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Old 20th April 2006, 09:37 PM   #1
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Default Question about automotive relays??

hey guys

i am working on a project where the ultimate goal is to have a few horns go off if the car door opens...the goal isnt all that important...what is important is knowing what to look for in an automotive relay...so help me out

i plan on splicing into the wire that runs to the little light that turns on when the vehicle door is adjar....this wire will be the signal turn on wire for a relay (i am thinking maybe 30 amps?) which will have power incoming from the battery (either directly or thru the fusebox) and sending it out to a couple of vehicle horns...

what power rating shoud i look for in the relay? is there any problems with my general explanation? what about fuses? cuz i am aware when it comes to an automotive audio amplifier, the main battery positive cable needs a fuse; so will i need to fuse the incoming battery power wire running to the relay? what size fuse to use? the link i have posted is where i was browsing different relays and i cant figure out how some of them differ, or why some 4 pin relays are better than other 4 pin relays just cuz the internal construction is a little different...someone please shed some light on this for me ...thanks!

http://www.mouser.com/catalog/625/1299.pdf
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Old 20th April 2006, 09:56 PM   #2
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The relay doesn't need to be all that big for a few horns. A standard 30A will be more than enough. Fuse the incoming positive wire with a 10A fuse, but this value may need changing depending on your horns. The horn current should be in the technical specs for them.
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Old 20th April 2006, 10:43 PM   #3
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thanks for the reply richie...lets just say each horn is rated at 5 amps...so i would buy a 10 amp fuse and put it in line with the main power wire? this would protect me from any fire damange? or should i get a slightly higher rated fuse? what is the possible risks of using no fuse in this application?
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Old 21st April 2006, 12:13 AM   #4
Eva is offline Eva  Spain
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Use the smallest fuse that does not blow during normal operation, that will provide the best protection, altough it may require some experimentation (or just measuring the actual current being draw by your load and selecting a slightly higher fuse).

Remember to place the fuse in the positive lead at the beginning of your power line in order to protect the entire line.
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Old 21st April 2006, 03:23 AM   #5
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thanks for the reply eva...ill be sure to do that
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