# Shunt Current divider



## Duncan (Dec 8, 2008)

Hi

You use that shunt to measure current

You put it in series with your main feed from the batteries - and you measure the voltage across the shunt

That one is labeled - 500 amps at 75 milliVolts 

So when you are drawing 500 amps there will be 75mV across the shunt
You connect that to a gauge where 75mV reads - 500Amps


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## brian_ (Feb 7, 2017)

Guzel said:


> Hy I was surfing for practical current divider applications where I found this piece of metal:
> 
> 
> 
> ...


The problem is that the label is incorrect: it is a high-current low-resistance resistor (used as Duncan explained for current measurement); it is not a current divider of any kind.


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## steveob (Nov 10, 2017)

well, it is a current divider, 5x100 amps, but then it joins them all back up again, so it is not the best term to describe the application, don't know who thought it needed misleading buzzwords.


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## brian_ (Feb 7, 2017)

steveob said:


> well, it is a current divider, 5x100 amps, but then it joins them all back up again...



True!

The parallel current paths are pointless for any purpose other than area for heat dissipation (presumably why it is built this way), but the current will divide about equally between the five bars.

My guess is that there was some awkward translation along the way to the French version.


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## Guzel (May 21, 2017)

brian_ said:


> True!
> 
> The parallel current paths are pointless for any purpose other than area for heat dissipation (presumably why it is built this way), but the current will divide about equally between the five bars.


Yeah, I too think the same way, provided all bars have same resistance.


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## MattsAwesomeStuff (Aug 10, 2017)

Not just heat dissipation, but, consider why it's not just a round bar?

You are trying to very accurately measure the infinitesimal voltage drop across a huge conductor, with an infinitesimal known, precise, calibrated amount of resistance, blindly.

That is, you build the shunt to a spec, or you measure it afterwards, and expect the resistance to be that precise amount. The calculated voltage drop across that very precise resistance standard is used to extrapolate and tell you what it thinks the current should be according to those parameters. But it's blind. It never measures the resistance, it's calibrated for what it's expected to be.

Resistance varies hugely with temperature. For extreme example, when a lightbulb starts up, cold, it has 1/16th the resistance and therefor draws 16x as much current as when it's hot (and bright).

If you calibrated a lightbulb as your shunt, and measured it when cold, you would be reading, for example.... 16 amps when there's only 1 amp flowing (a 120W bulb).

In the case of the car, your gauge might show you're drawing 1600 amps when you're only drawing 100.

Now obviously the shunt isn't getting glowing-white-hot, that's an extreme example, but a few degrees difference on an already-so-small-it's-hard-to-measure voltage drop will make your gauge lie to you. Maybe not 1600%, but, how unsatisfied would you be with even 10% or 25% inaccuracy?

That's why shunts are always thin, wide bands, or multiples thereof. Lots of surface area relative to their cross section.

It's not because you're worried about heat build up or inefficiency, it's because it's a highly calibrated measurement device that needs to be as consistent as possible.


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## Guzel (May 21, 2017)

Duncan said:


> Hi
> So when you are drawing 500 amps there will be 75mV across the shunt
> You connect that to a gauge where 75mV reads - 500Amps


I roughly tried making the designs. I think it will be easier to hook up the wires as in red connections voltage will remain same at both points though.


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## Duncan (Dec 8, 2008)

The big honking connectors for the 500 amps go onto the big screws

The voltage connection for the meter goes onto the two screws


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## steveob (Nov 10, 2017)

fwiw, those bars *should* be made of manganin, which has a very neutral temperature coefficient, i.e. keep it below 80C and it will be within 0.02%


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## MattsAwesomeStuff (Aug 10, 2017)

> fwiw, those bars *should* be made of manganin


Neat, never knew that.


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## brian_ (Feb 7, 2017)

brian_ said:


> The parallel current paths are pointless for any purpose other than area for heat dissipation (presumably why it is built this way), but the current will divide about equally between the five bars.





Guzel said:


> Yeah, I too think the same way, provided all bars have same resistance.


The problem as an educational exercise is that there is no way to independently measure the current in each bar, or even to accurately measure the voltage drop across each bar... so it seems pointless to me for the intended purpose.

Of course it's well-designed as a current-measuring shunt resistor. 



Duncan said:


> The big honking connectors for the 500 amps go onto the big screws
> 
> The voltage connection for the meter goes onto the two screws


Yes, and not because the voltages between pairs of screws will be meaningfully different, but only for ease and independence of connections.


