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I need a Amplifier to amplify the analog signal from ACS712

JackJTR

New Member
Greetings everyone, I wanted to ask something regarding the circuit that I am doing for my Final Year Project

So after setting up the circuit and the sensor, it was found out that the output generated by the ACS712 is 0.1mV (through Calculation). The reason why the output signal is so low is that because the current going through the circuit is low. Other than that, the microcontroller that I am using to receive the analog signal is Arduino Mega. As the resolution for Arduino mega ADC is10-bit. Thus the smallest value for the ADC is 4.88mV. Thus, I wanted to use an amplifier to amplify the Analog signal generated from the ACS712.

The circuit below shows the setup for the Amplifier (The amplifier used is LM741).

1621348001538.png

From the calculations using the formula (1+R2/R1), the gain is 1000. but when I run the simulation, the output voltage from pin 6 is 2.11V rather than the 0.1V (1000x0.1mV)? May I ask why is it like this, and is there any better way/Op-amp that I can use to amplify the 0.1mV input to a readable 0.1V? Or am I doing this entirely wrong?
 

rjenkinsgb

Well-Known Member
Most Helpful Member
I'd say something is fundamentally wrong.

The 741 is not a good choice, you would need a much better spec opamp.
And the signal level is just unreasonable; your current sense circuit needs rethinking if at all possible to give a more practical signal level out.

Can you give the full design and info on what you are measuring?

To clarify why an opamp such as the 741 is so bad, look at the specs on page 5 of the datasheet; it's input offset is typically 1mV and can be much higher - and the inputs have a relatively high bias current, so it's not suitable for circuits using high value resistors.
 

audioguru

Well-Known Member
Most Helpful Member
The 741 opamp is an antique that is 52 years old! It should be buried.
 

unclejed613

Well-Known Member
Most Helpful Member
looks like you were looking for a gain of 1000... try changing R1 to 100 ohms, and R2 to 100k. you have bias currents in your inputs, and using a 10k resistor is going to change that bias current to a substantial offset error... the high value of the resistor is also going to add noise to an already noisy op amp... i also recommend going to something like an LF353 or TL072 instead of a 741...
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Lots of things are "fishy" here. I thought most of the ACS sensors need +5V and usually 1/2 * Vcc is mid-scale.

Now that DOES NOT mean the output is 0 to 5V. Well it does, but not really. the output is usually ratiometric to the supply voltage,

We need a LOT more information.
 

JimB

Super Moderator
Most Helpful Member

audioguru

Well-Known Member
Most Helpful Member
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I got the first dose of the Pfizer Covid-19 vaccine 7 weeks ago and the 2nd dose will be soon.
I feel (and act) like I am 30 years younger.
So I am not ready to be buried, maybe in 25 years.:)
 

SPDCHK

Member
Question: What range of current do you want to measure? I would drop the ACS712.
I use a shunt resistor (0.01ohm) in series with my Solar panel to measure current drawn during charging on my MPPT charge controller.
I use an Inverting op-amp setup (mine is calibrated for 0-10Amp) but you can adjust the gain as required for your application.
Link: Inverting Op-amp calculator
Calculate the volt-drop expected over the shunt resistor and plug in the values into the calculator. I set my output voltage to be 4.5Volt (to allow for unforeseen over-current spikes). EDIT: I use the MCP602 Op-amp, connected to same 5V power supply as used for the Arduino
 

JackJTR

New Member
hi, thank you for every one response, after looking through again for the available current sensor on the market, I decided to use the adafruit INA219 current sensor, may I ask is that a good choice? Because I saw on google that the smallest measurable current value is 100uA.
1621431102183.png

Attach is one of the example circuits where I need to measure the current, voltage, and resistance of the resistor, and feed the result into the Arduino MEGA
AD0 and AD1 are used to measure the voltage across the R2&R3 (AD0) and R3(AD1). Then I will be able to calculate the voltage drop across each resistor.
1621431703381.png

SPDCHK. I would like to use a shunt to measure the current too, but the current flowing through is soo small that the Arduino MEGa can tell the difference between them. May i ask if i would like to do it using a shunt, what should i do / change ?


1621431905588.png

Sorry if the question looks dumb, but I would like to learn from my mistake and improve it!
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
You still really didn;t answer the questions unless this is "homework".

There is this https://www.eevblog.com/product/ucurrentgold/ device. Full design details are available.

and there are feedback ammeters or I-V converters which are mostly used for low currents. To measure low resisistances, you have to measure the voltage across and the current through the device. For say 1 ohm, 10 Meg-ohms in parallel with 1 ohms is negligible.

if your trying to measure the current through a 10M resistor with an ohmmeter with a 10Megohm input Z, then there are other issues.

I've set up systems to measure a few picoamps to 10 mA and designed analog I-V converters to measure +-100 mA and 3 decades lower with +-10V out.
 

schmitt trigger

Well-Known Member
Exactly
So we ask again;
What is your maximum and minimum current values which you wish to measure?
Do you plan on low side or high side sensing?
If high side, what is the maximum voltage.

Without concrete details, this thread will quickly devolve to noise. Seen this happen dozens of times, in this forum and others that I frequent.
 

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