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i want to drive a small put dat takes 12vdc at 1.2amp. i have a relay capable of switching its contacts at 5v so was goin to use the output pin of my arduino board as signal control to switch on the relay and an external power source for the 12vdc on the contacts of relay. my question is whether or not i have to use a transistor and a diofe across the relay coil to protect the board or will it be ok with just using the relay?
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Yes you will need a diode, but whether you need a transistor will depend upon how much current the relay requires to operate.
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drive a small put dat

OK, I give up. What on earth is a "put dat"?

The coils of 5V relays typically require more current than a microcontroller
pin can drive, so use an NPN inverter [etc] ckt to drive the coil.

Also, you **ALWAYS** need a diode across the relay coil. They develop a large
voltage spike when switching off, related to the formula V = L di/dt. The quicker
the turnoff, ie small dt, the larger the spike.

http://en.wikipedia.org/wiki/Inductor

You might also need some sort of spark quencher across the relay "contacts",
too, if you're switching any sort of inductive load.




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my guess is 'put dat' = "out put" (give or take a fair amount of mis-keying)
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will it be ok with just using the relay?
In most cases you do need a transistor. You always need a diode. What is your coil resistance if it is less than 100R you need a transistor.
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The "network" across the switch contacts was called a snubber, typically a cap between 100 -  500 nF and a 5 to 200 ohm resistor is series across the contacts. They work well at quenching the energy that causes the spark/spike to be developed across the contacts. It was used primarily to extend the contact life by reducing the "pitting" an actual ARC would be  reduces by quenching it. this pitting was the effects of the actual contact inductive switching load Currents.

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