Where to connect alternator voltage sensing wire
Discussion
Mr2Mike said:
Penelope Stopit said:
even the steel bodyshell can suffer from voltage drop at high currents
The above comment from you is what I am on about. Now who doesnt know what they are talking about????
You, unless you have a superconducting body on your car?The above comment from you is what I am on about. Now who doesnt know what they are talking about????
Edited by Mr2Mike on Wednesday 13th January 23:15
JohnMcL said:
OK everyone - I think I have it. Thanks to all, even those who do not understand Ohm's law or that all conductors have resistance and therefore voltage drop. I have found a good explanation of right/wrong sensor wire connection (and why) at
http://wilbo666.pbworks.com/w/page/39441708/Toyota... There is an informative picture if you scroll down.
I still recommend reading Part 2 in the link I posted back near the top.
Because it explains why the simple approach by wilbo666 may be improved by taking the sense wire to the main distribution point in the loom as long as the alternator output may be connected to the same place.http://wilbo666.pbworks.com/w/page/39441708/Toyota... There is an informative picture if you scroll down.
I still recommend reading Part 2 in the link I posted back near the top.
I know its a long read, but scroll down to the section after the "wiring" heading and all will be revealed.
I am embarrassed that my question has resulted in some helpful and technically correct answers being challenged in an unscientific way. Sorry. I shall not post again.
Penelope Stopit said:
Hence anyone having a dabble with electrics that does not understand that the main battery positive wire at the starter motor is true battery voltage, should not be having a dabble
You are assuming the starter wire comes from the battery. That's not always true. Many vehicles it piggybacks from the alternator. In which case, if it's anything like mine, at full load you'll be reading about 0.2v low compared to the battery/system voltage. More for vehicles on closer matched wiring.Perhaps you should not be near wiring yourself?
I'm very interested in this superconducting steel car body you have though, do you have it patented yet?
Edited by PhillipM on Thursday 14th January 22:13
Penelope Stopit said:
PhillipM said:
It will suffer from voltage drop at any current, so technically he's right.
You will not get a volt drop through a car body from front to back when drawing a low current through that body and I challenge you to prove me wrongIn the picture I have used 2 car bodies to put my test double in your favour, Two car bodies not one, volt drop test. The voltage that is measured at the battery end of the cars is equal to the voltage measured at the front end of the cars were a 5 Watt bulb is connected
There is no voltdrop across a car body with a 5 Watt load applied
Edited by Penelope Stopit on Saturday 16th January 12:31
Penelope Stopit said:
I see you have avoided my challenge
In the picture I have used 2 car bodies to put my test double in your favour, Two car bodies not one, volt drop test. The voltage that is measured at the battery end of the cars is equal to the voltage measured at the front end of the cars were a 5 Watt bulb is connected
There is no voltdrop across a car body with a 5 Watt load applied
Actually wherever you have current flow you will have some voltage drop even if it is only milli volts. If you had an open circuit you would measure the same voltages at all points in the circuit until of course you get past the open.In the picture I have used 2 car bodies to put my test double in your favour, Two car bodies not one, volt drop test. The voltage that is measured at the battery end of the cars is equal to the voltage measured at the front end of the cars were a 5 Watt bulb is connected
There is no voltdrop across a car body with a 5 Watt load applied
Edited by Penelope Stopit on Saturday 16th January 12:31
A guy called Dan Sullivan has written a very good book that covers voltage drop in great detail.
EFS
Edited by Oldred_V8S on Monday 25th January 11:00
Penelope Stopit said:
You will not get a volt drop through a car body from front to back when drawing a low current through that body and I challenge you to prove me wrong
Georg Ohm proved you wrong almost 200 years ago. Steel is not a superconductor. No matter how much of it you use to create a car body, it will have a finite resistance. Any non-zero resistance will drop a voltage when a current flows, the voltage drop being proportional to both the resistance and the current.In order to suffer no voltage drop when a current flow you need a superconductor (a material with zero resistance). All current superconductors need to be cooled to low temperatures to work, the best material available still needs to be cooled to -135 degrees Celsius.
