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Automotive circuit design.


skauldy
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Guys,

Anyone know of any good books where I can read about the basics of circuit design. The like of when I want to add spots and know what mm2 wire to use, what size relay amps wise and what rating of fuse to use. I have bought two books that were pure ****, and googled the life out of it. The majority of the Google search is fault finding and after reading a few pages in it says we're not going to talk about circuit design :(.

I did do it years ago when I was an apprentice but forgot most of it and now out of the motor trade 10 years,

Shane.

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Say for arguments sake I have a spot light that's 160w so 160w/13.8v = 11.6 amps. Does this mean any cable with a rating of over 11.6amps will do or is their a percentage value. Then if the consumer is drawing 11.6amps what is the calculation to work out the fuse ratings. It's a single circuit.

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The fuse is only there to stop the wire melting when there's a problem. So it also wants to be as close to your power source (battery/main feed wire) as possible but also where practical. Think of it all in loops helps ie. Positive feed-fuse-switch(or relay)-item(spots etc)-earth(negitive).

Mike

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This has been for me a very good source of information :

9781844253029.jpg

with this :

4267_node.jpg?itok=H_LeV7wp

We've been building looms etc. for loads of vehicles and so far no complaints...

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Thanks guys.

So back to my spots example. Power is 11.6amps, use 2mmsq cable which is 16amps capablilty with a 15 amp fuse. Ok cool. :)

I see theirs a chart to work out cable size depend on its length of run.

Thanks Arian, will pick up these books on a,as on and get reading. Hopefully going to start wiring up my truck soon.

Thanks for all the replies guys

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Thanks guys.

So back to my spots example. Power is 11.6amps, use 2mmsq cable which is 16amps capablilty with a 15 amp fuse. Ok cool. :)

I see theirs a chart to work out cable size depend on its length of run.

Thanks Arian, will pick up these books on a,as on and get reading. Hopefully going to start wiring up my truck soon.

Thanks for all the replies guys

Agree with previous posts, but would add that rating of the cable is only one aspect - as noted in your post, cable size should also take account of length of run and hence potential volts drop - viz standard headlamp situation on Defenders.

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What AMB said - voltage drop over a run is a factor too, thin cable may be rated for the current but drop a fair bit of voltage over a run.

I generally round up to the next cable/relay/switch rating if I'm much over 60% of the rated capacity, both for safety & reliability. Put another way, I never trust the rating on anything.

Fuse rating is deceptive, a 20A fuse running at 17A continuous may get hot enough to melt the fuse holder, but equally may not blow at 25A current (hence why your wire should not be rated too close to the fuse rating).

Old-style glass fuses can actually glow & singe the paper marker inside at half their rating (I found), took a while to find the source of the strange burnt-paper smell when the headlights were on!

Ratings on devices are also only a ballpark figure - your 160W spotlamps may well take a big kick when you switch them on which would blow a 15A fuse, but then settle down to a lower draw. Obviously, stepping up to a 17A fuse would be dodgy if your wiring was only up to the 160w/12A rating as there's a danger the wire would melt before the fuse blew.

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Electrics can get very very complicated if you reeeally want to get in to it. In all honesty, the very basics already covered in this thread just about sum it up. I generally upsize wire also, not in any scientific way. I know the minimum size and I go next up. Voltage drop hasn't been a problem with my vehicles, though they're also simple Series. If you start getting involved with CANBUS systems, then you may have more of an issue.

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Electrics can get very very complicated if you reeeally want to get in to it. In all honesty, the very basics already covered in this thread just about sum it up. I generally upsize wire also, not in any scientific way. I know the minimum size and I go next up. Voltage drop hasn't been a problem with my vehicles, though they're also simple Series. If you start getting involved with CANBUS systems, then you may have more of an issue.

Fuses are one thing you don't have to worry about on the actual CAN - they aren't fused.(And rarely go wrong) Just everything else...

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Would be nice if someone made an app and you Input watts, volts and cable run and it then told you cable size and fuse. Boom, there a million dollar idea.

If you can use Excel there are a numer of free spreadsheets on the net. Many supplied by manufacturers.

The problem that you have is that the solution must be a compromise between acceptable voltage drop, and cable size, with all the negative impacts of over specifying the cable - cost, weight, minimum bend radius etc. To decide what is an acceptable voltage drop requires sufficient knowledge (or experimentation) beforehand.

As mentioned above, many circuits can have significant peak surge currents on switch on. Too high a cable impedance and they may lock out. Likewise the type of fuse used ie a straight fuse, slow blow or ceramic. I used to design switched mode and other high power power supplies. Fuse specification could be a nightmare because performance was so variable. You could potentially run a 10A fuse at 20A for quite some time.

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Would be nice if someone made an app and you Input watts, volts and cable run and it then told you cable size and fuse. Boom, there a million dollar idea.

That would be really good. If it covered 12, 230 and 415 I'd even pay for that!

Well get your wallets out - I was bored:

http://fuddymuckers.co.uk/tools/cablecalc.html

Haven't done cable loss yet but how hard can it be? :rtfm:

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Can't zoom in enough to read/use and the sliders start going schitz when you touch the screen.

TBH from a UX POV, you maybe be better dumping the sliders and replacing with plain selects(probably what they are under the skin anyways), oh, and up the font size, a lot :) 16px really is the minimum these days, and 18 much better for readability.

:)

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