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Old 31-12-2010, 14:24   #1
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Amp Meter


Amp meter
Happy new year to all
I have had this nagging question for over a year and hesitated to bring it up because it sounds not possible but here goes there might be an answer yet.
I have an amp meter that displays the amp output of the alternator 0 to 120 amps. No bigy
Where it does not make since is I trace both wires on the amp meter back to the same output terminal on the alternator output to the Diode isolator. (In my mind it should probably go to a shunt or maybe + and but not both wires to same + terminal.) (Ps. At Isolator terminal there is one exception it looks like the previous owner ran out of room on the (Isolator Alternator terminal so he added what looks like a piece of copper buss bar about wide and 1 long 1/8 thick. So one wire from amp gauge goes to terminal on one end of buss bar(Under alternator output) and the other wire on the other end of buss bar) I hoped this might be a shunt bar but when I checked the resistance it was as I expected 0 0r .ooo4 ohms. So I give up
Any suggestions out there that could explain this paradox would be much appreciated
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Old 31-12-2010, 15:10   #2
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If it reads current then it is working correctly as a shunt. With a multimeter read the millivolts & recal You would have to have a very good multimeter to read that low resistance.
If it works all is well!!
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Old 31-12-2010, 15:31   #3
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Amp meter
Happy new year to all
I have had this nagging question for over a year and hesitated to bring it up because it sounds not possible but here goes there might be an answer yet.
I have an amp meter that displays the amp output of the alternator 0 to 120 amps. No bigy
Where it does not make since is I trace both wires on the amp meter back to the same output terminal on the alternator output to the Diode isolator. (In my mind it should probably go to a shunt or maybe + and but not both wires to same + terminal.) (Ps. At Isolator terminal there is one exception it looks like the previous owner ran out of room on the (Isolator Alternator terminal so he added what looks like a piece of copper buss bar about wide and 1 long 1/8 thick. So one wire from amp gauge goes to terminal on one end of buss bar(Under alternator output) and the other wire on the other end of buss bar) I hoped this might be a shunt bar but when I checked the resistance it was as I expected 0 0r .ooo4 ohms. So I give up
Any suggestions out there that could explain this paradox would be much appreciated
Traditional ammeters are designed for full scale deflection at 50 mvolts.
120 amps times 0.0004 ohms = 0.048 volts or 48 mvolts. Ignoring branch circuit calcs, as it's only 120,000:1.

John
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Old 01-01-2011, 18:01   #4
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Thanks Bill and John
I take it from your answer that I am not crazy and having 2 wires from the almost same post would deflect the current meter between 0 and 50 Milivolt ????
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Old 01-01-2011, 20:14   #5
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Traditional ammeter is 1 mA, 50 mV full scale deflection, and is 50 ohms.
When 120 amps comes out of the alternator you want 1 mA to go through the branch with the ammeter, since it is 50 ohms, V=IR, .001 x 50 = 50 mV. The voltage drop across both branches will be the same, 50 mV. 119.999 amps has to go through the other branch with a 50 mV drop. Using V=IR again, .05 / 119.999 = .00042 ohms is the value of the resistor of the other path, the shunt path. I'm surprised you could measure that also.

John
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Old 02-01-2011, 10:30   #6
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Once again I am impressed with the level of experiance on this forum.
Thanks agin John for sharing your knowlage
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Old 02-01-2011, 10:43   #7
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I have tried the same thing. Multimeters are not sensitive enough to read the voltage difference across a shunt to give you an accurate enough indication of the current running through the shunt. An multimeter will give you a little indication but it is not anywhere as accurate as a real ammeter....especially as you get into the larger shunts like the ones designed to handle up to 500 amps.

BTW, I find that having an ammeter that measures my alternators output is extremely valuable.
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Old 02-01-2011, 12:49   #8
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Sergy, an ammeter is really just a voltmeter that is reading a voltage drop across a known and calibrated resistance, which is usually a "shunt", in millivolts.

Shunts are usually built, sold, and matched to use with a specific ammeter, either as a packaged set, or for a specific scale.

So a "100 amp" shunt might be designed with enough resistance so that it has an internal loss of 1 millivolt per amp that it conducts. It would be sold with (or for) an ammeter which is actually a voltmeter, displaying a reading of 1-100 millivolts, but labelled as "Amps".

The voltmeter's two wires would be attached, one at each side of the shunt, to measure the voltage loss (drop) in the shunt.

If that's clear...there is no way that your ammeter's two wires should both be attached to the same place. It can't work that way.

You don't need a dedicated "shunt" although it is the simplest way to do things. For instance, a foot-long piece of 8AWG stranded cable might have a voltage drop of one millivolt per amp--and if it did, exactly, it would make a perfectly good substitute for a shunt although it would be about six times longer.

IF part of the isolator had a similarly convenient voltage drop, you could connect an ammeter across the two ends of the isolator and work that way as well--but I wouldn't expect the values it gave to be accurate. I'd use a real shunt, or a matched length of suitable cable. You might want to double-check your wiring, see if perhaps someone crossed wires or perhaps you have mistaken the wiring. A shunt typically looks like a two inch long piece of busbar with screw contacts at each end, and if should be in/under a plastic protective housing so it cannot short out.
Shunts are often placed in the NEGATIVE bus, at the negative common lead from the batteries, not in the positive lead at all. That depends on whether you are trying to monitor just the alternator--or the battery banks.
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Old 02-01-2011, 13:01   #9
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[QUOTE=hellosailor;589184If that's clear...there is no way that your ammeter's two wires should both be attached to the same place. It can't work that way.
[/QUOTE]


He has a shunt, he just didn't recognize it as such, he thought it was a bus bar.

