Why am I getting the wrong current readings?

[Tbolt33]

Hello. I am building a simple circuit with two parallel resistors, where I must find the currents I, I1, and I2 using MyDAQ. I have provided a picture of the circuit as well as a photo of the actual circuit that I built. It is made up of a 3V DC power source and an R1 resistor of 10K ohms and an R2 resistor of 3.3K ohms.

The problem that I am having is that I keep getting a reading of 0 Amps or 2.31 mAmps. I started off by trying to find I. What I did was I selected the DC Current option in the DMM and clicked "auto" for the mode instead of a specified range. Then, I plugged the red needle wire into the right "HI" outlet on the MyDAQ, as instructed by the DMM. 

Then, I took the red needle wire and made it touch the left side of the white jumper wire while taking the black needle wire and making it touch the right side of the white jumper wire. After about 5 secs, I got a reading of 0 mAmps. So I decided to move onto current I1. I did the same procedure. I placed the red needle wire onto the positive end of the orange wire on the on the left side of the R1 resistor, and placed the black needle wire on the negative end of the orange wire on the right side of the R1 resistor. I got a reading of 2.31 mAmps. So, I moved onto current I2. Here I mad ethe red needle wire touch the left side (positive end) of the R2 resistor while taking the black needle wire and touching the right side (negative end) of the R2 resistor. I got a reading of 2.31 mAmps.

The reason why these readings are wrong is because the current here needs to add up correctly. That is, I = I1 + I2. However, 2.31 mAmps (I1) + 2.31 mAmps (I2) does not equal 0 (I). With that said, does anyone know what Im doing wrong?

[AGJ_CR]

Hi,

I think that you might be making an error with the connections. When you are measuring current you need to make it in series and not in parallel. Before following with your circuit, I would recommend you take a look at the next tutorial:

myDAQ tutorial: Measure current with a shunt resistor and DMM voltmeter

https://decibel.ni.com/content/docs/DOC-12946

Regards,

AGJ

[natasftw]

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@Tbolt33 wrote:
 

The reason why these readings are wrong is because the current here needs to add up correctly. That is, I = I1 + I2. However, 2.31 mAmps (I1) + 2.31 mAmps (I2) does not equal 0 (I). With that said, does anyone know what Im doing wrong?

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As you learn to work with circuits, you want to avoid this mentality.  While you're right that I1+I2 should be equal to I, this isn't a good way to check your values are correct.  You should calculate what values you expect to see in the circuit and then measure the practical values.  Doing this, you can check your measurements against a known value.

As someone else here mentioned, you're measuring the current incorrectly.  You need to understand how a DMM works to see what you're doing with your circuit.  There are two measurements you're going to work with: current and voltage.  Let's look at each of those.

Voltage is the difference between two points.  For this measurement, you want to have your leads touch the two different points and let the reading be that difference.  In this case, we want to measure in parallel.  The DMM acts as infinite resistance.  By doing this, all of the current passes through your original circuit and you can see the difference.

Current is constant throughout a branch of your circuit.  This means you need to make the measurement within the branch.  The easiest way to do that is to "break" the circuit and replace the break with your DMM.  This allows the current to flow through the DMM.  In current setting, the DMM acts as a short.  Here, you can read the current. 

Take that step back.  Find the Req of R1 and R2 in parallel.  With that value, you can find the expected I by using Ohm's Law.  To determine expected currents across each resistor, you can either use Ohm's Law and a voltage divider or you can just use the calculations for a current divider.  Once you have those three values, your measurements will be easier to verify.