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simple motor driver

Hi, I am trying to drive a motor using a MOSFET(N-Channel). In my circuit I have connected an indicator for me to know if my mosfet is working. And it really does, but whenever I connect my dc motor, my indicator turns off. I've tried connecting my motor directly to a battery alone, and it works fine. My battery is 1.2V; 1900 mA. Therefore, it is safe to assume that my motor works within that condition.

Resistor values: --- 270 ohms (indicator / LED) --- 1M ohms

Question: Why won't the motor run? Is it because it lacks current?

PS: The overall circuit uses a 5V as a source.

Can anyone help me. I really don't know what's wrong.

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  • \$\begingroup\$ Is it really a 1.2v battery you are using? \$\endgroup\$ – HandyHowie Oct 19 '15 at 15:57
  • \$\begingroup\$ what gate drive voltage are you using? if 1.2v, that might be enough to light the indicator, but not enough to run the motor \$\endgroup\$ – Neil_UK Oct 19 '15 at 16:00
  • \$\begingroup\$ Yes, just for testing the motor. For the circuit i am using a 5V source; 1A. \$\endgroup\$ – random Oct 19 '15 at 16:01
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    \$\begingroup\$ @random The datasheet specifies the Vgs Threshold to be between 2V-4V. If your use case happens to have a threshold of ~4V, then the FET may not be able to turn on fully with your 3.17V. You should be using a logic level FET for this. Typical FETs like the one you are using are designed for ~10-15V on the gate. You could also buffer the gate signal with another FET and pull-up resistor to the 5V source. This would switch between GND and 5V. This would always meet the Vgs threshold value of 4V. \$\endgroup\$ – MadHatter Oct 19 '15 at 16:16
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    \$\begingroup\$ You need to be absolutely clear about the voltage you are driving the MOSFET with. At the moment you seem to be saying 5V, 3.17V and there's another mention of 1.2 volts. Be clear. Also visible LEDs won't work at 1.2V so here's another anomaly in what you are saying. \$\endgroup\$ – Andy aka Oct 19 '15 at 17:37
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Hmmmm. A gate voltage of 3.17V might not be enough to close the drain-to-source channel. Best option might be to use a buffer cause you should be getting an output closer to 5V. 3.17V might be due to your supply can't source large current. I won't suspect much that is because of the FET because it has high impedance. Try removing the pull-down resistor (1 Meg) and measure again the gate voltage.. It should lessen the current drawn.

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this is Karl. If you are using the battery to supply the motor and a power adaptor for the logic circuits, be sure to check if the signal and power grounds are connected. This FET should ideally work fine but if it loads the logic gate when connected to the gate (cause the logic gate is probably TTL), then use a spare op-amp in your LM358 and configure it as a buffer to your gate. At Vgs = 4V, it can sink 10 Amps per se. 5V from the logic circuit won't saturate it yet but should allow the required current to run your motor. A logic level just operates at cut-off and saturation, which this IRFZ44 is capable of. It can operate linearly also but since you only give it 5V and 0V, then it becomes logic-controlled.

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