From original post:

(Isolator Alternator terminal so he added what looks like a piece of copper buss bar about wide and 1 long 1/8 thick. So one wire from amp gauge goes to terminal on one end of buss bar(Under alternator output) and the other wire on the other end of buss bar
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Old 02-01-2011, 13:01   #10
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An isolator (a big diode) will not give you a proportional reading because of its internal resistance which is there regardless of the amount of current flowing. You need a shunt that is rated for the ammeter you will be reading from. Shunts and ammeters usually come as a kit so you get a combination that will work together.
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Old 02-01-2011, 23:48   #11
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Amp meter/shunt

Thanks you all for your comments they are appreciated, as John indicated I may have a shunt and did not recognize it as such. Up until now it did not make sense but I know the amp meter worked. I have had to install a new Isolation which made me revisit the ghost of the past which was how can it work because I thought both wires were common but if this buss bar is indeed a shunt then it makes sense.
As Hellosailor sais any literature I have read regarding shunts they are on the negative side if that is the case what would be the proper way to hook up an amp meter to monitor the output current of the alternator
David I do not know what BTW means but I agree with your statment
"BTW, I find that having an ammeter that measures my alternators output is extremely valuable"
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Old 03-01-2011, 12:22   #12
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Sergy, what you want to do is simple. An ammeter will measure "whatever" the shunt is inserted into. So if you place the shunt "at" the alternator output post, and then run the rest of the output from the shunt to the rest of your system, you'll measure the alternator output.

A couple of cautions apply: The shunt should be scurely mounted so it isn't moving, which probably means you run a cable from the alternator to the shunt, and install the shunt on a bulkhead. With protective cover on the shunt. Some heavy plexiglass will do if you can't get a fitted one from the maker.

Also, the alternator output wire MAY be a length of "fusible link" wire, or it may connect to one on the way to the starter. Fusible link wire is identified by printing on the insulation, it is literally a length of wire that burns up like a fuse to protect the rest of the system. Up to the 80's they were common, they are now considered a fire hazard (duh!) so if you have one, you might want to replace it with conventional wire and a fuse.

The two wires from the ammeter to the shunt should be twisted around each other, to protect them from picking up stray currents inductively. Probably not an issue with most installations--but easy enough to do it right.

The ammeter should read only positive when the alternator is running, if a diode blew it would actually show negative numbers when the engine was off. If the polarity is reversed, no harm, just switch the wires on the shunt.

You can, by the way, install a second shunt in the negative common cable, add a litle double-pole two-way switch, and then use the same ammeter display to show either the alternator or the system amperage. Just make sure it is a double-pole switch, so your shunt leads don't become a short circuit. (A small fuse would be a good idea if you do this.)
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Old 03-01-2011, 17:40   #13
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Thanks Hellosalor,
You make it sound like I can go and buy a second shunt?? Does this not have to be calibrated to match the 120 amp meter. Is it not possible that the shunt at the battery may be overloaded at 120 amps?
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Old 03-01-2011, 18:27   #14
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Quote:
Originally Posted by Sergy View Post
Thanks you all for your comments they are appreciated, as John indicated I may have a shunt and did not recognize it as such. Up until now it did not make sense but I know the amp meter worked. I have had to install a new Isolation which made me revisit the ghost of the past which was how can it work because I thought both wires were common but if this buss bar is indeed a shunt then it makes sense.
As Hellosailor sais any literature I have read regarding shunts they are on the negative side if that is the case what would be the proper way to hook up an amp meter to monitor the output current of the alternator
David I do not know what BTW means but I agree with your statment
"BTW, I find that having an ammeter that measures my alternators output is extremely valuable"
BTW means By The Way. Sorry, that was a little bit of UATTSOI. (Unnecessary Acronyms for Things That Should be Spelled Out Instead)

With digital ammeters, the shunt must be wired on the negative side. The problem becomes, you cannot wire a shunt to the negative side of an alternator because most alternators ground through the engine. So some smartie pants invented what is called a shunt shifter or a shunt adapter that allows you to attach a shunt to the positive side of an alternator so you can get a measure of how much current your alternator is generating.

Is this doo-hickey what you are looking at on your shunt that has you puzzled?

Blue Sea 8242 Shunt Adapter for DC Digital Ammeter
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Old 03-01-2011, 20:58   #15
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Sergy, a "120 amp" meter would have me puzzled. Usually digital voltmeters are set up as 1xxx digits or 1xx digits, to read "199" as a maximum. I don't know what you've got there but would *guess* that you either have a 100A or 200A shunt with a standard rate of 1 millivolt per amp. If there is any identification on it, any numbers, that would help. Otherwise you'd have to do some testing to check, but a 100A or 200A 'standard' shunt should be readily available. There are some other types, but that's the most common.

By the way, a shunt is usually chosen with a safety margin, so a 150A or 120A maximum design load would be normal for a "200A" shunt.
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