I've got a magnetic low voltage transformer (120V to 12V). The instruction book says that the transformer must be loaded at least 80% for proper operation. That's fine, but my mind wants to know why. I can understand overloading would be bad, but underloading? Why would too small of a load be a problem?

 

Depends what "proper" means. It may need that much load to pull the voltage down to ~12V. Could be up around 16V or more, unloaded. Otherwise, I'd say the wording makes no sense.

 

Small wall-wart type transformers are designed to not start a fire if short-circuited (overloaded). This makes for lousy load regulation, so a unregulated DC power pack advertised as 12Vdc under rated load will shoot up to 18V unloaded...

 

I've got a magnetic low voltage transformer (120V to 12V). The instruction book says that the transformer must be loaded at least 80% for proper operation. That's fine, but my mind wants to know why. I can understand overloading would be bad, but underloading? Why would too small of a load be a problem?

Do you have a mfr/part number for this transformer?

If this is a split bobbin transformer with no auxiliary or safety circuitry, then I don't understand why there would be loading requirements.