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OEM ECU data logger and interceptor idea 
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LQFP144 - On Top Of The Game
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How do you go about reading voltage/amps running down a wire without affecting anything? (I am an electronics noob which makes it hard to work out what to search for on google)

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Thu Oct 09, 2008 8:11 pm
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Voltage is all you need to know, and that is easy :-) Just connect it up.

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Thu Oct 09, 2008 8:44 pm
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If you don't know or trust your resistance measurements, you can do it via hall sensor. There are some you can simply clamp on a wire and come as a hand held guage.


Thu Oct 09, 2008 9:25 pm
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But if you wanted to know how much current a device is drawing, you need is a shunt in series with the load or to accurately know the resistance of a given bit of cable and measure the voltage drop along that exact length. from that you can calculate current flow.
If you wanted to get a totally passive method use one of the newer generation of hall effect current transformers and wrap it round the cable, the magnetic field is directly proportional to the current flowing in the conductor. Although this sounds perfect its really overly complex when the first method is the easiest by far and produces very accurate results.

for example, if a known stretch of cable has 1 ohm of resistance and there is 1 volt difference between the ends say one end the battery the other end a lamp the using ohms law:

V/R=I

V=Volts
R=Ohms
I=Amps
so
1/1=1 so the lamp is drawing 1 amp of current.

another example:

R=0.5 Ohms
V=9 Volts
I=18 amps being drawn by the load.

Simple eh?

Ideally you want to make sure that the cable has the lowest possible resistance otherwise that resistance creates heat which means more resistance and therefore more heat. All this equals lost energy in the cable that was meant to power the load and excessive wear and tear on joints and terminations. not to mention that your load is now getting less voltage than its designed to have it could mean dimmer headlights, injectors that don't open as quickly as they should or simply that the controlling circuit has less say over what the load is doing. Prime example a big speaker driven by a big amp on really thin cheap ass speaker cable.

If you want advice on big cables ask Fred. I believe he must also have shares in a copper cable company. ;)

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Thu Oct 09, 2008 9:46 pm
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LQFP144 - On Top Of The Game

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jharvey wrote:
If you don't know or trust your resistance measurements, you can do it via hall sensor. There are some you can simply clamp on a wire and come as a hand held guage.


Don't buy micky mouse DMM's then, Simple

Buy Fluke!

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Thu Oct 09, 2008 9:47 pm
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davebmw wrote:
Prime example a big speaker driven by a big amp on really thin cheap ass speaker cable.

So true, if you put thin cable on two speakers, and put double the length on one, you WILL hear that it is quieter! Speaker cables should always be kept short as possible. I have some 10m ones for party use, but you don't want that for hifi :-)

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Thu Oct 09, 2008 9:58 pm
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LQFP144 - On Top Of The Game

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Its not just the fact that they get quieter either, the amplifier pushes and pulls the cone and the back EMF generated by the speaker needs to be damped by the output transistors. If the cable has a higher resistance the amplifier has less control and the shape of the sound waves generated become distorted from the original intended signal.

So Fred is 100% correct speaker cables should be as short as possible and as thick as you can get away with.

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Thu Oct 09, 2008 10:05 pm
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The thicker you got the more capacitance you get! But I agree to a point. My biggest stuff is 3.5sq mm a side and my normal stuff is 1.8 or so from memory. The big stuff is nice for parties because it is durable as hell :-)

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Thu Oct 09, 2008 10:12 pm
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LQFP144 - On Top Of The Game

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Yeah I bet thats why you have such massive cables in the ute, it adds Ah to the battery!

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Thu Oct 09, 2008 10:54 pm
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davebmw wrote:
Yeah I bet thats why you have such massive cables in the ute, it adds Ah to the battery!

Look, it cranks like it's on steroids. I've had wheel spin from the starter motor alone. If I ever need to jump start someone it will have near full voltage to the front. And, last, but not least :

It's the right size per mathematics!

ie

50sq mm at 4m =
25sq mm at 2m =
12.5sqmm at 1m =
6.125sqmm at 0.5m

How many sq mm is the 0.5m cable going from your battery to starter?? ;-)

It's about the same as stock in terms of resistance from battery to starter which is what I wanted. It's not as much overkill as you will continue to give me shlt about for the rest of my life ;-)

Fred.

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Thu Oct 09, 2008 11:07 pm
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