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### Topic: 12v Relay (Read 1 time)previous topic - next topic

#### kb1ibh

##### May 25, 2012, 09:01 amLast Edit: May 25, 2012, 09:03 am by kb1ibh Reason: 1
Forgive me, but i've spent the majority of my life focusing on software development, and have a very weak knowledge of electrical engineering concepts, but hey, that's why i got an arduino!

now, this isnt what you're most likely expecting, i'm familiar with the reasons and construction of a transistor/diode/resistor setup for triggering a relay with the arduino, but i want to be 100% certain i'm setting up the 12v power source correctly, and that i ultimately wont set my car on fire...

I've got a TIP120 Transistor triggering a 40A 12v Automotive Relay, and if im certain of anything, its that taking a screwdriver and shorting out the terminals of a car battery is a pretty bad idea... as far as the construction of a relay is concerned, on the coil side, its simply a spool of wire wrapped around an iron rod... my question is, how is it that you properly power the coil of a relay in a car? does the relatively thin wire of the coil prevent the sudden and nearly instantaneous transfer of power that we see with Mr Screwdriver? am i required to somehow step something down to safe levels to prevent the relay from effectively shorting out the battery, or will it only draw as much current as is necessary to itself?

#### RuggedCircuits

#1
##### May 25, 2012, 09:14 am
The coil of a relay has a built-in resistance (all that thin wire really does add up in resistance). You will probably see a rating like "12V 100mA" which, if you do the Ohm's Law calculation, tells you that the resistance of the coil is about 120 ohms.

So, the answer to your question is what you wrote yourself: it only draw as much current as is necessary to itself

--
The QuadRAM shield: add 512 kilobytes of external RAM to your Arduino Mega/Mega2560

#### kb1ibh

#2
##### May 25, 2012, 09:17 am

The coil of a relay has a built-in resistance (all that thin wire really does add up in resistance). You will probably see a rating like "12V 100mA" which, if you do the Ohm's Law calculation, tells you that the resistance of the coil is about 120 ohms.

So, the answer to your question is what you wrote yourself: it only draw as much current as is necessary to itself

--
The QuadRAM shield: add 512 kilobytes of external RAM to your Arduino Mega/Mega2560

Awesome. please forgive my ignorance, I, my Insurance company, and my towns Firefighters thank you.

#### terryking228

#3
##### May 25, 2012, 11:38 amLast Edit: May 25, 2012, 11:41 am by terryking228 Reason: 1
Hi, In addition to the relay coil resistance, it has significant inductance.

"An inductance acts to oppose a change in current in a circuit". So the current in the relay coil will actually be lower at the moment voltage is applied, and rise to the value predicted by the resistance shortly thereafter (See http://en.wikipedia.org/wiki/Time_constant#Time_constants_in_electrical_circuits if you want gory details) .

But in the real world of automobile vibration, temperature extremes, flying salt slush etc. you may want to consider what happens if there is a fault in the relay or wiring. Almost every component in a vehicle has a fuse to protect the system if Bad Stuff Happens.  Your relay (and anything else you add) should be protected by a fuse or circuit breaker in case of short circuits or component failures. You may be able to tap off one of the many fused circuits in your vehicle, or add you own, as an "Inline Fuse" (See Car Parts Store or Radio Shack).

Heavy current devices in a vehicle, like the alternator, starter solenoid, etc. are often protected by "Fusible Links", which are short pieces of thinner wire that will melt under a serious fault condition. Usually the only thing that has no intentional short circuit protection is the starter motor.

You can try this with a 2 inch piece of #22 or so wire, connected in the middle of a large (say #12 or larger) wire. Try it out with Mr. Screwdriver AND gloves!

Lots of relay How-To here:  http://arduino-info.wikispaces.com/ArduinoPower
Regards, Terry King terry@yourduino.com  - Check great prices, devices and Arduino-related boards at http://YourDuino.com
HOW-TO: http://ArduinoInfo.Info

#### Eriba

#4
##### May 25, 2012, 11:57 am
Quote
a 40A 12v Automotive Relay

That 40A figure will be the contacts rating and not the current the coil will draw to make/break the contacts.
"Everything should be made as simple as possible, but no simpler."

#### kb1ibh

#5
##### May 25, 2012, 08:14 pm

Quote
a 40A 12v Automotive Relay

That 40A figure will be the contacts rating and not the current the coil will draw to make/break the contacts.

i'm aware of that, i just threw it in there just to be as descriptive as possible

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