Little help needed regarding shunt resistors

Hi!

matter is rather simple, well probadly most of you, but i'm confused in this matter. I bought this following item, and i have it currently. 50ADC meter along with shunt resistor. What i don't understand is the function of shunt resistor. If i read schematic correctly, it shows to install ammeter in parallel with resistor, and other bigger screws in shunt resistor are for main current. But, why is the ammeter in parallel? could someone tell whole story behind shunt resistor and whether i understood this whole right at all...
yes i know, ebay probadly isn't best place to buy this stuff, but i couldn't fine even ONE 50ADC meter locally, or even from europe.

**broken link removed**

"Shunt" means "Bypass".

In order to give you a reading, and not simultaneously burn out the ADC circuit (and, at the same time, not add a significant load to the circuit under test), the majority of the current is passed through the very low resistance shunt.

Thus the ADC (in parallel with the shunt and a very high impedance) only sees the very low voltage generated across an equally low resistance (the shunt). The readout circuit performs the necessary math to display the current flowing in the shunt.

It's not an ammeter in parallel with the shunt resistor, it's a voltmeter (calibrated to read amps when connected to the shunt). The shunt, in conjunction with the voltmeter, makes an ammeter.

They only let a small percentage of the current go through ammeter.

That way they don't have to make a super duty ammeter that can conduct 50 amps or more.

Just think of it as a "current divider", MOST of the current takes the HIGHWAY (the shunt) and a proportional amount takes the route of the meter. e.g 50 A where 49 of them takes the shunt and 1 Amp takes the meter path. If there were 25 A flowing, then 0.5A would take the meter path.

Well, not necessarily true, so let's do it another way: Everybody but a straggler goes through the shunt. The other meter is actually a micro ammeter with a series resistance which means it's a voltmeter. So, it measures the 50 mV or so voltage dropped across the shunt at say 50 Amps. You just put different numbers on the scale.

Now, why the big and small terminals. The Chinese do it a little cheaper, but the idea is the resistance between the small terminals is controlled or known. It then doesn;t matter where the big wires connect and what the voltage drops are between the big and small terminals.

A non-Chinese shunt: **broken link removed**

Current flowing through a resistor develops voltage V = IR (Ohm's law).

Voltmeter connected to the shunt measures V.

You then calculate current I = V/R

There's a little confusion running through this thread

This is because there's two simple ways of looking at how a shunt works:

1) The meter works as a voltmeter, and measures the voltage dropped across the shunt.

2) The meter works as a micro(or milli)-ammeter, with most of the current bypassed through the shunt.

Both are really the same thing.

Essentially all meters were micro/milli-ammeters - to make it into an ammeter you put it across a shunt, to make it in to a voltmeter you add a series resistor.

As you're not adding a series resistor in an ammeter you're not 'specifically' measuring the voltage.

However, it's common practice to add a swamp resistor in series with the micro/milli-ammeter - this isn't particularly to make it read voltage, it's to 'swamp' the movements own internal resistance, because different meters have different internal resistances. By adding the swamp resistor a meter manufacturer can compensate for different movements, or different values even within the same model.

There are also high current meters that work directly (no shunt), the movement is just wound with VERY thick wire old car ammeters are a prime example.

Nigel Goodwin said:

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Essentially all meters were micro/milli-ammeters - to make it into an ammeter you put it across a shunt, to make it in to a voltmeter you add a series resistor.

As you're not adding a series resistor in an ammeter you're not 'specifically' measuring the voltage.
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This is a nit, but I would argue that you are indeed measuring the voltage across the shunt. You can do that with a high impedance digital voltmeter or a relatively low impedance analog meter, but in each case it's the shunt voltage being measured, which determines the current through the shunt. In the analog meter case the series resistance may be just the meter movement resistance. The current through the meters is so small relative to the shunt current, that it has no significant effect on the current measurement in either case.

crutschow said:

This is a nit, but I would argue that you are indeed measuring the voltage across the shunt. You can do that with a high impedance digital voltmeter or a relatively low impedance analog meter, but in each case it's the shunt voltage being measured, which determines the current through the shunt. In the analog meter case the series resistance may be just the meter movement resistance. The current through the meters is so small relative to the shunt current, that it has no significant effect on the current measurement in either case.

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Like I said, it's the exact same thing - but the fact remains it's called a 'shunt' because it shunts the extra current past the movement - and the calculations for shunts were based on that premise (but again, it all depends how you want to shuffle ohms law round ).

