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Current Sensor

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gilardino

New Member
Hi, i really need help on my project. I want to lift 3 different weights of the objects. So I think of current sensor to differentiate the objects based on the current drawn when the object is lifted. For example, 50g object will drawn more current to lift the object compared to 30g object. So, how to implement this? And what type of current sensor to be used?

Thanks. Appreciate any help...
 

Sceadwian

Banned
The most common way to measure current is with a shunt resistor. This is simply a resistor of a known value usually very low (less than an ohm) to avoid heating or reducing power to the current be drawn. If the current you're measuring is small you can sometimes use resistors of a higher value. You measure the voltage differential across the resistor usually with an opamp calibrated to give you whatever scale you need. What you do with this voltage is entirely up to you, you could feed it into an ADC and let a micro controller do something with the information or design further analog electronics to produce the results you're looking for.

I'm going to assume you're trying to measure the current through a motor? More information about the circuit you'll be trying to add this to would be helpful. After you get the current measurement setup working you then put a known weight on the device for calibration.
 
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gilardino

New Member
Thanks for the reply. Appreciate it so much. Yes, i want to measure current through the motor. So heavier load (object) will drawn more current from battery to run the motor.

My question, does shunt resistor connected series to the motor? Does it need any other component? And the use of opamp to amplify the voltage across the shunt resistor to get the scale of voltage i want?
Yes, i plan to use build in ADC on the PIC to read the voltage value.
 

Sceadwian

Banned
Yes, it connects in series, nope no other components, that's it. Since it's from a motor you're gonna get a lot of noise you'll probably want to pass it through an opamp acting as an integrator, as well as an amplification stage, you might be able to integrate and amplify using single opamp stage but I'm not good with the analog stuff, just about any opamp cookbook should have schematics for all the integrator/filters/amplifier you need. It will take a bit of tuneing to get the desired result.
 
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dknguyen

Well-Known Member
Most Helpful Member
Give this a read:
http://www.electro-tech-online.com/custompdfs/2009/03/00894a.pdf

About current shunts specifically...the resistor can be on two sides of the motor- closest to ground (Simplest and you can use an op-amp but suffers can certain problems...mainly because it places a resistance in the ground connection it can cause extra noise) and closest to +V (requires an amplifier with special characteristics).
 

gilardino

New Member
i see. Thanks for both replies. And what do you mean by special characteristics? Can i just use non-inverting opamp to get the desired value (by changing resistance's value)?

My purpose is to differentiate 3 different weight of the objects and fed it to PIC. So i thought of current sensor on how much the current drawn. Apart from this idea, do you have any idea to differentiate the 3 different weights of objects?

Many thanks.
 

dknguyen

Well-Known Member
Most Helpful Member
THe special characteristics are that is has to be tolerate of high voltage since it experiences the voltage between motor +V and ground, and a high common mode input range since it is measuring a small voltage difference very close to +V.

From the sounds of your app, it sounds like low-side will be fine (which is the method where the resistor is connected closest to ground so the shunt resistor voltage is ground-referenced and easy to measure so you just use a simple op-amp as an amplifier).

Noise of placing this resistor in the ground plane is more of a problem for motors (which current sensing is usually for) that have high frequency switching. Only you know how your weight sensors work. If the frequencies are very low you will be fine with low-side sensing.
 
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MikeMl

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Most Helpful Member
Thanks for the reply. Appreciate it so much. Yes, i want to measure current through the motor. So heavier load (object) will drawn more current from battery to run the motor.

My question, does shunt resistor connected series to the motor? Does it need any other component? And the use of opamp to amplify the voltage across the shunt resistor to get the scale of voltage i want?
Yes, i plan to use build in ADC on the PIC to read the voltage value.
Use a ZXCT1009 high side current monitor to produce a ground-referenced voltage that is proportional to the current in the shunt resistor. By picking the shunt resistance, and the scaling resistor, you can keep the voltage within the PIC's ADC range. Been here, done this...
 

Sceadwian

Banned
I noticed those are SOT23 or SM8 packages, those aren't exactly easy to work with =-) But if gilardino thinks he can solder it it'd work nicely. There's an IC for every application nowdays.
 
