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Lets get AMP'd up

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In this latest installment of SOI University, our little friend the AMP meter comes under scrutiny.

Sitting there all shiny and pretty when it's new with its well defined, curvaceous bidirectional amperage scale all glistening in the sunlight -makes you forget how many problems the darn thing can cause in the proper charging of the tractors they endow.

Well one of those bad girls tried to ruin Christmas for SOI's neighbor and was consigned to the university autopsy bin within minutes.

As soon as I can find where my grandkids hid all my AA batteries for the camera, we will continue.

In the meantime, feel free to put up all the questions you might have about ammeters and I will personally see to it satisfactory answers will be given to all questions.

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Squirrel, I like you sign, and they may think they need Dr. Phil. What I think they need is a dose of reality send them to Gunnery Sergeant R. Lee Ermey. :ychain:

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:ychain: One of my all time favorite commercials

need a tissue? :D :D

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Well, I'm going to jump ahead of the Master and talk about The Law. Ohm's Law that is ! While not always necessary, it is one of those nice things to have in your back pocket. Here is the basic version

V = I * R

V = voltage (also sometimes called voltage drop - more later)

I = current (i.e. amps - I can't remember why they always abbreviate it "I" instead of "A")

R = resistance

Now in written word: The voltage (drop) across a device is equal to the amount of current passing through the device times the resistance of the device.

A simple example is a headlight. We all know that one side of a headlight is connected to the positive terminal of the battery (via a switch) and the other side is connected to ground (via the chassis which is of course connected to the negative terminal on the battery). With the headlight disconnected, using you DVM (digital volt-ohm meter, there is no excuse for not having one ! They are frequently on sale at Harbor Freight for less than $5. A nice one with rubber housing are also frequently on sale at Sears for less than $20) measure the resistance of the headlight.

Now plug the headlight back into the wiring harness and turn it on. Switching your meter to read volts, and placing the probes across the contacts of the headlight, read the voltage drop (it is called "drop" because the voltage is being lowered or dropped from a higher value to a lower value) across the contacts. It should be close to 12V because those contacts are connected to the positive and negative sides of the battery, (Quiz: Now measure the voltage at the battery terminals. Why might those 2 measurements be different ?)

So now we know the resistance of the headlight and the voltage drop across the 2 contacts. Remembering our Algebra (sigh - my second love - mathematics), we know we can re-write Ohm's Law to be I = R / V and know how much current that headlight is drawing.

So what you say ! Well, current draw is useful to know what size wire to use (See Table C) when replacing a bad wire (why waste money with a 12 gauge wire when a 14 gauge is adequate). Also, when adding (extra) lights, you should know what the total current draw is to make certain you are not exceeding the current capacity of the stator.

In summary

V = I * R

I = V / R

R = V / I

No matter how you write it, it's The Law ! Obey it !!

P.S. For those of you who are wondering how these 2 "nut cases" (hey, I made a squirrel joke) are related, we are brothers, from separate parents. Laurel and Hardy. Martin and Lewis. Batman and Robin. You get the picture.

Hey, SOI, I buy my AA's "by the pound" on Flea Bay !

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Fellow RS'ers, please forgive me for going a tad off topic for just a moment....Wiz, thank you for the wiring chart. I've tinkered around with power boats for years and never realized there was a color convention for marine wiring. From here on out my life will be much easier!

Now back to :D tractors, and thanks for your indulgence! :D

Duff :ychain:

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Duff, you are welcome !

Part of the reason I referenced that web site was to bring up (again) that there is a difference between stranded electrical "hook up" wire (like Radio Shack), automotive wire and marine grade wire. If you really want your next WH wiring project to last (i.e. not have to re-do it in 5 years), use marine grade wire.

Marine grade wire is pre-tinned. That is, each individual copper strand is coated with tin before being combined with other strands and encapsulate in plastic insulation. The benefit is almost zero corrosion even in wet conditions. Also the insulation has a higher temperature melting point (along with the tin plating makes it easier to solder) than regular "hook up" wire.

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I = current (i.e. amps - I can't remember why they always abbreviate it "I" instead of "A")

I stands for Intensity in regard to it's use as an abbreviation for Amperes.

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I have a question I have been wondering for a while. Most of the newer tractors went to a voltage meter. Why was the switch made away from the amp meters, even in GM trucks had amperage gauges and they went to voltage meters mid 70's???

