Author Topic: Reading high currents on PCB: Hall effect sensor vs shunt resistor  (Read 5599 times)

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Offline Siwastaja

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Re: Reading high currents on PCB: Hall effect sensor vs shunt resistor
« Reply #25 on: May 07, 2020, 12:08:19 pm »
However i can remember reading some articles about current always taking the path of least resistance, so i can imagine that even the slightest imbalance might cause the current to flow trough the lowest shunt.

Just clear that bullshit from your head. Such "information" is indeed often seen, even by teachers, and is very damaging because it easily gets stuck in your head. Obviously, it is not true, as shown by simple analysis with the Kirchoff voltage and current laws.

Something similar to what you describe - all current going through one device due to even slight imbalane - does happen with semiconductor devices which have exponential current vs. voltage characteristics, and which may have the temperature coefficient the "wrong way", so that one device using more current causes it to start hogging even more current. But these devices are not measured using the unit of resistance.
 

Online ejeffrey

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Re: Reading high currents on PCB: Hall effect sensor vs shunt resistor
« Reply #26 on: May 07, 2020, 06:10:58 pm »
Thanks. I guess 500uohm could work, but i can imagine the output would be very sensitive! 0,1A would be 0,50uV. I hope a INA219 would be up to this. I guess its mV vs dissipation.

50 microvolt/ 100 mA.  The INA219 has a typical DC offset of 10 uV, so equivalent to 20 mA.  That's pretty good.  The worst case is 50-100 uV, it depends on your actual target offset.

Quote
Anyway, i'm having a little trouble imagining the physics of 50A running trough such a small SMD component. I guess i could take 2 of 1,5mOhm and put them parallel on both sides of the PCB, but that is still a very small pad its sitting on!
My mind says i need huge thick trough hole shunts to pass that amount of current....

Its just not that big a deal.  A lot of the rules of thumb for current density are based on long traces with very conservative temperature rises such as 10C.  For something a short trace just the length of a pad and if you are willing to tolerate a bigger temperature change you can push a lot more current than the conservative estimates suggest.

Using copper traces for the shunt as suggested above is also an option.  It is more complicated since you need to add temperature correction but cheaper since a temperature sensor is likely cheaper than a good current shunt resistor.
 

Offline engrguy42

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Re: Reading high currents on PCB: Hall effect sensor vs shunt resistor
« Reply #27 on: May 07, 2020, 07:37:54 pm »
However i can remember reading some articles about current always taking the path of least resistance, so i can imagine that even the slightest imbalance might cause the current to flow trough the lowest shunt.

Just clear that bullshit from your head. Such "information" is indeed often seen, even by teachers, and is very damaging because it easily gets stuck in your head. Obviously, it is not true, as shown by simple analysis with the Kirchoff voltage and current laws.

Something similar to what you describe - all current going through one device due to even slight imbalane - does happen with semiconductor devices which have exponential current vs. voltage characteristics, and which may have the temperature coefficient the "wrong way", so that one device using more current causes it to start hogging even more current. But these devices are not measured using the unit of resistance.

I'm cornfused....

A true calculation of resistance includes considering the temperature coefficient of resistance, doesn't it? And if it's negative like semiconductors, that will change the value of resistance. How is that BS?
- The best engineers know enough to realize they don't know nuthin'...
- Those who agree with you can do no wrong. Those who disagree can do no right.
- I'm always amazed at how many people "already knew that" after you explain it to them in detail...
 

Online ejeffrey

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Re: Reading high currents on PCB: Hall effect sensor vs shunt resistor
« Reply #28 on: May 07, 2020, 08:19:30 pm »
What he is referring to is the common misunderstanding by those new to electronics of the phrase "current follows the path of least resistance" to mean that when there are resistors in parallel the current only flows through the smaller one.  In reality we know the parallel combination of to resistors is smaller than either and the "least resistance" path is to flow through both.
 

Offline superKrisTopic starter

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Re: Reading high currents on PCB: Hall effect sensor vs shunt resistor
« Reply #29 on: May 07, 2020, 08:19:45 pm »
Thanks all, I have done a lot of reading based on all your tips.

I don't think using the traces for current measurement is a bad idea, but i woud need to do a lot of testing and redesign. I think it will be a little to tricky for me at this point, and i do have limited time.

So with everything i learned I think I think have the following option:

Hall effect sensors
I can stick with my ACS712/ACS758 sensors. However to get them stable i would need to redesign to a differential ACD setup, as i prefer not to use the Arduino's ACD's. A solution would be adding another ADS1015 and a voltage divider with either a potentiometer, or precision resistors or a software offset in the uC's code.

Current shunt
The INA219 is a great device. Extremely easy to use and seems reasonably stable. I would need 1 per channel as chips like the INA3221 require higher shunt voltages. A small advantage of this might be that I can place them very close to every shunt. A 0,0015mOhm shunt would be perfect. A single one will cover the 0-20A range nicely. A double one (On both sides of the PCB) will be perfect for the 0-50A range.

I made a table to compare the advantages and disadvantages. I hope i got the data right.



So looking at this, there seems to be some advantages to switching to shunts. I'm not sure how these translate into the real world

For now i wanna assume the 2oz traces ans the small SMD shunt are up to the task of transferring the 50A. The dissipation however will be a lot higher for the 50A shunts. I fear the PCB traces and components will already add more heat than i would prefer. Will the additional dissipation be a problem?

Also a 0-37mV signal is MUCH more sensitive for disruption than a 0,5-4,5V signal. How stable can i get the signal at a LSB size of 10uV? I gues i dont really need that level of precission, but 40uV/50mA would be nice to have. Do you guys this is possible, or will there be to much noise etc.

 

Offline Siwastaja

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Re: Reading high currents on PCB: Hall effect sensor vs shunt resistor
« Reply #30 on: May 07, 2020, 08:31:35 pm »
I'm cornfused....

A true calculation of resistance includes considering the temperature coefficient of resistance, doesn't it? And if it's negative like semiconductors, that will change the value of resistance. How is that BS?

If a resistor has such strong negative temp coeff that it causes practically all current go through one resistor with only minor imbalance in resistance, such part is crappy beyond imagination and could hardly be called a resistor; it would be a semiconductor, or maybe an NTC.

I was commenting about the phrase "current flows through the path of least resistance", which is just simply untrue and makes no sense whatsoever. Whoever came up with that wasn't thinking temperature coefficients or other sophisticated details; in fact, they just were not thinking at all, and then it stuck. Current is shared between the resistive paths in a simple linear fashion, I = U/R where U is the same over all paralleled resistors. So if you apply 1V over 0.9 ohm and 1.1 ohm resistors, the currents are 1.11A and 0.91A, respectively. The sad thing is, this is so easy to analyze properly that no one needs to invent such a "helpful" phrase.
« Last Edit: May 07, 2020, 08:34:43 pm by Siwastaja »
 

Offline engrguy42

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Re: Reading high currents on PCB: Hall effect sensor vs shunt resistor
« Reply #31 on: May 07, 2020, 08:43:32 pm »
What he is referring to is the common misunderstanding by those new to electronics of the phrase "current follows the path of least resistance" to mean that when there are resistors in parallel the current only flows through the smaller one.  In reality we know the parallel combination of to resistors is smaller than either and the "least resistance" path is to flow through both.

AAAAhhhhh, that's what he meant. I never considered that some people think that ALL current flows thru the smallest of two resistors in parallel. Thanks.
- The best engineers know enough to realize they don't know nuthin'...
- Those who agree with you can do no wrong. Those who disagree can do no right.
- I'm always amazed at how many people "already knew that" after you explain it to them in detail...
 


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