondary is not supplying power. As mentioned, not all of this current is a loss, and the resistance of the primary wire and the quality of the core matter as well. Often, transformer manufacturers don't provide enough data for you to accurately compare their product with their competitors' products, at least not without doing a lot of calculations with the data they do provide.

Answer 2

Per comments, your measurement of no-load current isn't strictly zero, but somewhere between 0 and 1 VA (or ≤1 W). No-load current should be fairly low (proportionally) in a well-made transformer. A cursory search suggests that no-load currents are around ≤2%, 2-10%, or 6% of full-load current in a laminated-type power transformer.

Based on this, the no-load power consumption (eddy current loss) would be between 0.24 and 1.2 VA.

I wasn't able to find specifics on no-load losses for transformers I found such as Hammond 186C10 or Signal DP-241-5-10L, but either of those or ones like them might be worth trying. Note that those datasheets show other items in the product line, so you could consider a larger transformer (e.g. 20 VA) if necessary to overcome other losses, but no-load current would increase as well.

In my attempt to provide a good solution, I found quite a few interesting things about transformers, most of which are covered well by this All About Circuits article, Practical Considerations - Transformers. While no-load losses would be a useful metric to find in a datasheet, since there are many variables that affect it (frequency, voltage, core composition, etc.) I can understand better why manufacturers don't usually provide this data.

To achieve a truly low or near-zero standby power usage, I think you would need to convert your system to use an efficient switch mode power supply (SMPS), which would output DC.