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The quick and simple answer is, volt meters are cheaper to buy and cheaper to install. An Ammeter has to be wired in series between the charging system and battery and requires heavier wiring. A volt meter is wired between switched power and ground and can use much lighter wiring. There are other reasons, safety among others, but I suspect price and profit were the major motivation for the switch.

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There is a lot of truth in what Jim_M said. The automotive industry will do just about anything to save $0.10.

I have another theory. People like to see a gauge that moves, because then they know it really is working. With an ammeter, it reads zero with the key off, zero with the key off and the engine and accessories off and near zero when the engines is running. With a volt meter, it reads zero with the key off, approximately 12v with the key on, engine off and approximately 12V with the engine running.

BTW, ever notice that most gauges in cars do not have dials/numbers on the gauge ? That is because they are not calibrated. Calibration is expensive.

So if you only have a voltmeter, how do you know if you are really charging your battery even if you have no numbers on the gauge ?

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BTW, ever notice that most gauges in cars do not have dials/numbers on the gauge ? That is because they are not calibrated. Calibration is expensive.

A little off topic, but kinda related... :ychain:

I had an early 90's Ford Ranger for a long time that had an oil pressure gauge that read 0 when not running (obviously) and something like 45 when running. It read that same number at full throttle, at idle, hot, cold whatever.

I heard that people had complained about low oil pressure at idle, so to fix the problem Ford put a resistor in the circuit to make it read "normal" if there was any oil pressure at all. I dont know if this is true, but it sounds plausible to me...

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BTW, ever notice that most gauges in cars do not have dials/numbers on the gauge ? That is because they are not calibrated. Calibration is expensive.

A little off topic, but kinda related... :ychain:

I had an early 90's Ford Ranger for a long time that had an oil pressure gauge that read 0 when not running (obviously) and something like 45 when running. It read that same number at full throttle, at idle, hot, cold whatever.

I heard that people had complained about low oil pressure at idle, so to fix the problem Ford put a resistor in the circuit to make it read "normal" if there was any oil pressure at all. I dont know if this is true, but it sounds plausible to me...

Fords did do that a lot, my 85 Chevy luckily is pretty dead on as far as oil pressure, engine temp and voltmeter

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Valuable info, but something seems a little "squirrley" here............... :ychain::D

Oh and by the way...... With the automotive volt gauge.... O volts Key Off, 12 volts KOEO, and 14 volts KOER........ Pretty sure it is listed as 0, 12, and 12 in SOI/Wizards post........

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With the automotive volt gauge.... O volts Key Off, 12 volts KOEO, and 14 volts KOER........ Pretty sure it is listed as 0, 12, and 12 in SOI/Wizards post........

Well, actually, you just answered the question I posed !

So if you only have a voltmeter, how do you know if you are really charging your battery even if you have no numbers on the gauge ?

It was kind of a trick question, because that slightly higher reading KOER over KOEO means there is more "potential" (another word for voltage) available to charge the battery.

All 12V lead acid batteries are made up of 6 cells. Each cell, when fully charged, will produce approximately 2.2v for a total of 13.2v. To "charge" the battery you need to supply it with more voltage than its fully charged state.

Charging at high rate (say over 15V) will cause the battery to heat up and the acid to boil.

A battery with insufficient liquid or clogged vent CAN EXPLODE !

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Where is that little squirrel with the autopsy results !

Well, to continue off topic (briefly)

I had an early 90's Ford Ranger for a long time that had an oil pressure gauge that read 0 when not running (obviously) and something like 45 when running.

The volt/amp/oil pressure/fuel level gauges on most (all ?) Ford products are not "true" electrical gauges because, IMHO, they do not use a d'Arsonval galvanometer movement.

A voltage supplied by the Instrument Panel Voltage Regulator, is fed to the "gauge" and then to the sensor. The "sensor" provides a variable resistance to ground. Inside the gauge is a piece of bi-metal. The pointer is glued on the bi-metal at a 90 degree angle. The current flowing through the circuit cause the bi-metal to heat up and bend. This gives you a "general" indication of the item you a "sensing", but the reason there are typically no number is because of the wide variation in deflection of the bi-metal for a given current. Also ambient temperature affects the reading.

