I have looked everywhere online and I cannot find a clear answer.
I have a 50mV current shunt and I am running 50V though it.
I am wondering if the voltage limits on the ADS1115 in differntial mode are refrenced to ground or refrenced to the other differential pin.
E.G
pin0 50v
pin1 50.05v
The differential between the pins is only 0.05V does that mean it wont fry?
Everywhere except a datasheet?
The two ADS input pins must both be within the common mode range of the ADS1115.
Usually not easy with a directly connected shunt (must not go below ground).
Could be easier to switch to a chip that is designed for this, like the INA226.
Leo..
Wawa:
The two ADS input pins must both be within the common mode range of the ADS1115.
What exactly does that mean?
My brief read of the datasheet suggests to me that in differential mode it works the same as the Atmega chips (e.g. 2560) that have a differential ADC mode. In other words differential inputs that are not referenced to the chip's GND can be measured provided the differential is within the limits acceptable to the chip. For example it could (perhaps) measure the difference between 300.00v and 300.01v
The OP may be interested in the fact that the Atmega chips with differential ADC also have amplifiers to help with small voltages.
...R
Robin2:
For example it could (perhaps) measure the difference between 300.00v and 300.01v
Wishful thinking.
Second line of the specs, all analogue inputs must be between GND and VDD.
That could already be a problem if you connect one side of a 50mV shunt to ground.
-50mV with bi-directional currents is outside the specifications.
Leo..
Wawa:
Wishful thinking.
Second line of the specs, all analogue inputs must be between GND and VDD.
If you were to connect a 300v wire to it which was not part of the ADC ground circuit how would it know? It's perfectly safe (electrically) to swing from a single 10kV cable provided you are well clear of the ground. Birds do it all the time.
...R
Yes, floating measurements could work if the device OR the Arduino setup is not connected to any reference point, like ground.
But what if your 50mV shunt is 'floating' 300mV below ground of the ADS.
Possible, because that's below the voltage of the input claming diodes of the ADS.
Would it still work? Your guess is as good as mine.
Leo..
Wawa:
Yes, floating measurements could work if the device OR the Arduino setup is not connected to any reference point, like ground.
But what if your 50mV shunt is 'floating' 300mV below ground of the ADS.
In my simple world if it is floating it won't matter if it is 300mV or 300kV. However if I am over-simplifying I would be happy to have a proper explanation.
...R
Thanks for all the help, I had no idea that Power/Current Monitor chips existed. Ill look into getting something built for this application.
Cheers
This board can measure an uni-directional current that's 50volt above ground.
There are more chips in that INA family.
Leo..
I am looking to measure 0-500A at 0-50V using a 1000A 50mV shunt.
I chose such a large shunt because the datasheet mentions for continious operation the shunt must be 2x the size required. This effectivly takes the differential output range from 0-50mV to 0-25mV.
I had origionaly planned to use the INA169 IC but the datasheet shows terrible accuracy measureing below 25 mV. Is there an alternitive IC or method for getting good results from this shunt?
I should also add this is DC voltage.
There is a lot of important information missing like is it isolated from the mains? Are you measuring low or high side current? and many more. You have a tough problem that is not going to be easy to solve. Your Arduino A/D will yield a very low count at 25mV (it can be improved or an external A/D). There are ways to amplify the 25 mV signal but it is not a simple task. Keithley Instruments publishes a booklet called: Low Level Measurements Handbook, I would suggest you read all 239 pages then re examine your problem. This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!
Gil
Any chance of measuring the voltage drop across all of the wiring from source to destination instead of just the shunt?
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zoomkat:
Any chance of measuring the voltage drop across all of the wiring from source to destination instead of just the shunt?
The point of using a shunt is that you know its resistance and you can calculate the current by measuring the voltage across it.
To the OP ...
AFAIK you can detect the voltage across the shunt with any of the Atmega chips that has a differential ADC capability. Just make sure that the Arduino GND is isolated from the circuit feeding the shunt.
Nowadays you can get current-sensing chips so maybe there is no need for a shunt. However I don't know how accurate they are.
...R
To update I think I will change to a 500A 50mV Shunt anduse this IC I found to solve my problem.
The ZXCT1084 current monitor can handle up to 80V and the datasheet seems to suggest the output voltage is liniar between 0-50mV. The only issue is the error seems to be values under about 10mV have too large an error to be useable, I can live with that.
I would then plug this IC into my ADS1115 for more accurate readings.
Could someone please just sanity check this choice before I order them?
Unfortunitly the project is using an ESP based MCU and requres the wifi and bluetooth functionality aswell as being far into development of other features. I didnt realise there were current sense chips/CT's that could detect DC voltage. I will also research them further
You didn't tell us yet what you're measuring.
Is it uni-directional DC?
Can't you have the shunt in the ground line?
That would eliminate the high-side to low-side conversion chip and the error.
Leo..
Sorry, I am measuring a welder so there isnt an easy ground line for a single welder. It is uni-directional DC