Advice on wiring in compass light

[yerffoeg]

I have recently rewired my boat. I am now finishing the job off. I need to wire in the backlighting for my fixed Pastimo compass light.

I was intending to wire it across the circuit breaker (rated 5A) which turns on and feeds the navigation lights circuit. This is so the compass light comes on when the nav lights are turned on. The only problem with this is that the wiring for the compass light is fine and will therefore need its own lower rated fuse for protection. Does that sound right?!

 

[sarabande]

remember to twist the power supply wires in the vicinity of the compass to reduce magnetic influences.

 

[MarkGrubb]

I don't think a DC supply will have any affect on the compass. The metal in the wire may, but twisting won't help.

 

[Billjratt]

yes, twist it quite tightly to reduce the electro-magnetic field. I fed my compass light via diodes from two sources, 1, the masthead trilight, 2, the low level navlights, that way, when either is used the compass lights up.
You could incorporate a small inline fuse, but I've never seen it done. More importantly, you may like to fit a dimmer rheostat if the compass is mounted in your line of sight.

 

[john_morris_uk]

[ QUOTE ]
I don't think a DC supply will have any affect on the compass. The metal in the wire may, but twisting won't help.

[/ QUOTE ]I am afraid that you are wrong. Twisting the wires does help reduce the (very small) magnetic field that is produces by the few milliamps that run through the wires to the compass light.

The wires themselves are invariably tinned copper. Neither the copper, nor the solder used in tinning are ferrous, and hence the wire in itself has no influence on the magnetic field around the compass.

Don't wire your compass with Military Telephone cable - it has steel strands for strength with copper round them for conductivity.

 

[FAITIRA]

[ QUOTE ]
I don't think a DC supply will have any affect on the compass. The metal in the wire may, but twisting won't help.

[/ QUOTE ]

It does, definitly don,t forget to twist them.

I have the light linked to the nav lights, but a fuse is probably a good idea, it,s never been an inconvenience linked.

 

[VicS]

[ QUOTE ]
The only problem with this is that the wiring for the compass light is fine and will therefore need its own lower rated fuse for protection. Does that sound right?!

[/ QUOTE ] Yes that is right. It would be sensible to fit a fuse that reflects the wiring size. Make it large enough to supply the current the bulb uses with a bit of a margin to be sure that the bulb current does not blow it.

It is also correct that the wires would best be twisted to eliminate the overall magnetic field that surrounds them although in practice untwisted twin flex may not affect it.

To correct Avast Mark, it is DC that will create a fixed magnetic field that could affect the compass, (AC would create a oscillating field with no overall effect on the compass, although it can induce hum into electronic circuits). The copper wire itself will have no effect.

 

[MarkGrubb]

OK. I've learned something new again. I thought magnetic fields were only created by AC currents as the field is set up by the rate of change of current (di/dt), in a similar way that electric fields need a changing voltage (dv/dt). Better stick my head back in th books.

 

[yerffoeg]

Thanks Vic,

The other answers about wire twisting were interesting but didn't really answer my question!

 

[john_morris_uk]

[ QUOTE ]
OK. I've learned something new again. I thought magnetic fields were only created by AC currents as the field is set up by the rate of change of current (di/dt), in a similar way that electric fields need a changing voltage (dv/dt). Better stick my head back in th books.

[/ QUOTE ]AC currents produce magnetic fields that grow and decay rapidly. DC currents poduce fixed magnetic fields. Twisting the wire tightly helps to ensure that the fields cancel each other out.

 

[VicS]

A DC current flowing in a wire produces a circular magnetic field around the wire. (If the current is flowing way from you the field is in a clockwise direction around the wire.) If it did not then electric motors, solenoids, relays and simple meters etc etc would not work on DC!

Perhaps you are getting confused with the induction of a current in a wire by a magnetic field. A fluctuating field is required for that. That is why transformers only work on AC and why you have to spin a coil inside a magnet in order to generate a current.

 

[StugeronSteve]

Mine were wired into the nav lights and Bingo, the first night passage out goes the compass light as soon as I switched on the tricolour. As has been suggested above, you could wire it with supplies from both sets of lights run through diodes. I took the simpler solution and broke into the supply to the adjacent pedestal instruments, fitting a waterproof on-off toggle switch and in-line fuse.

 

[VicS]

[ QUOTE ]
out goes the compass light as soon as I switched on the tricolour.

[/ QUOTE ] Obviously it depends on how the nav lights are wired.

I have an overriding on/off switch for the lights, then a two way switch. If it is up the tricolour comes on, if it's down the side & stern lights come on. A third switch switches the steaming light on but only if the side lights are selected.

