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How to Wire a Shunt for an Amp Meter?

08/28/2011 11:28 PM

I wish to see how much amperage is being used by the different lights I repair.

I have a 12v car battery and some wire to a wall mount with two posts, one for each power and fround and wish to wire the meter in line so whatever light I test I can see how much amperage the lights are pulling.

I have a panel mount amo. meter and a shunt that came with it, but do mot know where to mount it.

If I hook up the meter by itself, evrything maxes out the 30A. meter.

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#1

Re: How to wire a shunt for a Amp meter?

08/28/2011 11:37 PM

The shunt should be in series with the load. The instrument parallel over the shunt.

If the Amp meter shows out of scale, the resistance of your shunt is too high. You are measuring the voltage over the shunt. If shunt and meter came together, you need to go for a higher amperage.

What kind of lights do you repair? It is quite some current you seem to draw?

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#4
In reply to #1

Re: How to wire a shunt for a Amp meter?

08/29/2011 5:56 PM

I buy, repair then re-sell those Lightbars that all the State DOT trucks have [the old mechanical ones with the amber lens & 5 lights], then re-sell them and some seem to pull 20-30 amps so the DOT people say although none seem to know for sure but say to put a 30 amp fuse on all of them, even the little ones. I also rebuild all the mechanics of the old "Wig-wag" lights too.

I guess I need to learn how to read a schematic too as I never did learn how. I've always been doing the mechanical stuff.

Do I put the shunt BEFORE the meter? [between the car battery and meter]

The 30 amp meter and shunt came together. Also, there is no wiring diagram or anything other than a template for the size of mounting.

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#6
In reply to #4

Re: How to wire a shunt for a Amp meter?

08/30/2011 12:00 AM

I think that the purpose of a fuse in a circuit is to protect the wiring. You should not install a 30-Amp fuse in a circuit (say for the "little lights") without reference to the current-carrying capacity of the wire. To be clear: the fuse does not protect the light bulbs, it protects the connecting wiring.

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#9
In reply to #4

Re: How to wire a shunt for a Amp meter?

08/30/2011 5:44 AM

TonyS in post #2 has got the picture, but in words it goes like this: connect one side of the shunt to one side of the meter and the other side of the shunt to the other side of the meter, and you have a made-up ammeter with a full-scale deflection of 30 A. Consider that as a single unit and don't take it apart again. I hope one side of the meter is marked with a + and the other side with a -, otherwise we are going to proceed cautiously.
Now take a small bulb (side or interior light). Connect one side of the ammeter to the + pole of the battery. Connect the other side of the ammeter to the bulb. Finally connect the other side of the bulb to the other pole of the battery. If the ammeter moves from 0 in the right direction, you are in luck. Disconnect and mark the ammeter, if not already marked, with a + next to the + pole of the battery. If the needle went the wrong way, reverse the connections to the ammeter, and mark the correct side.
You are now in business, but I would make up a board for testing which includes a switch and 30A fuse (to protect battery and ammeter): battery + -> fuse -> switch -> ammeter +. Then ammeter - -> bulbholder side 1. Then bulbholder side 2 -> battery 1.
As for fusing the light bar, you will be testing the individual bulbs, but unless the whole lightbar takes 15-20 amps a 30 amp fuse is too much. A single headlamp bulb is around 60 watts, 5 amps.

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#2

Re: How to Wire a Shunt for an Amp Meter?

08/29/2011 9:09 AM

A pictures worth a thousand words

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#3
In reply to #2

Re: How to Wire a Shunt for an Amp Meter?

08/29/2011 1:33 PM

Yes, but you should now be using your multimeter in voltage (voltmeter) mode instead of amperage (ammeter) mode now. So the circle should contain a V instead of an A. The shunt itself will have a rating of how many amperes of current will produce some amount of a voltage (millivolt) across the shunt. If you use the ammeter setting of your multimeter, the internal shunt of the meter will be in parallel to the shunt and giving a false reading. It will be linear and scalable to the actual reading but the scale factor will not be known.

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#23
In reply to #3

Re: How to Wire a Shunt for an Amp Meter?

08/31/2011 7:23 AM

OK it's reading mV but read the original OP

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#24
In reply to #3

Re: How to Wire a Shunt for an Amp Meter?

08/31/2011 9:41 AM

I think that the OP is using an Ampmeter Only. Therefore, the shunt is calibrated for this meter.

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#26
In reply to #2

Re: How to Wire a Shunt for an Amp Meter?

