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Topic: Controlling a Car Horn with a MOSFET? (Read 1 time) previous topic - next topic


Jul 07, 2015, 10:32 pm Last Edit: Jul 07, 2015, 10:42 pm by conrailandrew
Hello all!
First post here, so forgive me if I'm not posting this question to the right place.

Background info:
I'm trying to control a 12V car horn from my Arduino, using a MOSFET. The horn draws about 8-9 amps, according to my multimeter.
This is the MOSFET I'm using: http://www.futurlec.com/Transistors/IRF540.shtml
(The 30amp version).

This is how I'm wiring it up, with the exception that I'm using an Arduino Mega 2650, and the car horn as the load.
I'm using a 12v car battery as the power supply.

The problem I'm running into is that when the Arduino attempts to turn on the horn, all I get is a faint click from the horn, and the MOSFET immediately gets extremely hot.

Troubleshooting I've done:
I know the horn and batteries are good, since when I connect the horn directly across the battery terminals, it honks easily.
The Arduino is good, since it is also controlling LEDs on a separate power supply without any problems.
I replaced the MOSFET with another one (same kind), thinking that might be the problem, but I get the same result.

Any ideas? Is a MOSFET perhaps not the best way to control the horn?


How big of a heatsink do you have on the mosfet?



Firstly the IRF540 is not a logic level MOSFET so you'd need another device just to switch it,
which isn't so smart.  This is why it doesn't work at all from 5V.

Go for a logic-level MOSFET with a low Rds(on), the IRF540 isn't good enough without a largish
heatsink.  Aim for 10 milliohm or less.   Choose a 25V or 30V device, 100V MOSFETs have
much higher Rds(on), and are only chosen for high voltage use.

You never choose a MOSFET by the max current rating, always by the Rds(on) and calculating
the power dissipation.  In general you'll find you run a MOSFET about 20% of its max current
rating in practice, any more and you are probably having thermal issues.  You can get
MOSFETs rated at 170A, in practice you use them for 30A.

9A with 0.044 ohm is 3.5W - find a 5 milliohm device and you'll only get 0.4W dissipation
which is so much simpler.  These days you can get down to about 2 milliohm readily, but
not many that are logic level - look for 5 to 10 milliohm in logic-level.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


Ah, ok.  Didn't realize there were so many variables, I'll do some research and order up something that will work better.



There are literally 10,000's of power MOSFETs and IGBT's on the market as much power electronics uses them.
(triacs and thyristors make up the rest)  New ranges come out frequenctly as every manufacturer tries to
get the upper hand in the market. 

Find a proper electronics supplier that has a decent range (at least 1000) of MOSFETs then you can
search for a good match to your requirements.  In the UK I use Farnell because they have free next day
postage, other supplies are available.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


Here's a complete schematic that answers your question, and allows an MCU to drive it:


Taken from this alarm clock project:


The advice to use a woefully underpowered AC adapter is a bad idea (car horns will use far more than 2A), but the circuit is sound.  No heat sink needed in that application because you won't keep the horn blaring for long in an alarm application.  And depending on how you pulse the horn in other applications, you still may not need a heatsink, although adding at least a small one would be a good idea for circuit longevity.

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