I know that a horn requires a larger amount of amperes to operate than many other electrical systems.

I am reading a book and I stepped into this situation:

..shows a relay application in a horn circuit. Battery voltage is applied to the coil. Because the horn button is a normally open–type switch, the current flow to ground is open. Pushing the horn button will complete the circuit, allowing current flow through the coil. The coil develops a magnetic field, which closes the contacts. With the contacts closed, battery voltage is applied to the horn (which is grounded). Used in this manner, the horn relay becomes a control of the high current necessary to blow the horn. The control circuit may be wired with very thin wire because it will have low current flowing through it. The control unit may have only 0.25 ampere flowing through it, and the horn may require 24 or more amperes

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from "automotive electricity and electronics by Barry Hollembeak".

Why bother using a relay? I know that the relay coil completes the horn circuit using magnetism, and I know that the relay is necessary but I can't understand why. For me the relay is just a fancy way to close the path to the horn device.


You really aren't understanding how a relay works or what it does. It states right in the quote that the control wire can only handle .25A and is very thin. The horn itself may require 24A or more.

When the relay is activated, it can transfer the needed amperage to the horn through wiring which is designed to take the load. There is only so much space within a steering wheel and column. In order to run wiring for everything which is controlled there, a relay is the way to go.

Consider in most vehicles you need to control things like the turn signals, lights (to include changing from low to high beam), ignition system, wipers/washers (front and back if so equipped), cruise control, etc. If every item was to be controlled directly, you'd have to increase the thickness of every wire and thus the size of the steering column to house it.

Besides this, you should also consider the cost to run such wires. You'd have to extend wiring to these specific devices of the larger size, which all costs more. It would also make the vehicle heavier.

Another thing to consider is the contact points in the horn button. In order to handle the load which would be put through the horn button, you'd have to build all points in it to handle the amperage if ran directly. A typical relay can run millions of cycles. To build the horn button to take the same amount of cycles, you'd have to have some serious engineering efforts going on. The relay not only makes the circuit, it also handles the load. The horn button itself is not designed the same way.

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  • Your answer was very helpful because didn't understand why a device that requires more amperage would have thinner conductor either. But how exactly having a device that closes a contact by electromagnetism produces a high current? Are you saying that the current from RELAY TO HORN DEVICE is greater than RELAY TO GROUND? – Diego Alves Oct 28 '17 at 18:34
  • @DiegoAlves no he isn't - the relay is just a remote controlled switch and when you close it, the current flow is the same at all points in that circuit, as long as nothing else connected to that circuit is in use. If you want to know more search for Kirchoff's current law. – Solar Mike Oct 28 '17 at 19:27
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    In the early days, relays etc were too expensive so all the wires were thicker and the currents for lights, wipers and horns etc all went through their respective switches. Even the starter solenoid was still manual until the late 50´s on some cars. One job was to clean the horn contacts on the steering wheel push button and the brush and brass track behind the wheel... – Solar Mike Oct 28 '17 at 19:31
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    Thanks for the add, @SolarMike ... To the OP, one thing I didn't clarrify here is, there are two separate circuits: one for the relay operated by the horn switch; the second for the horn which is operated by the relay. The horn switch circuit is low amperage, while the relay provides a high amperage circuit. Hopefully that helps clear things a little. – Pᴀᴜʟsᴛᴇʀ2 Oct 28 '17 at 19:38
  • I know that the relay was introduced to provide a higher amperage but I still don`t understand how the relay does it. – Diego Alves Oct 29 '17 at 1:28

A relay is controlled by a switch or button or other electrical contact that causes an electric switch to make contact so that it now operates a device with a higher power demand although both are the same voltage.

Thinking out of the box for example only, now picture a sports field with flood lights and it is time to turn on the lights. Someone goes over to a simple wall switch, similar to those in your home, and flips it up. This causes a reaction in the relay, causing it to trip the bigger switches that actually turn on the field lights. Yes I know they are 2 different voltages but the actions are the same.

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A relay isn't 'necessary' as such, it would operate just fine through the steering wheel switch in most cases, but a relay is designed in the circuit for many reasons.

Voltage drop - running a wire from the battery into the steering wheel and back out to the horn is a long path, and causes voltage drop from the resistance in the length of wire. Voltage drop may not be noticed in a horn, but will be noticed in sensitive components like head lights and spot lights.

Wire size - small wire is lighter, cheaper and takes up less space in the car.

Electrical switch size - relays allow cabin switches to be tiny, silent, easy to operate and last almost forever.

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