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What Are the Limits on Powering Relay Coil Terminal 86?

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Member spacespace
Joined: July 06, 2020
Location: United Kingdom
Posted: July 06, 2020 at 6:57 PM / IP Logged Link to Post Post Reply Quote mcbeginner
Hello, I have a couple of what are probably very basic questions that hopefully someone can help me with. They are:
Is there a limit to the current that can pass through the coil (terminals 86 to 85) on a standard 40A relay?
Is it safe to connect terminals 87 and 86 together if different power sources are connected to terminals 86 (with a low current) and 30 (a higher current)?
I added auxiliary lighting to my motorcycle and used a relay so that the lights can be wired direct to the battery, but will only work when the bike is powered. As the bike provides a switched socket for accessories, I used a plug with the positive pin connecting to terminal 86 of a relay, terminal 85 then goes to a switch, which returns to the negative pin of the locker to ground it. The power to the lighting then coming from the positive terminal on the battery through terminals 30 and 87 of the relay. Obviously I also have fuses coming from both positive sources to protect the cabling,
I am sure that is a pretty standard layout, and it works perfectly. I cannot remember precisely why the switch was placed after the relay, but I read something which made it sound like a better choice. But the reason for using a relay was firstly so only a minimal current would go through the switch which is fastened to the handlebars, and secondly because the accessory circuit is shared, so it seemed better to power a relatively high current use directly from the battery.
The reason for my questions is that what I would like to do now is make it so that when the bike's power is turned off, the lighting will remain on until the switch is turned off to break the circuit. Essentially making the relay power the coil itself once engaged. I know that this can be achieved by connecting terminals 87 and 86 together. My concerns are that the current required by the lighting would go through the coil, which will be several amps rather than the milliamps the coil itself draws on its own circuit. So would this be too much or does it not matter? And it also means the coil, once engaged, would have both the switched connection and direct battery connection powering it. Is that safe? Both in terms of how it would affect the current they draw, but also it effectively means the battery's positive terminal will be connected to the positive pin of the accessory socket.
I realize that just connecting the terminals on a single relay would undermine my requirement for only a small current goes through the switch, as the current drawn by the lights will also be connected to terminal 86 and therefore go through the switch too. So I would look to use two relays, the one described above to only active the coil of a second relay, with a direct battery connection on terminal 30 to power the lighting.
Incidentally, I can see how three relays can be used to avoid all the things I am asking about, but I am sure there must be a simpler solution with two relays.
I hope that makes sense. I gave the full context of what I am trying to achieve to explain my questions, but I would like to understand the general principles rather than just get an answer to the specific problem.
Thank you.
i am an idiot 
Platinum - Posts: 13,367
Platinum spaceThis member consistently provides reliable informationspace
Joined: September 21, 2006
Location: Louisiana, United States
Posted: July 06, 2020 at 7:14 PM / IP Logged Link to Post Post Reply Quote i am an idiot
The coil only draws 160 milliamps. It will only draw that much current, no matter what the supply line is capable of. It will take care of itself.
Member - Posts: 4
Member spacespace
Joined: July 06, 2020
Location: United Kingdom
Posted: July 07, 2020 at 7:12 AM / IP Logged Link to Post Post Reply Quote mcbeginner
So even if the lights were drawing 6A through terminals 30 and 87, connecting terminal 87 to 86 would still only carry enough current for the coil and not the full 6A. Thanks. I guess that is obvious in hindsight, as you can kind-of think of the point where you split the cable as if it were the battery itself, so obviously the current will be different based on need rather than the total carried to that point.
But what about having two different 12V sources connected to terminal 86? If the switch is closed they would be going to ground, but if not then it would effectively be connecting the two positive sources together. Would that risk causing damage?
And one other question I should have asked, should I be using a diode across the coil? It is not something I had not known about before until trying to find answers to my questions, and now I am not sure if it is necessary. But as my source is a switched connection provided by the bike I do not know what if any components are in between the source and my relay.
The relay I am currently using shows no form of protection on the diagram printed on it, although it has not been a problem. However I was planing to replace it with a micro relay, as there is so little space on a bike that saving half an inch can be very helpful. But the diagram on the ones I was looking at show an internal resistor being used for protection. Would that be enough or should I look for one with a diode? The speed to power down does not matter, which I understand is the main disadvantage of a diode.
Thank you.

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