General 12v wiring question - aux lights

Duporth

Duporth

Joined
Apr 24, 2017
Messages
415
Location
Australia
Bike
2022 BMW R1250RS
I am wiring a pair of aux lights on an older
V-Strom and would appreciate help on a general question.

The lights draw 0.8 Amps each, according to the specs and I intend wiring them through an on/off handlebar switch rated at 6 amps.

Question: Should I use a relay switched by the 6 Amp switch or is it okay to wire the lights directly through the 6 Amp switch?

Thank you, D
 
I would recommend using a relay to take supply voltage as far away from load as possible, It will make the power wire as short as can be. Also use the switch as a ground side switch for the relay so if there is a short on that leg the worst that will happen is the light will stay on.
Absolutely, as St-Gerard says, wire it so it goes off when bike is off,
 
You don't need a relay (which is simply a switch activated by an electromagnet) if your switch is rated for 6 amps. I would not hesitate to wire the lite directly through the switch IF said switch is made in the USA or by a reputable company. I'm wary of electrical stuff bought on Amazon and doubtful their 'specs' are correct. Worst case if your switch is asian junk it will die an untimely death and you will have to replace it. Don't forget a fuse (3 amps should be fine) in the positive line to the light. I like running the fused power to the switch. This is the opposite of what @motornut suggested. This comes from 44 years as an electrician (line voltage is different than low voltage, but the lessons die hard).
 
Thanks very much helpers. I will go ahead and wire the lights directly.
I have installed a ‘Thunderbox’ distribution box that can be wired to switch power delivery on/off with the ignition.
Many thanks again, D
 
motornut said:
I would recommend using a relay to take supply voltage as far away from load as possible, It will make the power wire as short as can be. Also use the switch as a ground side switch for the relay so if there is a short on that leg the worst that will happen is the light will stay on.
Absolutely, as St-Gerard says, wire it so it goes off when bike is off,
Click to expand...
Can you try explaining this like I don't have 34 years experience in Auto Electric. I just want to understand it better, seems like an added failsafe using the ground to trigger?
 
I would do is as motornut stated . 1, don't like power wires going to the handle bars. Too much movement, if not properly insulated, secured and routed there is a possibility of a short. . 2, using a relay controls power so lights out when ignition is off.
 
Why is a 5- or 10-amp-fused control circuit any different than a 5- or 10-amp-fused lighting circuit?
 
Unless your signal wire is fused for (whatever you think is safe) with a 1.6 amp load, the difference between signal and power is essentially nonexistent. I get not wanting to put power on the handlebars. (Did you know honda put the fuse box on the top of the triple tree on the CB650 for a year or two? isn't that nuts?)

Having a relay lets you play with power logic, though it does increase complexity. But with the 3-5amp fuse this circuit would use? Your "safety" by going to a relay system is nonexistant. And in theroy, adding more joints is just as big of a risk.
 
Sorry to stir up so much controversy, I get bitchy about the time someone has spent as a whatever. I know 6 month mechanics that as good as anybody. My soap box is done and I am jumping off.
On a relay there are 5 spades on them that only four are generally used.
Pin 30 is the supply ( Battery or ignition supply) Fused for the expected load
Pin 86 is the coil control for the relay, frequently I jumper it over to the 30 wire so the coil has constant power to it. The wire can be connected to any control.
Pin 85 is the ground wire for the coil, this is what I connect to the bar switch and the bar switch to any ground
Pin 87 is connected to the load, in this case the light
The reasoning behind wiring this way is that when the circuit is not being used the hot wires only between the supply and the relay, from the realay to the switch and from the relay to the light is dead, there are no breaks between the light and the relay. The wire going to the switch has power until the switch grounds it.
If the switch wire shorts to ground, the lights stay on, no fuse blown. If the wire that goes to the lights grounds the fuse does blow. But there are only 2 connections from the relay to the light.
When you get used to using relays they get simpler.
That one pin that is not used is a constant "closed" contact internal. I have used this for alternating lights, but that is for a different time
 
Larry Fine said:
Why is a 5- or 10-amp-fused control circuit any different than a 5- or 10-amp-fused lighting circuit?
Click to expand...

That's the thing. The control circuit should be fused at the half amp, or 1amp level. Nobody's discussed that. But it's also not really useful in this case. You don't wanna be running 22 or 30 ga wires to the switch, just for durability reasons, and if you're stringing 16ga.. and you don't wanna tie it into other logic, might as well carry the load on it.

motornut said:
Sorry to stir up so much controversy, I get bitchy about the time someone has spent as a whatever. I know 6 month mechanics that as good as anybody.
Click to expand...
motornut said:
I have only spent 34 years as a heavy duty and automotive mechanic so you probably know more about electrical failures in automotive than me
Click to expand...

Your emphasis on "I've done this a long time" is a crutch. It's clear you know your stuff (knowing duetch standard pin numbers off the top of your head says something) Your accusation of "6 month mechanics" is also pretty nice. You're also using it to attempt to stop other conversation. Your appeal to expertise (as much as you might have) is not well founded in this case. It's clear you're aware it's bitchy, then maybe.. don't. You also discount the 40 year mechanics who can't wrap a wire harness, or won't touch an engine with variable valve timing.

