the thing is that the motor speed is substantially lower when i run the code (analogWrite the pin 9 to 255), than what it is if i directly connect the motor to the battery. I don`t have a way to really measure the speed loss (nor the current drawn). Is this normal? is there something wrong with my circuit? Is the transistor bad for the job?
Btw, it is an arduino UNO fed via USB, so as far as I know, the output voltage on pin 9 is 5V when the analogWrite is set to 255.
Btw #2. What i have is a thrust measurement stand. I get a thrust of around 22g when the motor is driven straight from the battery, and it drops down to 14g when driven through the transistor with the analogWrite at 255 (same propeller, same test conditions). That should give a general idea of how much power I am losing.
That FET has a typical "on" resistance of 0.024ohm.
That and the wiring could reduce speed/torque somewhat.
Try to shorten source/drain while the motor is running on full.
That will tell you if the FET is the culprit.
I don't see a back-EMF diode across the motor.
That could already have damaged the FET.
Leo..
The FET it self has a flyback diode built in(or at least i hope it does). That is why I've chosen not to add one in parallel with the motor.
I tried to short it like you said and there is a speed increase of the motor, but it is very small, nothing like connecting it directly to the battery terminals.
If you're driving the motor via power from the arduino's +5 output when powered by the usb cable, you could well be overloading the arduino's regulator... does it get hot?
find a seperate high current supply to drive the motor.
allanhurst:
If you're driving the motor via power from the arduino's +5 output when powered by the usb cable, you could well be overloading the arduino's regulator... does it get hot?
find a seperate high current supply to drive the motor.
Allan
No. I am feeding the motor from the 3.7V battery. The arduino pin 9 is connected to the gate of the FET.
What i mean by driving the motor by the arduino is controlling it, not feeding it
I think the stall current of the motor is somewhere around 1.8A to 2A.
arolds:
The FET it self has a flyback diode built in(or at least i hope it does). That is why I've chosen not to add one in parallel with the motor.
I tried to short it like you said and there is a speed increase of the motor, but it is very small, nothing like connecting it directly to the battery terminals.
Back-EMF generates a more positive voltage across the FET.
The FET's inbuild diode only shorts negative voltages across the FET.
Then you must have volt-drop somewhere in the wiring.
Time to get out the DMM.
Leo..
756E6C:
The FET's body diode is NOT a flyback diode, flyback has to be in parallel with the motor (cathode toward +Ve), do you have a spare FET (and diode)?
I do. Will try to switch transistor and add a flyback diode in parallel with the motor to check if the problem was a damaged transistor.
Its only a 20V FET, and a very low 8V Vgs limit, it could easily be damaged by static when handling, MOSFETs
don't have protection diodes so can be much less robust that CMOS devices. It certainly should have
no problem handling the current for a small motor so I would conclude its fried.
Thanks for the response everyone. I followed Tom's advice and purchased some through hole FETs.
Also gonna add flyback diodes to the circuit. Hopefully it works this time around
arolds:
I tried to short it like you said and there is a speed increase of the motor, but it is very small, nothing like connecting it directly to the battery terminals.
If shorting the FET didn't help, then replacing the FET also won't help.
Follow the motor circuit: +batt>>wire>>motor>>wire>>FET>>wire>>-batt.
You tested the motor, the battery, and the FET.
What is left is three wires.
Too thin?, too long?, broken?
Get the DMM out.
Basic faultfinding. Don't replace anything until you're sure what's wrong.
If you start replacing things you're no sure of, you might introduce a second problem.
Leo..
Wawa:
If shorting the FET didn't help, then replacing the FET also won't help.
Follow the motor circuit: +batt>>wire>>motor>>wire>>FET>>wire>>-batt.
You tested the motor, the battery, and the FET.
What is left is three wires.
Too thin?, too long?, broken?
Get the DMM out.
Basic faultfinding. Don't replace anything until you're sure what's wrong.
If you start replacing things you're no sure of, you might introduce a second problem.
Leo..
Hi Leo. The wires I am using are 22 gauge (2.54mm diameter) 20cm long, with prototype board pins at the ends. None of them seem to be broken. I will test with a DMM as soon as i have the chance to get one.
2.54mm is more than enough for 2Amp.
Then all that is left is connections.
Measure voltage (when motor is running at full speed) across motor, across battery, across FET,
from +batt to +motor, from -motor to drain, and from source to -batt.
Post back.
Leo..
The wires I am using are 22 gauge (2.54mm diameter)
This does not sound right. 2.54 mm diameter is a hefty wire. AWG 22 is not. I suppose that "gauge" could refer to some other system but I suspect that the wire diameter is wrong.
What does "gauge" mean and what is the real wire diameter?
