resistance of relay?

Hi everyone, I want to connect a relay, 85 to constant +12v, 86 to chasis ground. It's hard to explain why I gonna do this, but it turns out to be. I don't know the resistance of the relay, so I'm worrying if the circuit may cause overheating or short. Can anyone show me how to know the resistance of a relay? Many thanks to my rookie question~ Info about the relay: AGT 12V 30/40A 14VDC MAH-112-C-1 (that's all written on the relay plastic shell) I bought it from http://www.amazon.com/gp/product/B005HFYE1O/ref=oh_details_o00_s00_i00 Many many thanks!!!

That relay's coil resistance should be 85 to 90 Ohms. By convention, +12V should go to 86 and GND to 85, but since that relay does not seem to have any internal anti-spike diode it should be ok.

Incidentally, X2 with Oldspark ref 85 and 86. By the way, a CONSTANT 12v+ to 1 side and a constant ground to the other will run flat the average car battery overnight, from the relay coil current draw.

thank you so much

thank you very much, howie! but i'm too new to fully understand your post. what is the X2 stuff you mentioned? and the overnight thing means? is there gonna be a problem if the relay is on for several nights? so many thanks!!! just want to make sure everything is correct and my car won't be ruined...

Yes before K.Pierson or Mr. I jump in with the short erudite explanations and Oldspark gives you the long but erudite explanation:- X 2 on any post by the way means in agreement. When a "standard automotive relay" (of the type in your photo) is engaged, the coil draws current apart from the draw from whatever it's powering, thus it's perfectly OK to run these things for a short time or when engine is running etc. Not to be activated and left when vehicle is sitting say overnight, = flat battery the next morning. Common sense says there are obvious exceptions such as horns, headlight flash but for instance I wouldn't do fog lights unless they were integrated with the low beam headlights, most (European) cars have headlights that only work with the ignition on plus lights on door buzzers. That's the ONLY real reason cars have ignition control for wash/wipe, h/AC, windows etc.

What exactly are you trying to do?  A Single Pole Double Throw relay may be able to be used and have it break the normally closed contacts when the relay is energized. 

Damn, Mr I beat me...

tinecidy627 wrote: and the overnight thing means?

I thought it was clear that that meant...

howie ll wrote: ... a constant ground ... will run flat the average car battery overnight from the relay coil current draw.

Over several nights or whenever the car isn't run and charging, it will flatten the battery. A battery left reasonably discharged for a longer period is a big compromise on battery life. Plus you need a spare cranking battery.

thank you guys so much for teaching me everything basic, I really appreciate that. btw, how do you know the resistance of the relay, based on the parameters written on it, or just some common sense? thanks

Thank you howie, my knowledge of web language is so limited. Just curious that, is it unreasonable at all to have a relay constantly on when the ignition is off? I understand totally that this flattens the battery, but how bad it flattens? If the relay is 80-90 ohms, then it's less than 2 watt, sounds not big to me. I believe there are something that flattens the battery this way. For example, a starter kill relay is such that on when ignition is off (alarm is on), so it "flattens" the battery several days and nights as long as you don't drive the car. So, is my relay totally crazy? BTW, I use the relay as part of an all-time switch, which I want to be working all the time regardless of engine on/off, so I guess I have to sacrifice the battery?