The history of the pin numbers is fun too. The pin numbers come from the germans deciding everything needed the to be labeled "the same". It's useful!. But also not relevant here. I'm glad you know how to use a relay. We all ~should~. Honestly, given the loads here, using an automotive relay is silly, it's bigger, uglier, and unless you get a good, actually weathersealed one, not any better than a PCB mount version. Larry asked "why". The reason you'd use a relay in this case, is to enable relay logic, and to tie it into ~some other system~. Relays are another point of failure, cost more, and take up more space. For 1.6amps? Even OEM's don't use relays.

I'm going to give you the benifit of the doubt here. Since you've been doing this for so long, I assume you're accustomed to foglights, and driving lights, taking ~several amps~ per lamp. With the LED revolution, that's just not true anymore. 10-15 years ago, a driving light setup will be pulling a minimum of 8amps, and likely as much as 16. I'd be exactly with you here. If you wanna hear a fun one? The LED headlights on my Cherokee, due to how the grounding works on the car, will stay turned on if I don't turn them off before shutting off the engine. The relay is self energizing because of where the relay is grounded in reference to the headlight (I didn't do the install...) but that's ~another~ reason why relays can be funny.

Lets be clear here, you're not "wrong." A relay is one way to do it. But direct wired switch is another, correct, way.
 
Larry Fine said:
1.6a is nothing
Click to expand...

And...

nerobro said:
A relay is one way to do it. But direct wired switch is another, correct, way
Click to expand...

There you have it. The relay (as a device) was created to switch higher currents, via a lower (signal circuit) current. In this particular case, due to the low load current (1.6Amps) and the switch rating (6Amps) one can wire this either way as mentioned. And per the NEC (@Larry Fine - please correct me if I misstate this, it's been a while) it is recommended to load devices up to 80% of the rating, in this case 6 Amps x .8 = 4.8 Amps, so it's all good.
 
draser said:
And per the NEC (@Larry Fine - please correct me if I misstate this, it's been a while) it is recommended to load devices up to 80% of the rating, in this case 6 Amps x .8 = 4.8 Amps, so it's all good.
Click to expand...
The general rule is to limit continuous loading to 80% of the circuit's capacity.

Or, to say it another way, a circuit should be sized to 125% of a continuous load.
 
There is no reason to fuse a ground side control wire since it intended to be grounded when in use. I have relays as large as 3/8s by 3/4 inch as well as the larger ones and are weather proof, I also use a relay block which has relays about 1/4 inch square, although they are not weather proof and must be located differently which in this case does not apply. You are right about relays being used for heavy loads. but they are used for lighter loads to protect their source Ie Ecms which aren't used in this application so I use relays out of my knowledge and believe it stands alone.

As for the rule of thumb, the manufacturers use the equation of .01 of voltage loss per wire per connection. I am using 2 connections between the light and the relay on the load side which should be fused at the power source, 4 between the relay and the switch, but these are intended to be grounded, any short to ground will cause the lights to stay on. Fusing them would be unnecessary and accomplish nothing as this side uses so little amps to trigger the coil, the control source wherever it is used is already fused.
My intention with the experience use is that a idea should stand by its own, I try to use that as a basis on my recommendations, rather than how long I have been doing it for a living, I have gotten info from new mechanics that was valuable even though they had little time in the field and I have used it as I am sure anybody would and does. I am sorry, it was not intended to be anything but a knee-jerk response on my part. Any criticizing by me is not intended or belittling of anyone's experience at all as I read the info by its content, not its presentation.
A direct connection to the lighting will work fine for the loads used, but I prefer to have live wires exposed to environments as little as possible. After using relays I don't find them complicated and offer the info as a recommendation..... not a demand.
 
A quick primer for those who need it:

Relays are used to allow a low-current circuit to control a high current; basically a wired remote-control switch. The idea is to relieve the switching/control wiring of the burden of carrying the entire load. The greatest example of this on a bike is the starter relay.

There are other factory-installed ones like bikes that come with headlight relays, and ones we add, like for after-market horns and accessory circuits. Sometimes, we add this feature to bikes that don't come that way, like I did for my '96 Nighthawk 750 headlight.

Some people use them as power isolation devices, even when the bike's wiring can easily handle the load. My trailer has all LED lighting, so I wired the connector directly to the bike's wires. If I wreck, a couple of blown fuses will be the least of my concerns.

Relays can be used to reverse switch state, like when you want presence of power on one wire to turn off the power on another wire, or you want the loss of power on one wire to energize another wire. This is of use in exclusive (either/or) controls like interlocks.

Lastly, they can be used to switch "polarity" like suggested above, to allow a switched ground wire to control a hot wire. This is how car horns are usually wired, so only one control wire has to be run to the switch, and the other switch wire is grounded locally.

My point is that, unless your intention is the polarity inversion, there is really no reason to use a relay for a small load when the control wiring would be exactly the same as that for directly controlling the load. A relay coil is simply another low-current load.

Thus endeth the lesson.
 
You must log in or register to reply here.
Back
Top Bottom