The sticker on my 240V:110V step down transformer says it is 600W but JakeHuston says it is rated for 300W? (I'd previously posted to JakeHuston asking the question, but no reply).
600 or 300 is a rather big difference so I'd like to know the answer and not plug in a 110V device that is too big for this transformer.
I have read somewhere you should add 50% to a device's rated capacity as a safety margin i.e. if the 110V device is 500W then best to use a transformer with >750W. (I know that's an oversimplification as there are other factors at play e.g. high energy start-up items like hair dryers etc but I'm just looking for a general rule-of-thumb).
Any comments or enlightenment will be appreciated.
Actually, the rating specified is misleading. Transformers are not rated in watts but in VOLT AMPERES (VA). Which is simply the product of voltage applied across the transformer and the current drawn by the transformer with the load connected.
The peak value of current and voltage occurs at different time instants as determined by the POWER FACTOR of the load; which again can be LAGGING or LEADING. In short if current peak lags the voltage peak, it is lagging power factor (motors, unloaded transformers, old fluorescent tube ballast have lagging power factor) and vice versa.
A transformer should never be loaded beyond its VA rating for anything more than a few seconds. Now Wattage is related to volt amperes as
Watts = VA * Power factor
Note: power factor value lies between 0 and 1.
So assuming a typo, the transformer is rated 600VA which means you can draw 600/110 = 5.45A - losses in transformer; so you should be left with about 5A to use.
Remember that not all loads have the same power factor, motors and cheap chinese SMPS/adaptors have substantially lower power factor than incandescent bulbs or high end IT equipments, which means you can extract far less "Watts" from your transformer. Higher the power factor higher is the wattage available (at PF of 1 wattage = 600).
Excuse me if it made no sense; English is my third language.