General Electronics Question

[Lexus-IS]

I have this custom wiring loom with a relay for a project i am working on. The loom has a big red wire thats meant to connect directly to the car Battery to be the power source. This loom was originally made to power a bulb which uses high wattage. But I want to use this loom to power low consumption LED lights instead. And because of this I dont really want to directly take power from Battery but instead from the accessory socket. My question is that the wire i'm going to run from the accessory line is going to be much thinner than the thick red one that is currently on the harness. Is this a problem? should I maybe remove the big thick wire and replace with thinner one?

[steve2006]

It shouldn't be a problem perhaps just a little unsightly to leave the existing wire in place and connect to your power source.

What I would suggest is fitting an inline fuse between the red wire and the wire you are tapping into.

The connections should preferably be soldered then well insulated with several layers of insulating tape and/or heat shrink sleeving.

Low power consumption devices such as LEDs generally can be wired direct to the supply voltage and do not necessarily need a relay which is designed to switch higher current devices such as headlights,starter motors and the like.

[Lexus-IS]

Thanks for your reply.

Is it safe to power a set of LED units which in total consume 13W at 12V directly from a car Battery? I will be using the relay to turn them on/off. But just wondering if its safe to run a thin automotive cable (Rated at 1.5a or 2a) directly from the Battery? (and putting some kind of inline fuse on the wire aswell)

[steve2006]

Yes 13 watts at 12 volts is just over 1 amp of current so the wire will be fine, fit a fuse rated at 2A.

[Lexus-IS]

Yes 13 watts at 12 volts is just over 1 amp of current so the wire will be fine, fit a fuse rated at 2A.

Thanks for your reply. If I have 2 different relay harnesses connected which will both draw power directly from the Battery but the relays will be switched via two different circuits but they will turn on the same device. Will this be a problem? technically even if both of the relays were to accidentally be switched on, the power drawn by both harnesses will still be coming from the same Battery and going to the same device, so it shouldn't be a problem should it?

[Geoffers]

Thanks for your reply. If I have 2 different relay harnesses connected which will both draw power directly from the battery but the relays will be switched via two different circuits but they will turn on the same device. Will this be a problem? technically even if both of the relays were to accidentally be switched on, the power drawn by both harnesses will still be coming from the same battery and going to the same device, so it shouldn't be a problem should it?

Its a little vague regarding the origins of your different circuits so you will need to make sure either circuit doesn't end up back feeding the other circuit if you see what I mean. This could be a hazard or even damaging to the switching circuit if it comes from an ECU. I would put a diode inline of both circuits just to prevent this.

[Lexus-IS]

Its a little vague regarding the origins of your different circuits so you will need to make sure either circuit doesn't end up back feeding the other circuit if you see what I mean. This could be a hazard or even damaging to the switching circuit if it comes from an ECU. I would put a diode inline of both circuits just to prevent this.

Thanks for the reply Geoffers. below i've attached a picture of what I am planning to do with the wiring. Hopefully my drawing makes sense and you or someone can advise me on if this is ok or if i will still need some kind of diode

[steve2006]

Thanks for your reply. If I have 2 different relay harnesses connected which will both draw power directly from the battery but the relays will be switched via two different circuits but they will turn on the same device. Will this be a problem? technically even if both of the relays were to accidentally be switched on, the power drawn by both harnesses will still be coming from the same battery and going to the same device, so it shouldn't be a problem should it?

Its a little vague regarding the origins of your different circuits so you will need to make sure either circuit doesn't end up back feeding the other circuit if you see what I mean. This could be a hazard or even damaging to the switching circuit if it comes from an ECU. I would put a diode inline of both circuits just to prevent this.

An IN4007 or some thing a little higher rated I think?

[Lexus-IS]

An IN4007 or some thing a little higher rated I think?

i take it IN4007 is a diode. at which part of my wiring diagram above will i connect it? i'm guessing just before the two red wires meet and become one before they go to the device? Also, I take it the diodes need to be the same amps and wattage as the device ?

[Geoffers]

As that stands, it look you are just powering each relay coil from the supply. As that's the case for each relay, they are isolated so no need - you won't be backfeeding anywhere. This circuit will work fine.

You could remove a relay though and make it a little smaller. Are these lighting circuit negative different from the cars other grounds? Some ECU switched lighting is. If not, you can do the same circuit with two diodes and a single relay.

If they are normal earths, you will just be using the positive from each circuit to trigger. Check continuity to earth from the negatives. If they are the same, you can build the circuit below.

It allows current to pass through the relay but not return to the other circuit as it hits the other side of the relay. The same goes for the other side. 1n4007 diodes are coppers to but.

Steve - yes, 1n4007.

[Lexus-IS]

You could remove a relay though and make it a little smaller. Are these lighting circuit negative different from the cars other grounds? Some ECU switched lighting is. If not, you can do the same circuit with two diodes and a single relay.

If they are normal earths, you will just be using the positive from each circuit to trigger. Check continuity to earth from the negatives. If they are the same, you can build the circuit below.

It allows current to pass through the relay but not return to the other circuit as it hits the other side of the relay. The same goes for the other side. 1n4007 diodes are coppers to but.

Thanks for that info Geoffers. Its very informative.

I've done a continuity check and all the grounds seem linked. Your suggestion of just using one relay seems much smaller and tidier. I do have a few more questions:

1) How do I calculate what kind of diode I need? Or is a 1n4007 the actual spec of the diode? and seeing as it is only going to be used to switch the relay I guess it doesnt have to be very high in amps either?

2) Do i just solder the diodes and heatshrink over them? or is there a cleaner way?

3) I'm thinking of storing the relay in a small waterproof project box and then mounting it in the engine bay, is that ok to do?

[steve2006]

Use the diodes already suggested they will be fine for the application, just make sure you fit them the correct way around,the end with the stripe must be nearest the relay as shown in the diagram and picture of the diode.

[Lexus-IS]

I've been looking on maplin website and the only 1n4007 i can find is labelled as 1n4007G and the link for it is here

Is this the one i need?

[steve2006]

Yes they are fine but if you PM me your address I'll pop some in the post for you for free.

[Geoffers]

Steve's answered your questions and thrown the diodes in to boot - you can't get better than that!

If you are putting it in a small waterproof project box, that's ideal. You could take it a stage further and fill the box with potting compound (a resin that sets solid) but that's going a bit far. You'll probably not need the heat shrink tubing as you would form a 'V' shape with the diodes coming off so the wires at the ends on the diodes won't touch but you can never be too safe so its always a good idea.

<Boring bit.>

Yes you are right, the diodes don't need to be of high current at all. 1n4007 are very common, cheap and reliable. The 1n4001-4007 range are all quite similar but the 4007 has the highest spec for reverse voltages, etc. (ideal in a car were there is the odd spike). They handle up to 1A which your relay coil obviously is nowhere near.

The 1n4007g is the same in operation. The G ratings are becoming more mainstream due to mass production. The G mean it is glass passivated meaning the semiconductor surfaces are protected against mechanical and chemical penetration by melted-on glass layers.

<End boring bit>

[Lexus-IS]

Thanks for that generous offer steve, i'll pm you my address now.

And thanks Steve & Geoffers for all the advice and help. very much appreciated.