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Resistors for a project, easy question! Answered

Hi all,
This is an easy question about resistors, its been years since I use any.

So, I have a replay which is rate 12v DC and 2amp.
The power supply is 12v DC and 6 amp.
V = I X R
R =  V / I

so R = 12 / 2 = 6ohm

So I need a 6ohm resistor? Is that correct?
I can't believe I have forgotten this!




Your calculation is correct, it is ohms law, but the 6ohm result is the resistance of the coil of the relay. You don't need any further resistance to operate it. It will operate at 12volts and draw 2amp from the 6amp supply. You should put a diode across the coil terminals though to prevent damaging any other circuitry. The bar symbol on the diode goes to the positive side of the coil.

No, I would guess the CONTACTS are rated to carry 12V at 2A. The coil is ALSO possibly 12V operation, but is VERY unlikely to pull more than 150mA

Ooopppsss ... Guess I should read the question a little closer next time. Either way though, no other resistor is required.

no you need a 4 ohm resistor in parallel to relay

Thank Jack, ye I meant relay, l+p close to each other on the keyboard!
I thought that would be the case, I had a slight panic that I needed a resistor for some reason. Solved!

Most electronic devices are designed to work from a constant voltage supply. That is to say the device wants,or expects, the supply voltage to be approximately constant, and then the device draws as much current as it needs.

Devices are rated in terms of the maximum current they will draw. 

Power supplies, i.e. power bricks, DC adapters, wall warts etc., are rated in terms of the maximum current they can comfortably supply. 

A power supply capable of supplying 6 A, will be totally happy supplying only 2A, or even 0 amperes in the case where nothing ( an open circuit) is attached to it.

So to make a long story short, you just connect the power supply and the device together, and the device decides how much current it wants to draw.  Assuming this amount of current is less than the maximum rating of the power supply, then both device and power supply will be happy.