Could we see a picture of this superconducting car body with it's exotic refrigeration system?
I'm also curious why you have specified "low currents" and have used a 5 watt load when most modern alternators are capable of producing 100Amps or more?
Edited by Mr2Mike on Monday 25th January 11:23
If OP wasn't confused prior to asking his question he probably is now with the over complication.
Lets see if we can clear up a few points for you.
A "steel" car body is not a "superconductor" It possesses all the same conducting properties of "Steel" - No more No less. Indeed it has a large cross section of matter hence it can carry far more current than the vehicle will ever demand.
The Body of a car only needs to conduct the same amount of current that the Starter Motor plus all Consumers will demand.
The Current Carrying Capacity of any circuit will be dictated by its weakest link, always the wiring and connectors.
It matters not how good of a conductor the Steel Body is because the thick cable(s) attached between the battery Negative terminal and the Engine Block and Body will dictate Volt Drop, they are the weakest link !! Same goes for the Battery Positive Wiring.
There will always be some volt drop in vehicle wiring circuits, there are acceptable limits and we measure those in Millivolts. These acceptable "limits" change for different circuits. We could be happy with 500mv across a Starter Motor circuit whereas that same amount of drop would cause havoc on an ECU sensor / actuator voltage supply, signal or ground.
If Your vehicle is wired up correctly and all your Volt drops are within acceptable levels then it does not matter where you take your voltage sensing feed from, of course you need to calculate the current draw of your new circuit to ensure it does not overload the wiring you attach it to. For example, it would not be right to wire up a new consumer that demands 20amps to a circuit that is wired and fused to carry 10 amps.
In your case I a guessing that it is just a Volt Sense Wire which will carry only a few milliamps.
I must agree with Jakg
"Assuming appropriately sized cable, the voltage should be the same everywhere in the system. If it's not, you probably have a problem."
.
In Fact, not "Probably" - You definitely have a problem and you need to fix that first
Just for reference, your Battery voltage will at best be 12.7v rising to at least 13.1v as the alternator wakes up and goes to work. That is a massive 400mv (worst Case Scenario) rise in voltage available to your Voltage Sense Wire.
The Golden Rule of Volt Drop is that it can only be measured if current is flowing in the circuit. PERIOD
Lets see if we can clear up a few points for you.
A "steel" car body is not a "superconductor" It possesses all the same conducting properties of "Steel" - No more No less. Indeed it has a large cross section of matter hence it can carry far more current than the vehicle will ever demand.
The Body of a car only needs to conduct the same amount of current that the Starter Motor plus all Consumers will demand.
The Current Carrying Capacity of any circuit will be dictated by its weakest link, always the wiring and connectors.
It matters not how good of a conductor the Steel Body is because the thick cable(s) attached between the battery Negative terminal and the Engine Block and Body will dictate Volt Drop, they are the weakest link !! Same goes for the Battery Positive Wiring.
There will always be some volt drop in vehicle wiring circuits, there are acceptable limits and we measure those in Millivolts. These acceptable "limits" change for different circuits. We could be happy with 500mv across a Starter Motor circuit whereas that same amount of drop would cause havoc on an ECU sensor / actuator voltage supply, signal or ground.
If Your vehicle is wired up correctly and all your Volt drops are within acceptable levels then it does not matter where you take your voltage sensing feed from, of course you need to calculate the current draw of your new circuit to ensure it does not overload the wiring you attach it to. For example, it would not be right to wire up a new consumer that demands 20amps to a circuit that is wired and fused to carry 10 amps.
In your case I a guessing that it is just a Volt Sense Wire which will carry only a few milliamps.
I must agree with Jakg
"Assuming appropriately sized cable, the voltage should be the same everywhere in the system. If it's not, you probably have a problem."
.
In Fact, not "Probably" - You definitely have a problem and you need to fix that first
Just for reference, your Battery voltage will at best be 12.7v rising to at least 13.1v as the alternator wakes up and goes to work. That is a massive 400mv (worst Case Scenario) rise in voltage available to your Voltage Sense Wire.
The Golden Rule of Volt Drop is that it can only be measured if current is flowing in the circuit. PERIOD
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