I bought a 12V 30W horn over the internet. When I connected it to a 12V transformer (4A), it doesn't work. The seller says it will only work off a 12V car battery and is declining a refund. I don't have a 12V car battery to test. Are they correct?
A 12V 30W device will need 2.5A to work, assuming that it is rated correctly.
However, it might take more than 2.5A to start up. Can you measure either the current being taken or the voltage being supplied when connected to the horn
Another thought. Is the transformer outputting AC or DC voltage ?
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Just a transformer? That will give you AC - the horn will want DC.
Not true. It neither knows nor cares where the 12V DC comes from - so long as it is a good, steady 12V DC.
Perhaps the "30W" refers to an audio 30W?
It would likely take a lot more than a 30W input supply to generate 30W of audio output ...
This!
You don't have a friend or family member with a vehicle? Raise the bonnet, connect the horn to the battery observing polarity and see if it sounds.
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If you bought this horn
The specifications state:
The working current of the horn should be above 30A.
Thanks for your help and suggestions. I decided to go to an electronics shop where the guy tested it for me. It requires 7.2A (which I think is what car batteries supply).
Perhaps the 30W did mean 30W of sound output?
No problem triggering it from a transformer at all once you supply the correct current 
Yes, car batteries can definitely supply very large currents; eg, the starter requires hundreds of amps!
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