How to tell whether a converter includes a transformer

I'm trying to buy a 200W step down transformer/converter here in the US before I go (for various reasons - so Paul's links won't do it for me), and was wondering how to tell whether a converter includes a transformer? Some of the ones on Amazon seem to be using the two words interchangeably! Thanks!

I'm afraid that a lot of promotional material you'll find is very imprecise and doesn't tell you whether it's a proper transformer or an electronic converter.

I'm trying to think of a simple way to describe how to tell the two types apart, but it's not quite as easy as looking for one specific thing (it's a case of being easy with experience, but rather hard to explain adequately to cover every eventuality). 

A traditional transformer is relatively heavy, since it contains a lot of steel and copper.  Size and weight increases with power rating.  It will also be more expensive than an electronic converter of similar rating.

Some units are sold where covers over each side of the actual transformer are used to hold the power cord and outlet, like this:

http://www.dvdoverseas.com/store/index.html?loadfile=itemthg200.html

Anything looking broadly similar to that is a real transformer.

Except for the smallest units (50 watts etc.), a real transformer tends to be too large and heavy to be built into one of those boxes which plug directly into the wall outlet.   Anything which is in a "wall wart" box similar to those commonly used as phone chargers and such like and which purports to be rated at several hundred watts or more will almost certainly be an electronic converter and not a real transformer.

You're looking for something which will come in a case with a separate power cord to plug into the wall outlet, e.g.:

http://www.dvdoverseas.com/store/index.html?loadfile=catalog6_0.html

Thanks Paul. Yes. the seven star/simran one was actually one of the ones I looked at. I'll be using it to blow up an aerobed (120W) and then off and on to play the music system (40W). So I was looking at the 200W transformer. A weight of 5 pounds doesn't sound too bad!

I'll be using it to blow up an aerobed (120W)

I take it that's a fairly standard sort of compressor motor?   For the sake of an extra 1 lb. or so to pack, I'd suggest you go up to the 300VA model.  It will handle the motor's switch-on surge rather better, and for the sake of a few extra bucks it might be handy to have the extra capacity one day.

By the way, it appears from the pictures that the transformers on that site are supplied with a Continental plug of the type which fits both German & French receptacles (used throughout much of Western Europe).  They suggest a plug adapter, but the one for the U.K. - http://www.dvdoverseas.com/store/itemss414.html - doesn't appear to provide a ground connection for the German/French plugs, so I wouldn't advise using that adapter if you buy from there.   Better to just cut the plug off and fit a British plug when you get here.

Thanks Paul, yes I did find another seven star transformer that has the UK plug either attached or as an additional attachment. And, I was also wondering whether 200W would be cutting it too close - I'll go for the 300 then. Thanks again!

I was also wondering whether 200W would be cutting it too close

With the type of motor you propose using, yes, it might be, depending upon how well that particular transformer will cope with the switch-on surge.  I'd always go for a transformer rating at least 2½ to 3 times the motor rating. 

It doesn't matter for non-inductive loads like lamps, radios, hair curling irons, etc., where you just need a transformer which matches or exceeds the rated load.