Transformer issue

I recently bought a step-up/down transformer to use with my UK stereo.  So far it works fine.  This morning I tried plugging my UK hair straightener in it.  When I turned it on, there was a horrible buzzing sound so I just switched it straight off.  I tried it again and it made the same noise.  Any ideas why this might happen?   I don't have its specs with me but I can't imagine it's too much for this transformer...it just seems mad!

I know it sounds mad but you're overloading the transformer with the hair straightener.

We have one to use with my sewing machine. Depending upon the size you bought, it is only rated for certain loads. It should have a number which tells you the wattage. For example: ST-200 is a 200Watt unit which will draw 1.7 amps at 120 volts.

Hair straighteners, dryers or anything with a heating element draw a lot of power. My hairdryer is 1800 watt and can draw something like 15 amps upon start-up. That's a lot!

We lost power during hurricane Ike last year and were running a generator for lights, the fridge, etc. I couldn't believe how I could hear that thing strain when I plugged in the toaster! Shocking how much power it takes when something has a heating element. 

You definitely want to check (a) the rating of the transformer and (b) the rating of your hair straightener.   I think you'll find that the second exceeds the first!   

Hair straighteners, dryers or anything with a heating element draw a lot of power. My hairdryer is 1800 watt and can draw something like 15 amps upon start-up. That's a lot!

We lost power during hurricane Ike last year and were running a generator for lights, the fridge, etc. I couldn't believe how I could hear that thing strain when I plugged in the toaster! Shocking how much power it takes when something has a heating element. 

Yes, anything which involves generating a substantial amount of heat involves much more power than just running a couple of lights or a radio/TV/similar device.

In over-simplified terms, watts = volts x amps, or by rearranging the formula, amps = watts / volts.   Hence 1800W / 120V = 15A.   

It becomes more complicated when things like motors are involved, but for basic heating elements that holds true.  So a 1200-watt toaster, for example, will draw 1200W / 120V = 10A.  Or if it's a 1200W toaster designed for 240V operation in Britain, 1200W / 240V = 5A.

Give a transformer plenty of ventilation & in particular don't stand them on a carpet; you can get discoloration due to the heat, made worse because it sinks into the pile and gets less air circulating around it.

Seems it's more than meets the eye. (Sorry, Geeta! That's the first thing that ran through my mind! Now the Transformers theme is in my head!)