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 https://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.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

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.


-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

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:

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and the overnight thing means?

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I thought it was clear that that meant...

howie ll wrote:

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

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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?

An Ohm meter. 

Also it isn't just the relay coil load, it's the load up to 40 amps it's switching.
At that load average battery fully charged to flat about 90 minutes.
I remember some years ago installing a system in a VW Sirocco, 3 x Soundstream or Nak amps, 2 x subs and satellites, 19 minutes with engine off and I needed to jump start it.

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

I just read your previous post again.
A starter kill WON'T give you a flat battery if wired correctly.
You can read my article in the "Car Security Hot Topics" section on this site or look at this diagram.
6C4_starter_cut_dei.bmp

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

tinecidy627 wrote:

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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?

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No. That's called connecting the load full time to the battery, and disconnecting the battery when you don't use it.

Otherwise you will need another battery to start the vehicle, and you will probably be buying new batteries every month or few.

Besides which, the relay de-energises with the flat battery so the circuit is pointless anyhow (unless it retriggers).


Such circuits should NEVER require a relay to be "normally" energised - ie, on when left idle and not running.

But as Howard wrote, yours doesn't anyhow, so why the concern?
If it does, just connect it and see since you are willing to sacrifice the battery...

Thank all of you for your time, and the patience for my dumbness. I figured out my goal can be achieved by a diode, so no need for a "constant" relay. However, since I learned some electric stuff in college, I want to discuss it with you guys and make sure I have the right picture:

For this relay, the coil resistance is about 80-90 Ohm. So when the relay is on, the voltage on it is 12 Volt, current will be ~0.14 Amp, so the power would be ~2 Watt. The 30/40A stuff is the maximum allowed current on the load side, in which 87 and 30 connect, right?

If this is all true, then let's consider the case howie mentioned, with a lot of high-power equipment on and the battery exhausted at ~90 min. In this case, current 30A, voltage 12V, power 360W. So the load power is about 200 fold higher than the relay itself. If I use this relay to drive something low current, say a LED, say it ~1W. So the battery won't be depleted for couple of months under such a low power, right? I think that's why a starter kill relay won't flatten the battery so much, since it only consumes about ~2 W on the relay it self (85-86), and it cuts the starter circuit so there is no power on the load side (87-30).

So if all my understandings are correct, then I'm happy with all these. I totally agree that a relay is designed to use a low current signal to control/switch a high current circuit. Actually I was trying to install a indicator LED, which turns on when the alarm system is on. So a single diode should work, no need to use a relay since the circuit is low enough.

Please point out if anything is illogical here so that I could learn more about the electric stuff. Thank you for reading this and helping me. My gratefulness~

Your first 2 paras are correct.

If you were to drive a LED, you would not use a relay. Wire the LED direct. Why use a 0.14A load to drive a 20mA (0.02A) load?

You shouldn't even need a diode for the LED since the LED itself is a diode. (That can depend on other circuits, but rarely.)

You just need to connect the "on" signal to the LED and a resistor.
Whether the other side of the LED & its resistor go to +12V or GND depends on whether the "on" signal is GND or +12V (or +5V etc) respectively.

A starter cut relay DOESN'T draw any power if wired correctly.
This is the THIRDtime I've posted this drawing since Saturday:-
starter_cut.bmp
And if you know so much what made you think you'd need a relay to power an LED?
By the way your calculations don't take into account diode induced (minor) voltage drops and circuit inefficiency adding to more draw.
It's STILL bad practice to have a relay constantly on.

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

In practice about 800ohms for a 2.5 volt LED

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.