|
I'm talking about those really loud horns which has a 12V air pump and 2 or 3 trumpets connected with a hose. The expensive ones can also play a tune, very popular in the 80's. But if you change your factory horn (or rather tooter) to an air horn, the factory relay is not rated for the high start-up current and the contacts often gets welded closed. Especially when you give the horn button a quick blip and the horn keeps going while you are in rush-hour traffic and have to pop the hood to pull off the wire to the pump (don't ask how I know...). A reverse diode across the pump power terminals will conduct the back EMF when power is removed, so the relay contacts will not see this back EMF.
Back EMF is not always a bad thing. In fact, it is what makes you car ignition work. You have current flowing through the coil, and when the contact breaker opens the inductance of the coil tries to keep current flowing by increasing the voltage (back EMF), which results in a spark across the plug. There is also a small spark across the contact breaker, but it is rated to handle it. The horn relay in your car is rated the handle the back EMF from the factory horn, but not from an air horn.
The factory horn is an electromagnet with a hammer which hits a metal disk for the sound and opens a series switch, the hammer retracts and the switch closes again to again energize the electromagnet and the hammer hits the disk again, and so on. I have not opened one, but there should be a reverse diode across the electromagnet to protect the contacts inside the horn (this diode can not be fitted externally, must be internal). Perhaps the early ones did not have a diode or it was underrated and did not last long, which might be why the contacts would often fail. And by the way, there is a screw on the outside which you can adjust to change the resonant frequency of the disk which indirectly changed the loudness.
Have fun...
|
