In article ,
robgraham writes:
I have an older Ikea quad spotlight system in my workshop for lighting
over the wood turning lathe. It uses a toroidal transfomer. I was
checking out a portable infra red heater which has a 700w/350w switch
- when switched to the lower setting, the transformer in the light
hummed noticeably. Any ideas why ?

My guess is:

a) You have significant voltage drop in your workshop with the
heater on (do the lights dim when the heater comes on?), and
b) the heater has only one element, and for the low power setting,
it is using either phase control or half-wave rectifier to generate
the low power setting.

These two mean the mains sine wave fed to the toroidal transfomer
is distorted, and mains toroidal transformers can get a bit
unhappy when that happens and start drawing a strange current
waveform. In theory, this can cause the transformer to overheat,
but I doubt the effect in your case would be bad enough for that.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]