next topic »

[on:]

Hi all its been a while,
     I have a shunt resistor, http://www.ebay.co.uk/itm/Shunt-Resistor-DC-200A-75mV-Current-Meter-Ammeter-/320842522051?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item4ab3b45dc3#ht_3107wt_1139 , I wish to integrate it into my near complete project.
I only need 1A precision, I am reading 3-4 other analog pins at 0-5v and am not sure how to read it, any help would be appreciated.
cheers, Neo 

[Reply #1 on:]

I am reading 3-4 other analog pins

Your problem is that the arduino's analogue input pins are referenced to ground. Therefore the shunt must be in the ground path of the current you are measuring, this may or may not cause you trouble.
So one end to ground and the other end to the analogue input and the ground of what you are trying to measure.

But the voltage you will get off this is tiny.

I only need 1A precision,

There is no way you will get this without using an op amp to amplify the voltage.
Raw each step is 5/1024 = 0.0048828125 Volts that means for 200 A you will get a reading of only 15, that is 13A per step. The arduino's A/D is not that accurate anyway only +/- 2 least significant bits.

So what range of current are you interested in?

[Reply #2 on:]

i'm looking to utilize the full 200A range but as of now 0-100A, could i use 2n222a transistors to boost the voltage to a usable range? I understand about reading the low side, thats where it is  in the system right now, any idea which op amp i would need to use to get 0-5v from 0-75mv i have google'd but there's a bit to much info to let me pick something out

[Reply #3 on:]

5V/75mV means a gain of 66.
Simple opamp like OPA353 should do it.
http://www.ti.com/lit/ds/symlink/opa2353.pdf

Wire it up as a non-inverting amplifer.
http://www.electronics-tutorials.ws/opamp/opamp_3.html

[Reply #4 on:]

  okay some im on a scavenge for a suitable part, i'm looking at a couple of lm393n.
http://www.ti.com/lit/ds/symlink/lm393-n.pdf

 would i be able to use this it has a non inverting input, I will order a few when i get the chance, but just to proto this i have stacks of pcb from various electronics.

   cheers

[Reply #5 on:]

No, you really want an op-amp, not a comparator.

[Reply #6 on:]

If you're looking for 200A range with only 1A resolution, then you would be better off using a Hall current sensor such as http://uk.farnell.com/allegro-microsystems/acs758ecb-200b-pff-t/ic-current-sensor-200a-3cb/dp/1791391. This produces an output of 10mV per amp, enough to read directly from an analog input pin, and also provides isolation between the Arduino and the current being measured.

[Reply #7 on:]

lol i was just about to post that exact sensor and page, luckily i may be able to pick one up from my local farnell, 2 miles rather that pay 15 postage.

Great minds think alike

[Reply #8 on:]

i may be able to pick one up from my local farnell, 2 miles

You from Leeds then?

[Reply #9 on:]

no Northampton, they have a warehouse just down the road, whats the best place in your opinion for parts in England?

[Reply #10 on:]

I'm sure Mike will reply in due course; but I use Farnell, RS, Rapid Online and Proto-Pic. I generally find that Farnell has the widest range and is easiest to search, but occasionally I find that RS has better prices or can supply something that Farnell cannot. Rapid has a very limited range, but their prices for small quantities are often better than Farnell. Proto-Pic holds stock of some useful SparkFun components such as SMD breakout boards.

Just occasionally I want something that I can't find in the UK, and then I usually use Digikey.

[Reply #11 on:]

just found an alright deal from ebay, its a bit more expensive but alot easier than dealing with the guys at the warehouse,

http://www.ebay.co.uk/itm/150653948018?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649#ht_725wt_1163

Not quite as high rating as i wanted, but hey 200A was 100 over what i needed lol. I will eventually buy another three of the 100A units to monitor 3 phases of my bldc motor.

[Reply #12 on:]

Hi I've just received the sensor but the only access to the net is my phone, does anyone know what I shoud map the input values to get a 1A resolution, lol I need an ammeter.
Its the 150A version, just building the wiring harness if anyone can give me an idea what input voltage per A I should get would be awesome.
cheers, neo

[Reply #13 on:]

I will eventually buy another three of the 100A units to monitor 3 phases of my bldc motor.

You do realize that making a current measurements from a BLDC motor winding is different then making a simple DC current reading as the motor windings are being driven with high current PWM pulses, so somewhere in either hardware after the hall effect sensor or in software you will have to 'filter' the measurement value over time to get a average current value for a averaged period of time.

 

[Reply #14 on:]

With Vcc = 5v the output at pin 3 has a sensitivity of 13.3 mV per amp. At zero current the offset of the output from the ideal value of 0.5 Vcc may be up to +/- 5mv at 25C.