That squirrel better get back before I go off on something really crazy ! Nuclear fusion, perpetual motion, making diamonds from coal; Wait that last one has been done !

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Sounds like a Ford man to me :ychain:

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Sounds like a Ford man to me :ychain:

Guess the KOEO and KOER gave me away......... :D

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Where is that little squirrel with the autopsy results !

That squirrel better get back before I go off on something really crazy ! Nuclear fusion, perpetual motion, making diamonds from coal; Wait that last one has been done !

So has fusion, at least in an "uncontrolled" way! :D

Regarding meters. Ideally one should have both a volt and an amp meter on a tractor or other vehicle. See my Avatar...

When the first cable across the Atlantic was installed they were ignorant of Ohms law, not to mention the effects of capacitance. The available meters were very crude, and when trying to detect the small amount of current flowing from Nova Scotia to Ireland (over a single conductor with a ground return, mind you) it was nearly impossible to see the meter move, particularly in a ship at see.

The solution was to mount a mirror on the meter needle and using a lantern, and a metal with a slit in it, project a beam of light onto the mirror.

The reflected light then bounced onto a screen where the meter movement was made more obvious ("amplified"?) due to the geometry of the setup.

True story. My great gr gr gr gr grandfather (Cyrus Field) was the man responsible for the project. William Thomson was knighted for his work on the project and became known as Lord Kelvin. He went on to become quite a famous physicist.

:D :ychain:

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Where is that little squirrel with the autopsy results !

You guys seemed like your having a great time so I decided to take a little vacation away from all the hustle and bustle

squirrelinfl.jpg

I'm back and I have my batteries.

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Below is a diagram representing three different charging conditions possible on ammeter equipped tractor.

The top diagram indicates battery in a full charge state and no appreciable load on the tractor

The middle diagram indicates a heavy accessory load, and the battery is now contributing to the overall running of the tractor. The charging system can no longer keep up with demand for accessory current.

The bottom diagram indicates normal accessory current demand with the battery absorbing power from the charging system .

magfieldammeter.gif

This is probably not news to anyone here.

Look a little closer and you will see a COMPASS representing the ammeter function in each of these diagrams. This substitution is not as wacky as it may first appear. The purpose of this post is to show that a magnetic field is present around a current carrying wire. The DIRECTION of the current flow will either add to or subtract from the compass needle magnetic properties.

With no current flow as in the top diagram, a small weight (shown as a green dot) will cause the compass needle (ammeter gauge pointer) to point straight upward.

So no current = the ammeter points to 0 indication. (in real life, the return to a 0 indication could also be accomplished by a clockspring inside the gauge).

When the battery is discharging, the magnetic lines of force around the wire start to oppose the magnetic force of the compass needle and allows the indicator needle to fall into the DISCHARGE area of the compass / ammeter. This is done with the help of the clockspring tension.

When the battery is charging, the magnetic lines of force around the wire change direction and ADD to the compass / ammeter pointer magnetic properties and force the needle against the clockspring and push the indicator into the CHARGE zone.

So now we know what makes the ammeter needle move.

Next installment will show how we transition the physical parts of a compass into the physical parts of an ammeter gauge.

Be back later with autopsy pics.

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Alrighty. So I plan on running a set of headlights on a 854 tractor. The battery is new as of last year, and I haven't needed a tender as of yet. I replaced the bearings in the Statrer/Generator, and expect it to last a while. The lights draw 2.5 amps each. ( I powered them up from my DC Variator, and it gave the amp reading directly at 12volts.) I am planning on using 14gage wire, SPST toggle switch, and a 7.5 amp fuse. Not sure yet if I will power it off the ignition switch, or a seperate cuircut directly from the battery. Thoughts from the teachers?

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So a little morphing of the "wire across a compass" setup will yield a functional ammeter.

In the lower diagram, the North. South markings are dropped in favor of -20 to +20 amp markings.

magfieldammeterbentwire.gif

The green dot weight is converted to either a clockspring to hold the pointer to a 0 indication or in this case, a small pendulum weight at the bottom of the pointer needle.

The "wires" crossing the face of the compass are flattened out into a "bracket" to support the indicator needle of the gauge. The pointer is connected to a small cylindrical magnet assembly which takes the place of the magnetized compass needle.

That should complete the theoretical morphing of the compass into an ammeter.

The actual photos of the inside of the defective ammeter gauge will follow.

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