Electronic – Why does a transformer for halogen bulbs burn when bulbs are too strong

powertransformer

If I have a transformer for 4x20W but I use halogen bulbs let's say 4x60W, why does the transformer burn then? I always thought that the bulbs are only as strong as the transformer gives, but it's the other way around. The transformer tries to give 4x60W and burns after short time.
Why is this so? I'd like to understand the physical background. Is this because the current density becomes too high or something like that?

Best Answer

Too low resistance of the bulbs causes too high current in the transformer secondary winding and so the transformer gets overheated. If you short the transformer (extreme of what you have with a too powerful load) the transformer gets burnt even faster (don't try that).