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OutToLunch

New Member
i would suspect that in the coming years, hobbyists will have to start getting used to surface mount components if they want to use parts that aren't more than a few years old. Through hole in many cases is going the way of the dinosaur and everyone is trying to cram more features in a smaller package. Hopefully, most common parts will still be in packages that are at least somewhat friendly to the hobbyist - SOT23, SOIC, etc are cool but QFN, BGA, etc (at least with a soldering iron) can be quite the pain...
 

Monkey

New Member
I quote OutToLunch 100%
It's not only due to my poor eyes that building stuffs got so complicated,
getting new components for your project is sometimes impossible.

Even SMD is getting obsolete, and how do we mount FPGAs in BGA format?
 

gilardino

New Member
Thanks for the replies. I asked for other solution because i have search some semiconductor companies in my area through online. And they do not have shunt resistor.
I also got try to find load cell and force sensor. And they cost expensive.

Thanks for the help by you all. If i can find it, sure i will give it a try.
 

Sceadwian

Banned
Monkey, all you need is solder paste laid out from a mask (which you can print and cut out yourself) and a toaster oven =) I've never done it myself but reflowing a board for BGA or surface mount isn't out of reach of the hobbyist. The hard part is the PCB's themselves. But you can get one off multilayer PCB's from at least a few shops now. There's really nothing holding back a hobbyist from making devices as complex as modern PC hardware.
 

Monkey

New Member
Hi gilardino (are you a football fan?),
shunt resistors are normal resistors mounted as shunt.
It's normally a low value resistor in series to what you want to control.

If you want to try something, be sure that the device (in your case the motor) doesn't create a lot of overvoltage that kill the electronic, then use a 1 ohm resistor in series, and an opamp where the inputs are connected to the resistor, do choose a reasonable feedback resistor for the opamp and take some measurements.

Enjoy electronic!
 

ccurtis

Well-Known Member
Another option is to pass the wire supplying current to the motor through a torroid with a gap in it. Locate a Hall sensor inside the gap. The output of the Hall sensor is amplifed and scaled as required. Some calibrated DC current probes are made this way.

Here is another single chip solution, including sense resistor, using the Hall principle. http://www.electro-tech-online.com/custompdfs/2009/03/0712.pdf
 
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gilardino

New Member
Yes, i agree with sceadwian, surface mount component can be easily soldered on multilayer PCB.

monkey, yeah, im a football fan. but not exactly gilardino fan.
erm regarding the current sensor. I think alot on it. For running the motor, i have built the 2 relays as switches to run a motor. It has 2 conditions. So wherever i put the shunt resistor (series to motor) closer to +V or to Gnd, when the swithcing is performed, the position of shunt resistor will be reversed.

Are you agree if i put shunt resistor in the negative side of the battery?
 

Sceadwian

Banned
Negative side of battery would measure current of the entire system, not just the motor.
 

power58

Member
shunt

My favorite Current sense resistor is a .1 ohm 1% Dale RH-25 25 Watt metal case resistor. The Voltage drop across the resistor is scaled so that you don't have to do any calcs. 10a = 1 volt . Just mentaly move the decimal on
your meter at lower currents.
 

simon400

New Member
Problem when connecting opamp circuit to amplify the voltage across Shunt resistor

Hi Guys,
I have to control a motor bidirectionally and measure the current through it.
I am using the connection as shown here

The voltage Vo is applied as input signal to opamp in noninverting amplifier mode.(i.e one terminal to pin noniverting while the other is connected to common ground of the opamp) The problem arises when the motor is operated in the reverse direction as the common ground of the opamp ( connected to shunt resistor) is connected to the high voltage side of the motor driver. I noticed that the motor is uncontrollable at this stage and runs at full speed.

COuld someone suggest an alternative method to measure the current bidirectionally using a shunt resistor itself?
Thanks
 

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MikeMl

Well-Known Member
Most Helpful Member
Either use an a high Common-mode rejection "instrumentation amplifier", or build one yourself using a conventional op-amp by matching four resistors to 0.1%.
 
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