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## brian_ (Feb 7, 2017)

steveob said:


> fwiw, those bars *should* be made of manganin, which has a very neutral temperature coefficient, i.e. keep it below 80C and it will be within 0.02%


Thanks Steve, I had never looked up what alloy exists for near-zero temperature coefficient; I only remembered Nichrome for high resistance (for heating elements) and PTC rubber for high positive temperature coefficient (for self-regulating resistance heating). Good to know.


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## Duncan (Dec 8, 2008)

The current will NOT be the same in each of those "blades"

If you look closely you will see small sawcuts in one of the blades

These are used to "tune" the assembly to the correct resistance for its function as a shunt


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## steveob (Nov 10, 2017)

Meh, voltage dividers don't always divide evenly either, but these are gonna be pretty close to each other (probably a bad idea not to). They probably just put these cheap ones in a band saw sideways to tune. You can see the notches go through each bar.


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## rbzig (Oct 15, 2015)

Not intending to hijack the thread but. What effect does voltage have on a shunt and is there a practical voltage limit? Also, what are the pros and cons of a hall effects voltage sensor?


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## brian_ (Feb 7, 2017)

rbzig said:


> Also, what are the pros and cons of a hall effects voltage sensor?


A Hall effect sensor does not detect voltage - it detects magnetic field. It can be used to measure current by measuring the magnetic field resulting from current flow in a conductor. Since this type of sensor responds to the magnetic field resulting from the current, and not to the voltage driving the current, a current measuring device based on the Hall effect does not need to have any resistance (beyond that inherent in the required conductor), so it can dissipate less power as heat due to resistance. I assume that Hall effect current sensor cost is higher (due to materials used and electronic complexity) and accuracy is lower, compared to a simple resistance shunt, but I haven't checked prices or specs to confirm that.


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## frank harvey (Jun 4, 2008)

Yours is a precision resistor meant to be mounted _in_series_ with the main power line between batteries and motor. "In series" means your diagram is inappropriate as it shows a "parallel" layout.

I'm not sure where the "shunt" name came from. Your original diagram matches a shunt setup but that's not what this precision resistor is used for.

It does not use Hall Effect (or there would be other wire leads exiting from the part, wires that gave a meter value representing the Hall Effect output).

The power wires attach to the 2 large screws and the meter's wires attach to those as well. Nothing should be wired to the parallel plates (the red in one of your diagrams).

The resistor itself will "drop" a small amount of voltage for your vehicle's current and it'll do that drop wherever it's placed in the whole (batteryGnd-motor-batteryPlus) circuit. You probably want to mount the part to the 
ground side of the circuit as that voltage also goes to the gauge and you probably don't want any part of your gauge sitting at battery+ (it's just messy to work with. You constantly have to remember it can shock you).

I've used such a resistor on a homemade electric truck.


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## brian_ (Feb 7, 2017)

frank harvey said:


> Yours is a precision resistor meant to be mounted _in_series_ with the main power line between batteries and motor. "In series" means your diagram is inappropriate as it shows a "parallel" layout.
> 
> I'm not sure where the "shunt" name came from.


The term "shunt" suggests a parallel connection, in which the shunt provides an alternate path for current, and that is in fact why this is called a shunt. The shunt provides a low-resistance path in parallel with a meter; the meter is a voltmeter (a high-impedance device, ideally of infinite resistance but we know that is not possible).

This may make more sense if you keep in mind that originally all meters were current meters, typically with a needle attached to moving coil in a magnetic field and restrained by a spring, so that the current in the coil provides a torque and the balance between electromagnetic torque and spring force determines the reading. A traditional analog ammeter has very low resistance (to minimize the voltage drop across the meter and thus minimize effect on current in the measured circuit), while the corresponding voltmeter is the same design but with a very high resistance (to minimize the current through the meter and thus again minimize effect on the measured circuit). The shunt for current measurement shunts current past a voltmeter, and thus allows a small (volt)meter to measure the current rather than using a huge (am)meter.


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## brian_ (Feb 7, 2017)

frank harvey said:


> The power wires attach to the 2 large screws and the meter's wires attach to those as well.


That doesn't make sense to me. The small screws are there for a reason (when used as intended as a current measurement shunt), as Duncan said:


Duncan said:


> The big honking connectors for the 500 amps go onto the big screws
> 
> The voltage connection for the meter goes onto the two screws


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