irf840

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uaefame

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Hello everyone,

I am trying to use a pic 16F84 to send high and low signal.
I check the pic signal high and low and it work fine.
I have a transistor irf840 i want it to act as a switch for the motor.
i want to drive the motor using pic signal.

Note: 24 is an external power supply.

For example high signal motor runs. low signal motor stationary.

That all i want it to do.

I examine the circuit nothing happen anyone have a clue what wrong!!

Thanks in advance
 

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Hi Hesham,

many things to check:

* Who design this circuit for you?

* in your circuit NPN while IRF840 is N-Channel MOSFET ( datasheet attached )

* post the code for more details

* It good practice to use FreeWheeling diode across any inductive load.

* Is this circuit to switch the motor ON / OFF?

........................................
 

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Last edited:
Although the picture below is a relay (you can use a motor, solenoid etc) You'll have to adjust the parts for your 24V motor but the basic layout would be the same.
**broken link removed**
 
hi uaefame,

The transistor is a n Channel MOSFET, it requires a Gate voltage of about +3V to operate.

Remove that LED, its clamping the Gate drive voltage to +2V.

Remember: a power diode across the motor..
 
Last edited:
The transistor is a n Channel MOSFET, it requires a Gate voltage of about +3V to operate.

Remove that LED, its clamping the Gate drive voltage to +2V.

Remember: a power diode across the motor..
Click to expand...

Hi ericgibbs, I tried it i think there is another problem rather than this. Even when the microcontroller power is off that mean no signal is send the motor keep rotating. As if i am feeding it directly.

I am confused
 
uaefame said:
Hi ericgibbs, I tried it i think there is another problem rather than this. Even when the microcontroller power is off that mean no signal is send the motor keep rotating. As if i am feeding it directly.
Click to expand...

If you haven't got a diode across the motor then you've probably already blown the FET?.
 
uaefame said:
Hi ericgibbs, I tried it i think there is another problem rather than this. Even when the microcontroller power is off that mean no signal is send the motor keep rotating. As if i am feeding it directly.

I am confused
Click to expand...

hi,
Its important that you do not have that LED on the Gate of the FET.
Look at the datasheet it says the Gate voltage must be between +2V and +4V to switch ON the FET.

Did you run the motor without a protection diode across the motor.?
If you tried switching the FET/Motor, you may have blown the FET with the back emf from the motor.

Do you have the IRF FET connected correctly, Source,Gate,Drain.?

How much current does the motor require to run.?

For a test disconnect the resistor from the PIC pin to the Gate and connect the end of the resistor to 0V... the motor should stop if the FET is OK..
EDIT: that is, resistor from the Gate to 0V

Tell us what you see.
 
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ericgibbs said:
Look at the datasheet it says the Gate voltage must be between +2V and +4V to switch ON the FET.
Click to expand...

No.
2V to 4V is the threshold voltage where it barely conducts only 250uA.
It and most Mosfets turn on completely with 10V on the gate which cannot be provided by a PIC.
A Logic-level Mosfet like an IRF3711Z turns on pretty well when its gate is only 4.5V. Its threshold voltage is from 1.55V to 2.45V.
It conducts much much better than the high voltage IRF840.

The output voltage of a PIC drops when it supplies current to an LED so have another output drive the LED.
 
In addition to the previous advice, keep in mind that a MOSFET is very easily damaged by ESD, (static electricity) Just picking a mosfet up w/o being properly grounded, will often cause the mosfet to fail.

I do alot of ESD testing at my work; most people have a substantial amount of static electricity built up on themselves at any given time. The threashold detection level of an ESD discharge is between 2000-4000 Volts for most people. Any discharge of static electricity, less than that, can't be normally felt. I think an ESD charge of between 50 and 200 volts can fry most mosfets.
 
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