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James Stewart
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Current Measurement

10/17/2016 3:44 PM

I need to measure current accurately within a range of 1-16 Ampere on a system that will be using a 12 VDC supply capable of 19 A. I will have a 15 amp fuse (slow blow) in the fuse holder on DC output, because I am still uncertain of how warming up will affect current draw in this system, but I suspect it will increase.

For current limiting, I will try a network of large value 8 Ω resistors (4 in parallel), and this gets me to 6A maximum, if cell resistance is low compared to this.

I can put the resistor network on the down leg from the electrolysis cell with no issues, but I need to be able to measure voltage drop over 2 Ω to ground for input to Arduino board data logger. I also have a couple of Arduino Hall effect sensors, but I am not sure what they are for as yet (need to view the CD on that kit). They might be proximity sensors.

I understand a ferrite open segment with the conductor passing through the ring opening with a Hall effect is also useful for current measurements. Any recommendations there?

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Rixter
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#1

Re: Current measurement

10/17/2016 4:31 PM

Are your 8 ohm resistors capable of dissipating at least 18 watts each? (P = V2/R)
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James Stewart
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#7
In reply to #1

Re: Current measurement

10/18/2016 9:07 AM

They are 50 watt resistors as I recall.

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JRaef
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#2

Re: Current measurement

10/17/2016 6:46 PM

Shunt resistor? That's the simplest method.

http://www.rc-electronics-usa.com/current-shunt.html

After that, a Hall Effect Transducer is cool, but needs a little more hardware to facilitate it, unless you just buy a pre-made unit.

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James Stewart
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#11
In reply to #2

Re: Current measurement

10/18/2016 9:21 AM

I think the Hall sensors I have in hand are your basic robotics proximity switches, so I would not use them. Hall effect current sensors are great, but pricey compared to shunt measurement, so shunt it is.

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Winfield Hill
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#3

Re: Current Measurement

10/17/2016 9:30 PM

Most of us doing high-current DC measurements use very small current-sensing resistors, and use 4-termial connection techniques. For example, a 0.01-ohm resistor would create a 200mV drop at 20A, and dissipate 4 watts. We'd probably pick an even smaller resistor value, say 2.5m-ohms (0.0025 ohms) and 1W dissipation.

It's useful to place the resistor in the positive bus, but take the measurement at ground. I like Linear Technology's LTC6101 current-sense IC, which lets you translate the sense voltage down to ground. If you use G=40 with a 2.5m-ohm sense resistor, your full-scale 20A will create a 2.0V signal.

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dkwarner
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#4
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Re: Current Measurement

10/18/2016 12:08 AM

GA!

I haven't used that particular chip, so I could easily be mistaken, but aren't the +IN and -IN terminals reversed in your diagram? I'd expect the +In to be at the source side of the shunt resistor...

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JohnDG
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#6
In reply to #4

Re: Current Measurement

10/18/2016 6:32 AM

Think the output transistor will invert the difference amp output, giving a +ve going output for increasing IN- vs IN+.

The circuit as drawn above is as drawn in the LT datasheet.

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James Stewart
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#10
In reply to #4

Re: Current Measurement

10/18/2016 9:19 AM

No, that is the inverting input typically used in voltage follower configuration, and it provides a high degree of signal isolation.

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James Stewart
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#9
In reply to #3

Re: Current Measurement

10/18/2016 9:18 AM

Yes, I have seen that used, and it works well with op-amp circuit.

I have to interface to an Arduino analog input in this case, so I will be using a grounded shunt directly connected to analog input. It will be good enough for first pass on this system. Second pass, I might well incorporate what you provided.

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Mr. small
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#5

Re: Current Measurement

10/18/2016 2:14 AM

Rule of the thumb does not work here, either?

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James Stewart
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#12
In reply to #5

Re: Current Measurement

10/19/2016 9:18 AM

Confucius say " He who stick thumb to high current circuit, may draw back a nub."

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James Stewart
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#8

Re: Current Measurement

10/18/2016 9:14 AM

Thank you to the tenth power!Ω

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