But the calculations, for example for a 1A meter using a 1mA movement, were done from 1mA though the meter, and 0.999A through the shunt. It doesn't make any difference if you do the calculations from a voltage perspective, you're still sending the exact same currents each way.

Where using voltage makes more sense is where you're using an electronic meter, so aren't constrained by the current requirements of the movement, and can boost the sensitivity by any value you like.

Nigel Goodwin said:

exact same currents each way.

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Generally not. The mili/microammeter type of voltmeter has impedances in ohms/volt. The whole point is the measuring circuit doesn't load the shunt.

Why is it called a shunt rather than a series resistor, probably depends on your point of view. To the ammeter it's a shunt. To the circuit, it's a series resistance.

KeepItSimpleStupid said:

Generally not. The mili/microammeter type of voltmeter has impedances in ohms/volt. The whole point is the measuring circuit doesn't load the shunt.

Why is it called a shunt rather than a series resistor, probably depends on your point of view. To the ammeter it's a shunt. To the circuit, it's a series resistance.

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It's called a shunt because that's what it does, the series resistor makes a voltmeter.

Bear in mind it's relative to the movement, NOT the circuit - an ammeter is still an ammeter even if not connected to a circuit

It's not like this is anything new - it's VERY, VERY, VERY old - 1800's?, we did it in Physics at school, it's not even electronics

As i said, depends on point of view:

You can't get any simpler than this:

A low-resistance precision resistor installed in parallel with an ammeter between a source of electrical energy and an electrical load to allow the meter to read a flow of current which exceeds the limit of the instrument. Most of the current in the circuit flows through the shunt and produces a voltage drop. The ammeter, acting as a millivoltmeter, measures the voltage drop across the shunt and indicates, on a special scale, the amount of current flowing through the circuit.

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https://www.datwiki.net/page.php?id=497&find=ammeter shunt&searching=yes

There's probably 4 ways of measuring current: The feedback ammeter, the shunt ammeter , the current transformer and the hall effect sensor.
The current transformer won;t work for DC.

I've dealt in currents below 2 PA at 100 V and 3000 A at < 6 VAC. I've also dealt with measuring electron beams currents (invisible wires so to speak) in scanning electron microscopes. I've also seen currents measured around 100 mA at 100 kV and 1.5 Amps at 13 kV. I've also personally had to deal with both accurate voltages and currents below 100 mA where the voltage drop across the device was <0.6 V.

One of my coolest projects that I built was a feedback ammeter capable of +-10V out in 4 ranges below 100 mA full scale. Biasing was +-10V or +-5V if suppression was 50 mA. This was a front-end to a lock-in amplifier. I enjoyed building it, but it did have some issues that were not resolved. The AC measurements were dead on, but the DC measurements had about a 50 pA offset. I was unable to spend the time to work on a fix.

I forgot that a zero volt output from a D/A converter wasn't. 500 uV is not zero. I expected to do all of the zeroing in software dependent on two measurements with a zero volt input, but it didn't workout. The design, however worked out great for the AC measurements.

Nigel Goodwin said:

It's not like this is anything new - it's VERY, VERY, VERY old - 1800's?, we did it in Physics at school, it's not even electronics

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Perhaps there is something new to it If you use a digital voltmeter where the voltage is measured by ADC, all you have is a charging capacitor in the ADC. In this case, there's no fixed current through the meter and capacitor will charge only to the voltage developed accross the shunt, regardless of the current through the meter.

For all of it, a mechanical ammeter, in its most basic form, is a galvanometer, routinely referred to as an ammeter.

As we all know, it is the current flowing in the meter that generates the magnetic field that causes, by its interaction with a magnet, a physical deflection of the magnet.

Current does this, of course, not voltage. Can't separate the two, but we know what is the motive force in the case of a galvanometer.

No real galvanometer has zero resistance, although some SIM ammeters (ideal) DO have zero ohms, so here is how I see it:

Adjust the meter face divisions of A2 to read 2 Amps FS.

Or:

Adjust the meter face divisions of A2 to read 20 Amps FS.

Long ago they towed a car in for me to repair.
The positive alternator output wire was on the ground terminal.
Pieces of the ammeter were in the back seat!

No one mentioned that analog meters 'pulse with life' and are quite attractive when executed in brass, walnut and crystal...

And let's all of us make sure we use the proper prefixes, yes?


and...



I believe you fellows answered the OP's question nicely, plus or minus side issues.