Automotive relay question

[frankd]

I've never wired up a relay before so this is probably a dumb question but was hoping to get some insight. I'm wiring up a relay to power an LED light bar on my UTV. Currently its wired directly to the battery but I dont like that set up as the kids will be messing around with it and I'm sure someone will leave it on.

I looked up some wiring diagrams but I'm a little confused so I was hoping to get some clarification. From what I can see, terminal 30 to 87 is where the higher amperage line would go and terminals 85 to 86 are the switch. The confusion is that in some diagrams, 85/86 or 30/87 are swapped but I'm assuming it doesnt matter, right?
For example, one diagram shows 85 going to ground and 86 going to the switch. Now if I swapped that (85 to switch and 86 to ground) it shouldnt make a difference right? Same goes for 30 to 87. As long as I'm using those 2 terminals for the higher amperage wire, it shouldnt matter which one goes where right?

 

[jmillican1983]

You are correct.

 

[yatg]

85,86 go to the coil and is typically controlled by the switch.
connect one of the terminals to ground and supply 12v through the switch to the other.

30,87,87a are the contacts.
30 is common, connect 12v to that.
87 is NO normally open, connect that to the light.

 

[frankd]

Thanks. I figured it wouldnt matter but having never wired one before I wanted to make sure.

 

[dcg9381]

I dunno how much power your bar is pulling, but I had lots of failures with standard 20A automotive relays over time... I started upgrading them to larger and don't use the 20A versions anymore if I can help it.

 

[Wrench97]

I dunno how much power your bar is pulling, but I had lots of failures with standard 20A automotive relays over time... I started upgrading them to larger and don't use the 20A versions anymore if I can help it.

Led light bar or conventional?

The problem usually is finding a decent brand Hella and Bosh both make decent ones when you can find them, locally it's always the no name $3.99 specials in stock.

 

[AffableCurmudgeon]

Same goes for 30 to 87. As long as I'm using those 2 terminals for the higher amperage wire, it shouldnt matter which one goes where right?

In your case, it will not matter but in other cases it would matter.

Specifically, 30 is common and goes to 12v. 87 is normally open and 87a is normally closed. When there is no control signal on 86 (assuming 85 is grounded), 30 is connected to 87a. When control signal is high on 86, 30 no longer connects to 87a and instead connects to 87.

In your case, what you asked would work but if you wanted to light to go off when control signal goes high, you would want 12v to go to common, 30.

 

[dogdog]

I think this site is the best when ever I needed to do my Alarm or wanted a specialized relay setup

https://www.the12volt.com/relays/relaydiagram47.html

85 / 86 are the working coils that when energized it pulls the contacts from position 87a to 87. 85/86 are not polarity sensitive unless you put in a diode to suppress the sparks it might generate. (magnetic flux break down voltage it generates when it de-energized)

30 is the common control input terminal... the outputs are

87a is the default normally closed (N.C.). means when the coil is not energized... it shows continuity between 30 and 87A.

87 is Normally OPEN (N.O.) it means when the coil on 85/86 is energized... it will pull the contact close.. this N.O will show continuity between 30 and 87... when coil is energized.

the rest of the configuration you can get from this.

https://www.the12volt.com/relays/relaydiagrams.html

 

[frankd]

I dunno how much power your bar is pulling, but I had lots of failures with standard 20A automotive relays over time... I started upgrading them to larger and don't use the 20A versions anymore if I can help it.

its an LED light bar. Not sure how much current its pulling. I have an inline fuse and was going to start at a 10amp fuse and see if that didnt blow. And work my way up to 20amp if needed.
Not sure of the brand of light bar as it was already there. the existing light bar wiring is 14 gauge. I had removed a 30amp relay from another accessory so I was just going to use that.

 

[a_whale]

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[frankd]

Do you know how many watts the light bar is rated for? Assuming a 12V light bar, your current draw will be wattage / voltage (i.e. watts / 12V).

With unknown/chinese relays, I try to use a relay rated for at least twice what my current will be. Relays run hot close to their rated current, and operating it in the hot sun in high ambient temperature can push them past their rated temperature. You can calculate how hot it will get if you know what the resistance of the relay contacts are, but you won't get this info with a chinese or off-the-shelf relay from an automotive store.

Unfortunately I don't know the wattage and can't see any sort it name/brand on it. The wiring on it is 14 gauge. I had pulled a 30 amp relay off the machine (which was powering bluetooth speakers that didn't work) so I plan on using that and starting with a 10amp fuse. Assuming the fuse doesn't blow then the relay should be adequate right?

 

[dogdog]

Do you know how many watts the light bar is rated for? Assuming a 12V light bar, your current draw will be wattage / voltage (i.e. watts / 12V).

With unknown/chinese relays, I try to use a relay rated for at least twice what my current will be. Relays run hot close to their rated current, and operating it in the hot sun in high ambient temperature can push them past their rated temperature. You can calculate how hot it will get if you know what the resistance of the relay contacts are, but you won't get this info with a chinese or off-the-shelf relay from an automotive store.

Do tell us how do you calculate this on non-chinese off the shelves relays from an automotive store? I don't think I have seen any specifications from Chinese or Non-Chinese automotive relays. Something about a fool with a tool blames the tool and not the fool....

For generic automotive relays 30AMP are common, 40 AMP are common as well... 10 / 20 or any other are not much...

 

[a_whale]

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[dogdog]

Dumb it down for me... show me a automotive relay that give you this parameter.

 

[a_whale]

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[dogdog]

The point is, unless you are ordering from some supplier that caters to specific industries, there probably won't any data sheet... Those probably will be xxxxx more $$$$... If you are into that much of engineering details, you can specify the contact points to be platinum / Iridium that will not erode as easily from arcing than from regular contact points that is made out of "other" metal. Adding diode and capacitors to suppress your arcing. again it's all about cost.


NY temperature can get as hot as 110degF, rare but there are occasions. I have not seen any of the relay that drive my fog lamps 2x55Watt died because of bad relay due to heat generate from the load.. Arcing across contact points on a remote starter because some one too cheap to install an 10 cents diode , Yes., corroded contact yes.


Power dissipation calculation of a relay in an automotive application ? Ok... you know that is insignificant to the load right ? beside that your Ptotal is for power consumption of the relay, not the same as heat dissipation capability of that relay. In other word, for engineering documentation it's a thumbs up for details and CYA, practical use = 0.

I think this would have been more useful for the OP.

https://www.google.com/search?q=measure+amp+draw+with+multimeter+across+a+fuese

 

[DoorBreaker]

One item I didn't see mentioned is to look over any "spare" relays terminal for any special items. The common "ice cube" style Bosch units can be had as 4 or 5 pin parts, normally 20 or 30 amps and there are versions with a damping diode or a resistor across the coil as well. Those are polarity dependent when it comes to how they connect to the triggering voltage. The diode is a good idea on many items to prevent the voltage spike from the field collapse as the voltage is switched off from damaging anything controlling the relar or connected to the voltage supply for it.

 

[a_whale]

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[Wrench97]

A data sheet for Cheapo relays is useless, want a good one stick to Bosh, Siemens or Hella for the amp load you have.
You can find them in 10/20a, 20/30a(the only ones I buy, and even 30/60a but connectors/terminal ends are harder to find locally for the larger terminal 60's.

 

[a_whale]

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