Why is there a voltage drop when I connect my motor to Mosfet controlled by PWM signals?

Thread Starter

Adarsh Shanmugam

Joined Jul 27, 2018
9
So that means the MOSFET is working fine.
The problem appears to be a voltage drop somewhere from the battery to the MOSFET.
Measure the voltage drop between the battery and the motor, and between the motor and the MOSFET drain.

What wire size are you using for the connections?
Hi. How do I measure the voltage between the battery and the motor? I kinda don't understand that part. However, the voltage between the motor's positive pole and the MOSFET drain measures 2.79V. The voltage between the motor's positive and motor's negative pole is 2.71V (Reduced from previous post's reading 2.77V because of the battery discharge from running the tests I guess). The voltage between the motor's positive pole and the battery's negative measures 3.05V. (If that's what you meant by voltage between the battery and the motor). I'm testing these connections in a breadboard with a couple of jumper wires. The jumper wires have a resistance of approx 0.6 - 0.7 ohm. Could the resistance of my breadboard and jumper wires affect the voltage drop? Thanks!!!:)
 

crutschow

Joined Mar 14, 2008
28,507
How do I measure the voltage between the battery and the motor?
You measure between the positive battery pole and the positive motor terminal.
Also you measure between the motor negative terminal and the MOSFET drain terminal.
All measurements with 5V on the MOSFET gate-source.
The jumper wires have a resistance of approx 0.6 - 0.7 ohm. Could the resistance of my breadboard and jumper wires affect the voltage drop?
Of course.
Do you know Ohm's law , V=I*R?
What would be the voltage drop for the 3A motor current through the 0.6-0.7 ohm resistance?
 

Thread Starter

Adarsh Shanmugam

Joined Jul 27, 2018
9
You measure between the positive battery pole and the positive motor terminal.
Also you measure between the motor negative terminal and the MOSFET drain terminal.
All measurements with 5V on the MOSFET gate-source.
Of course.
Do you know Ohm's law , V=I*R?
What would be the voltage drop for the 3A motor current through the 0.6-0.7 ohm resistance?
Hey man! I did as you said.
Results:
Positive battery pole and the positive motor terminal: - 0.95V (My positive probe of DVM attached to the positive pole of the motor)
Motor negative terminal and the MOSFET drain terminal: 0.4 V(My positive probe of DVM attached to the negative pole of the motor)
The wires I'm using have a diameter of roughly 0.5 mm. (22 AWG I guess)
 

danadak

Joined Mar 10, 2018
4,057
22 AWG Resistance is 16.14 ohms/1000 feet, or .0164 ohms / foot.
Thats with a perfect connection at each end to source/load, solid
core. Copper of course. If wire is copper coated steel whole other matter.

Regards, Dana.
 

crutschow

Joined Mar 14, 2008
28,507
Positive battery pole and the positive motor terminal: - 0.95V (My positive probe of DVM attached to the positive pole of the motor)
Motor negative terminal and the MOSFET drain terminal: 0.4 V(My positive probe of DVM attached to the negative pole of the motor)
Based upon that you are losing about about 1.35V total in your wiring and contact resistances.
Suggest you use some copper wire (22 AWG or larger) directly from the battery positive to the motor positive, from the motor negative to the MOSFET drain, and from the MOSFET source back to the battery negative.
Do not go through the breadboard push-in contacts for these connections.
 

Thread Starter

Adarsh Shanmugam

Joined Jul 27, 2018
9
Based upon that you are losing about about 1.35V total in your wiring and contact resistances.
Suggest you use some copper wire (22 AWG or larger) directly from the battery positive to the motor positive, from the motor negative to the MOSFET drain, and from the MOSFET source back to the battery negative.
Do not go through the breadboard push-in contacts for these connections.
Hey man! It works!!! I soldered the components directly and totally eliminated the breadboard. Looks like the problem was with the breadboard's resistance. The voltage drop across the motor now measures 3.58V. Thanks a lot!!!:)
 

Thread Starter

Adarsh Shanmugam

Joined Jul 27, 2018
9
22 AWG Resistance is 16.14 ohms/1000 feet, or .0164 ohms / foot.
Thats with a perfect connection at each end to source/load, solid
core. Copper of course. If wire is copper coated steel whole other matter.

Regards, Dana.
Hey man! The wire I'm using is a single stranded one made of steel I guess (not copper for sure). Anyways, I removed the breadboard and soldered the components directly and it works. The voltage across the motor measures 3.58V. Thanks a lot!!!:)
 
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