08/31/2011 7:22 PM

Good reply. I add one caution which protects Galvano in circuits using External shunts.

Always be careful Not to connect Galvameter [Ammeter] to Main Circuit connections but on two small connection points a little inside on the Shunt to avoid burning of Galvo if Main connections become loose

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#5

Re: How to Wire a Shunt for an Amp Meter?

08/29/2011 11:06 PM

the ratio of the resistance is the inverse of the current ratio.

So if you have a 1 milliamp meter = 1 ma drives it full scale, and it has an internal resistance of 1000 ohms E= I x R, then E = .001 x 1000 or one volt.

If you place a shunt of .001 ohms across the meter and drive it to full scale = 1 volt, and since the shunt is .001 ohms.

That means it will draw 1000 amps E = I x R or 1 = 1000 x .001.

So now your scale needs to be changed so that the 0 to one volt scale is replaced with a 0 to 1000 amps scale. And of course, 0.1, and 0.01 give corresponding ratios.

In this 1000 amp case, total current is 1000 Amps plus 1 Ma, and so on with other ratios.

This works with AC, but for precision, they use current transformers.

For more details. Ammeter

Shunts etc

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#7

Re: How to Wire a Shunt for an Amp Meter?

08/30/2011 12:29 AM

As a mechanical person myself, I understand your confusion. As has been explained, you are measuring the voltage drop over the shunt (simply a resistor) to determine the current (Ohms Law). This finally dawned on me when I placed a number of resisters in series with a battery and measured the voltage difference either side of the resistor, you can do this using light bulbs. In your case you have two resistors, the shunt with a small voltage drop (millivolts) and the equipment you are testing. e.g. if the total is 12V then the shunt may drop 20mV and the equipment 11.98V. If you open your meter you should see the shunt it uses inside.

Hope this helps.

Tony

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#8
In reply to #7

Re: How to Wire a Shunt for an Amp Meter?

08/30/2011 3:06 AM

Please, Please, Please do not use the term "Amperage". There is no such thing. Please use the term Current when referring to Amperes. Also Please do not use the term "Wattage" for the same reason, Power is measured in Watts and should be referred to as Watts, not "Wattage".

Regards

A grumpy Electrical Engineer.

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#13
In reply to #8

Re: How to Wire a Shunt for an Amp Meter?

08/30/2011 4:35 PM

And are we to abandon "voltage" in favour of "electromotive force?

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#28
In reply to #13

Re: How to Wire a Shunt for an Amp Meter?

09/01/2011 3:16 AM

No need to abandon any correct term.

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#32
In reply to #13

Re: How to Wire a Shunt for an Amp Meter?

09/02/2011 11:58 AM

Yes, and cabbage is also too long.

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#10

Re: How to Wire a Shunt for an Amp Meter?

08/30/2011 6:16 AM

The guys are helpful, but together a bit confusing. You can build your shunts without much fuss with pieces of wire. This recommendation is for DC to 60cycles AC only. Higher frequencies are a different matter. Please go with Tony's drawing. It will take a bit experimentation on your part, no problem, you will get the hang of it.

Before you can measure something, you have to get something you know, you can trust. In this case 3 resistors, $2-5 apiece. You either go to the friendly electric supply house or .digi-key.com. The choices are bewildering. But your needs are simple.

For various currents, you need:

1 Ohms 5 Watts or higher for 1 Amps or less. Voltage drop is 1 Volt at 1 Amps

0,1 Ohms 5 Watts or higher for 10 Amps or less. Voltage drop is 1 Volt at 10 Amps

0,01 Ohms 2 Watts or higher for 100 Amps or less, Voltage drop is 1 Volt at 100 A.

You see the pattern.

I do not care, what make or model the resistors are. For example at digi-key a 0,1 Ohms is: PWR4412-2SBR1000F-ND. Go from there. Wire these resistors in their individual circuits with thick enough wires. Measure the current directly from the resistor in VOLTAGE mode on your meter.

Good luck, and ask if something unclear.

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#11

Re: How to Wire a Shunt for an Amp Meter?

08/30/2011 10:41 AM

All the other replies have ample information on the connections of the shunt & meter If this solves the problem thats great but something I have found with some digital amp meters is that they require an isolated power supply to run the display.

This means they can not be powered by the same supply they are measureing. There is a solid state power supply isolator available that can be used to make it posible for the meter to operate on the same supply

If not used it will either do damage to the meter or show full scale depending if the shunt is used on the +ve or the -ve side of the load .

