2 years ago
How can I power a car horn that requires 12v @30A or 24v @ 12A without a wall wart?
And what kind of relay should I use if I want to control it with a 5v logic signal? I have a car battery, but that thing puts out 800A.
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M$1 Answer
You are correct - a battery is the right solution.
The car horn has internal resistance and so it is safe to connect it to any 12V source. It requires 30Amps so any battery with a higher rating can safely supply it.
Likewise the relay must be one with a 5V coil and with a switching rating exceeding 30 amps. 50 would be safer.
The 5V logic signal will have a maximum current that it can deliver. So you must choose the resistance of the relay carefully so it doesn't draw too much current and blow your 5V logic board.
Say the relay was 120 ohm and you are driving it with 5V then its current draw will be (Ohms law) 5/120 = 41milliamps
Some micros can only deliver 20ma or so. In this case you would need a transistor buffer to drive the coil.
A 250 ohm coil would be 20ma.
The car horn has internal resistance and so it is safe to connect it to any 12V source. It requires 30Amps so any battery with a higher rating can safely supply it.
Likewise the relay must be one with a 5V coil and with a switching rating exceeding 30 amps. 50 would be safer.
The 5V logic signal will have a maximum current that it can deliver. So you must choose the resistance of the relay carefully so it doesn't draw too much current and blow your 5V logic board.
Say the relay was 120 ohm and you are driving it with 5V then its current draw will be (Ohms law) 5/120 = 41milliamps
Some micros can only deliver 20ma or so. In this case you would need a transistor buffer to drive the coil.
A 250 ohm coil would be 20ma.
You can leave an optional "tip" with Mahalo's virtual currency, Mahalo Dollars. If you are asking a difficult question that might require some research, or if you'd like a wide variety of feedback, a higher tip often leads to more answers to your question.
M$
Thank you very much for that!
Now I'm wondering how to go about connecting to the battery and about safety in general... could the car battery potentially kill me if i shorted it across my chest? I have sweaty hands so I have a higher risk of electrocution. Does the surface area of the leads that I use to connect to the battery matter or can I just use Radioshack jumper cables?
Use terminal clips appropriate to your battery terminals. Generally there is a bolt and a lug. The lug will need to have the right sized hole to fit. If you get crimp lugs then you can get them the right size for the cable you need and connect them directly.
At 12V and 30amps the min wire size is 6 or 7AWG or thicker. That's a 4mm (0.16inch or 5/32inch approx) diameter copper cable. From: http://www.powerstream.com/Wire_Size.htm
Presumably you also need a switch.
The 12V battery will not kill you it is safe to handle. Jumper cables will be fine for temporary use.