12.   Some Measurements

I measured the characteristics of a small selection of transformers to give some comparative data. I excluded regulation from this, as it is difficult to make a suitable load at short notice, and loads tend to get rather hot even with short usage. Most manufacturers will provide this information in their specifications, but be warned that this refers to a resistive load, and regulation will be much worse when supplying a conventional rectifier and filter capacitor (see above, and the Power Supply Design article for more details). It is also worth noting that an inductance meter is often of little use with large iron cored transformers, unless it operates with a sinusoidal waveform at (or near) the design frequency of the transformer. The inductances shown are calculated, since the measured values with my meter were a long way off.

Type   Rating   Inductance   Resistance    Turns/Volt   Magnetising   Core Loss   Reactance    Mass (kg)
Toroidal   500VA   34.7 H   2R4   2   22mA    5.28W   10.91k ohms   5.0
Toroidal   200VA   63 H   5R1   3   12mA    2.88W   20k ohms   2.7
E-I   200VA   4.36 H   6R6   2   175mA    42W   1.37k ohms   3.2
Table 7.1 - Measured Characteristics of Some Transformers

The toroidals are clear winners in terms of core loss in particular, but it must be said that the E-I transformer tested is not really representative of the majority.