I could supply the compass light from the first on/off switch so that it comes on automatically with the nav lights whatever the combination but I have in fact chosen to supply it independently. It is a fixed light over a grid compass and is useful for illuminating the cockpit floor (or pee bucket) when not actually sailing.

 

[jackho]

I would wire to a seperate circuit - All new boats that I know off does this as best practice. If you ever had the experience of losing your compass light you'll know what I mean. I've got the tee shirt and it's not a pleasant experience - all be it before gps. Anyway there are too many potiential faults that can occur with nav. lights being exposed to short circuits etc. but only my opinion.

 

[dmayes]

The amount of magnetic field around a wire depends on the amount of current flowing through it and the length of the wire (or the number of turns in a coil). The amount of current is very small and will be constant for the small bulb that you are using therefore the field will be very weak. Having the wires lie alongside each other will double the field produced as the direction of the current will influence the polarity of the field. The opposing directions of current flow will act in addition making the field stronger. Twisting the wires tightly together will actually strengthen the field as you are getting close to creating a coil and using a longer length of wire which will induce the field to flow along the length of the coil. The weakest field will be produced by seperating the the wires and having one at 90 degrees to the other. The very weak field that this produces will not affect the compass, unless you are using something like a 1000 watt bulb.

 

[chewi]

Sorry no.
The corkscrew rule: Thumb = current , fingers = field.
supply creates a field in one direction, return creates a field in the other so they nearly cancel, but don't quite because of the distance between the wires.
Twisting the wires puts the centre of the two fields in the same line so they cancel just that little bit more.

 

[VicS]

[ QUOTE ]
The amount of magnetic field around a wire depends on the amount of current flowing through it and the length of the wire (or the number of turns in a coil).

[/ QUOTE ] The field strength will depend on the current but the length of the wire has got nothing to do with it. A coil though is like having a number of wires side by side all with the same current going through them in the same direction so the field strength is proportional to the number of turns
[ QUOTE ]
The amount of current is very small and will be constant for the small bulb that you are using therefore the field will be very weak. Having the wires lie alongside each other will double the field produced as the direction of the current will influence the polarity of the field

[/ QUOTE ] True the current will be small and as a result the field will be weak. Having the wires side by side though will not double the field strength, it will more or less cancel it out as the current is flowing one way in one conductor and the opposite way in the other.
[ QUOTE ]
The opposing directions of current flow will act in addition making the field stronger

[/ QUOTE ] No! as I just said the opposing directions of the current will more or less cause the fields to cancel each other out. [ QUOTE ]
Twisting the wires tightly together will actually strengthen the field as you are getting close to creating a coil and using a longer length of wire which will induce the field to flow along the length of the coil.

[/ QUOTE ] Twisting the wires together is the normal practice. In no way does it resemble a coil. The length of the wire is not significant. If it was actually wound into a coil then the number of turns would be the relevant factor. [ QUOTE ]
The weakest field will be produced by separating the the wires and having one at 90 degrees to the other. The very weak field that this produces will not affect the compass, unless you are using something like a 1000 watt bulb

[/ QUOTE ] I don't understand the logic of this statement at all. How can you have a pair of wires between the supply and the bulb running at right angles to each other? Two separate wires will each have a circular field around them (in the direction I indicated earlier in the thread... The rule for remembering the direction of the field in relation to the direction of the current flow is "Maxwell's Corkscrew rule"). At the point where these two hypothetical wires cross there will be an interaction between the fields around each wire resulting in a simple distortion of their direction and strength. The strength of the field around a single wire is proportional to the current flow and is given by Laplace's law or Ampere's Law (q.v.) I agree though the field will be weak from the current flowing to a low power bulb and may not affect the compass significantly. With a 1000 watt bulb the deflection of the compass would be only one of several concerns.

In short then the answer to the original question about twisting the wires is yes the best practice is to lightly twist them together.

 

[roger]

What about using an LED

LEDs take so little current you could wire one up (with suitable series resistor and a switch) to a couple of AAA cells.

 

[pvb]

Bet you almost wish....

Bet you almost wish you hadn't asked! There's a lot of confusion in the responses you've had about twisting the wires - I think people are getting mixed up with the need to use twisted wires to reduce magnetic interference in the wire. This doesn't apply for the compass light, so the wire needn't be twisted.

I'd certainly suggest you wire it to a separate circuit, with an appropriate fuse.

 

[chewi]

Re: Bet you almost wish....

So why would the compass manufacturers twist them then? As an industrial engineer in Electronics I know we wouldn't twist in manufacture unless it needed it, its a nuisance to do.
Its to make the two fields concentric and hence self cancelling.

If it doesn't matter its only 'cos the current is so weak, but even Plastimo do it!