Class 2 transformers: ubiquitous, hidden, and inefficient

Class 2 transformers are small line-frequency transformers that are widely used for control systems that require 24 VAC signaling, including residential and commercial HVAC systems, industrial control systems, doorbells, and much more. In this work, we sampled and tested seven Class 2 transformers, each across different operating conditions, in order to characterize their efficiencies and note their shortcomings. We also provide possible improvements and solutions. We see on average a peak efficiency of 84.43% with 5.37 W of power loss when operated at 75% (30 VA output power) of their rated power, a 1.84% efficiency drop from the temperature rise that occurs at steady state when operated with full load, 2.8 W of no-load loss at 120 VAC input, and a no-load loss contribution of over 50% when operating at less than 75% load power. With these values, there is a clear goal to strive for in order to improve or create an alternative to these Class 2 transformers.