I have purchessed them in the past from E bay to use with panel meters if you do a search for "isolated power supply" it will come up with 5v & 12v output voltages depending on what voltage the meter operates on.

I hope this helps if the other answers haven't solved your problem

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#12

Re: How to Wire a Shunt for an Amp Meter?

08/30/2011 3:24 PM

Why not do it the easy way?

Just measure the ohms (with the power disconnected) and divide that into 13.2 volts. The results is the amps used.

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#14
In reply to #12

Re: How to Wire a Shunt for an Amp Meter?

08/30/2011 4:38 PM

You CANNOT, successfully, that is. Light bulbs change their resistance 10fold from cold to hot. Other resistors change too. Hence the standard resistors, not changing much, by design.

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#15

Re: How to Wire a Shunt for an Amp Meter?

08/30/2011 9:44 PM

Ok everyone, let me say some things that might help as i don't understand some of what is being said, afterall I'm just a woman:

1) The shunt came with the meter. [See photos]

2) The meter IS marked + and - on the back

3) Whatever you wish to call it, the meter says "Amps" not volts. I already know that these lightbars [which is what I am MOST concerner with] are for 12VDC use on commercial vehicles not in AC circuits.

4) as can be seen, it is a analog not a digital meter.

5) there are both 5 loghts AND 5 motors in each lightbar that are used to turn the reflectors.

6) I wish to see how many AMPS that each lightbar uses in total so I can know what size fuse to put with each lightbar when i sell them.

7) Some of the lightbars are only 4 or 2 light'rs but ALL have the motors and reflectors in them.

Does this clarify anything for you all? I Hope so, cause I am STILL confused.

Thanks Ya-all.

Tami-Lynn

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#16
In reply to #15

Re: How to Wire a Shunt for an Amp Meter?

08/30/2011 10:20 PM

Hi Tami-Lynn

"afterall I'm just a woman" I've worked with women who run rings around me, but my wife says that's not difficult.

I couldn't see the meter in your pictures.

As has been explained by a previous contributor, both lights and also motors have a low starting resistance i.e. draw more current when starting, maybe 10 times. Fuses came in two main types, slow and fast blow the former being for your type of application so that they will only blow if the device fails after starting, either by a short circuit or a motor jamming etc.

Another thought to add to your confusion, is to use a cheap oscilloscope to measure the voltage over the shunt. This will give a diagram of voltage (which represents current) over time. You would need to take advice on this setup from someone local.

Good luck - Tony

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#19
In reply to #16

Re: How to Wire a Shunt for an Amp Meter?

08/30/2011 11:25 PM

Yes, an oscilloscope reading the small voltage across the shunt can precisely record the actual peak current but this is likely not really needed to be known here. Common automotive filament fuses do not blow open the circuit as soon as the current exceeds the rated current. An ATC type fuse rated at 3 amperes will nominally take a tenth of a second to open if the actual current draw is 15 amperes. Most filaments will be quite hot in a tenth of a second.

You do give me here an opening to introduce something to probably not do here. There are marvelous circuit protection devices called PTC fuses. These are self resetting devices that have many very useful applications. I do not recommend them here though. When a short circuit happens with these devices in place they quickly heat up and become a virtual open. When they cool back down again they reset and permit full current to flow. The trouble here is that current albeit a small current continues to flow through a hot PTC device. Imagine this scenario; a light-bar is wired directly to the battery of a vehicle through an added aftermarket toggle switch with a PTC fuse to protect the wiring. The light bar shifts and pinches the supply wire. The lights go out because the short circuit and the PTC device heats up to a safe condition. The operator vows to fix this in the morning but because the lights are not glowing he forgets to throw the toggle switch OFF. The battery drains the entire night and may not have enough power in the morning to start the vehicle. In contrast a real fuse will actually open the circuit and save the battery along with the wires.

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#20
In reply to #19

Re: How to Wire a Shunt for an Amp Meter?

08/30/2011 11:41 PM

Thank you for your input, redfred, I use 12V motors and micros in my mechanical designs, but I'm conversant not learned, so I'm learning from this input too.

Tony

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#21
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Re: How to Wire a Shunt for an Amp Meter?

08/30/2011 11:54 PM

Glad to help.

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#18
In reply to #15

Re: How to Wire a Shunt for an Amp Meter?

08/30/2011 10:48 PM

Here's a few critical clarifications for you. The shunt and the meter work together as a matched set make an ammeter. The meter alone actually measures very small voltages and not current but when it is wired in parallel (like the drawing TonyS made) with the shunt, the needle movement accurately shows the amperage through the load (light-bar) of the circuit.

