Most PC power supplies have a legend on the casing which will tell you
what colour has what specs. It shouldn't take much to work out which
one will provide you enough load.
We recently built a box to house a car stereo with amp and sub's for
use on our back patio. Two PC power supplies makes the whole thing work
nicely, can't see how it wouldn't provide enough for a car horn to
If it's only going to be sounded once in a while, you might be better
off buying a 12 volt sealed rechargeable "burglar alarm battery" and
using an scrounged 12 volt DC wall wart to keeped that battery trickle
That way you'd be able to have the horn sound even if the line power was
You might need to play around with a current meter and a handful of
resistors to limit the battery charging current to an acceptable
continuous level, but it isn't eggsackly something you'd need help from
a Nobel laureat for.
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