Current - Time measurement tool

Thread Starter

nerdegutta

Joined Dec 15, 2009
2,684
The circuit is breadboarded, and I've run the motor a few times. I got some readings, but I think something is strange. It reads a constant 12v when the motor is not running. When the motor is running, it reads near 0v. Is that correct, or have I done some mistakes on the way?

I also burned off one of the legs on the "10x1R" resistors. I twisted and soldered 3x3 resistors to 1, so it would be 10 in total. After the test, I noticed the pin hole was a bit burned. In the attached picture, the green lines indicates the 10 resistors, and the red arrow is pointing at the pin hole I had one side inserted.

I will make some more test later today.
TCbreadboard1.JPG
 

Reloadron

Joined Jan 15, 2015
7,501
I see the problem. The amp circuit is basically fine. The pinout is what I missed with how the battery/supply, motor and shunt (1ohm x 10) are configured. I'll ge another drawing done this morning. As drawn you full supply was across the shunt which should be in series with the motor.

Ron
 

Reloadron

Joined Jan 15, 2015
7,501
Here is what you want. Motor voltage measured across the motor terminals and motor current amplified across the shunt. The shunt is in series with the motor and the shunt is the ten 1.0 ohm resistors in parallel.

Shunt Amplifier.png

In time we may want to place a 100K pot in place of R3 allowing for adjustable gain of the amplifier (calibration).

Ron
 

Thread Starter

nerdegutta

Joined Dec 15, 2009
2,684
Thanks for your time and effort!

To you mean a POT between pin 1 and 2, or pin 3 and the shunt?

I'll breadboard it again. :)
 

Reloadron

Joined Jan 15, 2015
7,501
No, in the circuit as drawn R1 & R3 determine the gain of the amplifier. So things work a little like this. The shunt consist of ten one ohm resistors for a total of 0.1 Ohms. The shunt is in series with the motor so the motor current passes through the shunt. This motor current produces a subsequent voltage drop across the shunt. Lets say the motor is drawing 200 mA (0.2 Amp). So we get .2 x .1 = 0.02 volts or 20 mV. The problem is that 20 mV is small and hard to measure with the DAQ so we amplify it. To get 20 mV to for example 2 volts would require a gain of 100. R1 and R3 should be giving the operational amplifier a gain of 100. However, since things aren't perfect we could replace R3 with a small 10 turn 100K pot and place a 1K resistor in series with it. It would look like this:

LM158 Amp.png

The added pot allows us to adjust the circuit for the Gain of 100 we want. However, lets just get things working as is first.

Ron
 

Thread Starter

nerdegutta

Joined Dec 15, 2009
2,684
I'm putting this on hold for awhile. The motor have a small gearbox, and some of the teeth on the gear which is connected to the motor shaft, are gone. I've checked some other gearboxes, and they have the same problem.

I'll get back on this when I get some new gearboxes.

Thanks for the help nad guidance!
 
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