Now you have some additional complications because a DC motor and a filament lamp will draw a different amount of current at start up. The filament as it heats up will increase in resistance and then draws less current. Fortunately most filament lamps heat up very quickly. A DC motor as it is starting to move a mass will also draw the maximum amount of current. Once the motor starts to move, the current drops until the only power drawn is what it takes to fight friction. As you've probably guessed by now, both the motor's peak current and the filament's peak current actually happen very briefly. This transient maybe so quick that you never see the needle of an analog ammeter jump up to a higher number. This is why a running lamp and motor display that draws 2A normally can blow a 2A fuse. As a "rule of thumb" if you double the running current and then step up to the next standard value for the value of your fuse, you will have a reliable system that won't blow fuses on start up.

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#22
In reply to #15

Re: How to Wire a Shunt for an Amp Meter?

08/31/2011 4:20 AM

I have to respect females too - my daughter is a civil engineer. Now that we have a picture of your problem and can see that headlamp bulbs are involved, I will comment that your ammeter is suitable for measuring the current of a single bulb or bulb/motor combination, but it could be overloaded by a 5-lamp bar if all the bulbs are lit together.

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#25
In reply to #15

Re: How to Wire a Shunt for an Amp Meter?

08/31/2011 12:56 PM

Sorry I automatically thought of a panel meter being digital as it's more comon these days to see digital rather than analogue

the diagram that TonyS has put up there is what you need to do to connect the meter

I hope TonyS doesn't mind I added a little to his diagram to make it clearer for you incase you don't understand the symbols

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#17

Re: How to Wire a Shunt for an Amp Meter?

08/30/2011 10:28 PM

No, Tami. You are way overcomplicating things. Please, pretty please, follow my recommendations first, and I promise not even notice you wearing a skirt. Scouts honor.

And I still will answer your questions.

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#27

Re: How to Wire a Shunt for an Amp Meter?

09/01/2011 12:02 AM

Regards all.

It a Panel Ammeter using a shunt.

Shunts have specs mV at the FSD for which the shunt is designed. Say .2 V or 200mV at Say 100A [if the shunt is designed for 100A

The meter used with this shunt should be printed on dial .... to 100A and have its sensitivity of 200ma to deflect the poit 100A on the dial.

For any system using ammeter the shunt is selected and meter is marked and calibrated for the output of the shunt.

To extend the range of a multimeter the recommended shunt unit by manufacturer may be used. In this case the Voltage-range is indicated by the manufacturer [normally ≈ 2.5V].

And an important factor is the Burden to which the shunt handles for accurate measurements.

The burden is the termination impedance of the measuring instrument.

And care in connecting the meter: See my reply#26 to #2.

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#29

Re: How to Wire a Shunt for an Amp Meter?

09/01/2011 4:31 AM

This maybe the answer to a maidens prayer. Available here but probably cheaper elsewhere.

http://www1.jaycar.com.au/productView.asp?ID=QP2257&keywords=automotive+blade&form=KEYWORD

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#30

Re: How to Wire a Shunt for an Amp Meter?

09/01/2011 1:58 PM

Guess I'm an old man from the old school, but.

If the person making the current measurement doesn't know how to use an ammeter, the best procedure would be to try a low current time delay fuse.
Turn the lights/motor on and off ten times. The voltage should be the maximum possible operating voltage, maybe 12X1.1=13.1 volt.

The voltage should not droop. Connect an analog voltmeter and small wattage light bulb across the 13.1 volt. If the voltmeter changes or the light bulb blinks, the voltage is drooping.


If the fuse opens, try the next larger time delay fuse.
Repeat the above until the fuse doesn't open.
Use a fuse with twice the current rating of the last fuse tested.

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#31

Re: How to Wire a Shunt for an Amp Meter?

09/02/2011 3:09 AM

I've done this, you don't need to take a lot of measurements. How many fuses are you going to install per light bar? Are you also providing the main cable that goes through the roof? to the control box. The control box sometimes has the sizes written on it. Also the lamps have a part number, with that you can determine the amps. Same for the motors.

At any rate, check the wire gauge on each circuit to be protected, determine the ampacity or current rating of that wire.

Install a fuse of 75% of that capacity. The fuse (s) is to protect the wiring not the lamps/ motors.

Buy a good power supply with a built in amp meter. If it is variable voltage, variable current it will save you a lot of fuses while troubleshooting.

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