Hello everyone,

I know this is a very basic question but Is there a difference (when it comes to the place of the resistor R) between circuit 1 and 2 ?

because I dont understand why why would put the resistor connected to the ground !

Thanks alot

 

To ensure the transistor turns off, prevents false triggering pulses.

 

Circuit 1 is used to ensure the FET is turned off if there is no input signal, like from a microcontroller port that has nor been initialized.
And it is fairly normal to include a small series resistor too.

 

but why the Resistor in the circuit 2 would not prevent the false triggering pulses ?
what is the purpose of connecting the resistor to ground here

To ensure the transistor turns off, prevents false triggering pulses.

 

so in the Circuit 2 , the resistor would not be put off even if we have a resistors on series ? bcz from my understanding , both they will consume current the same why , my problem is that i dont see the difference between the two circuits, and why it is better to use the resistor connected to ground
Thanks alot

Circuit 1 is used to ensure the FET is turned off if there is no input signal, like from a microcontroller port that has nor been initialized.
And it is fairly normal to include a small series resistor too.

 

so in the Circuit 2 , the resistor would not be put off even if we have a resistors on series ? bcz from my understanding , both they will consume current the same why , my problem is that i dont see the difference between the two circuits, and why it is better to use the resistor connected to ground
Thanks alot

The resistor in circuit 2 is only there to keep the gate from "ringing", Google gate resistor for more information. The resistor to ground is the only way to completely assure the gate won't turn on until a higher voltage is present, not just a "spurious" voltage, that can partially turn the mosfet on.

 

Because a FET's gate requires essentially zero direct current, if it is left open circuit it can "float" to a voltage high enough to turn the FET on, at least partially. Even the extremely low conductance over the surface of a printed circuit board can supply enough current to make the FET turn on.

The resistor to ground, or more generally between the gate and the source, is needed in circumstances where the thing that is driving the FET's gate may be high-impedance when the power for the FET's load is present. For example, if you are driving the circuit with an output pin of a microcontroller, this is a very common situation. Configurable I/O pins on micros are often set to be high-impedance (essentially open-circuit) inputs when the micro is reset. The firmware then configures the pin to be an output, but it may delayed by tens to hundreds of milliseconds after the power is applied. Once it is configured as a normal output that can both source or sink current ("totem pole"), the resistor serves no useful purpose. You might also have a situation in switch mode power supply where the controller that operates the FET takes a fraction of a second to come up to operating voltage after the power is applied to the FET, just because of the way the start-up circuit is designed. If the FET is allowed to turn on in that time it can be catastrophic because the current in the FET could reach destructive level in just microseconds. Again, the resistor to ground keeps the FET turned off until the control circuit is up and ready to go. Motor controllers have similar issues - you don't want the motor to start running until everything is ready to control it properly.

For high-power FETs that are being switched very fast, the series resistor is normally used to limit the current that is required to charge the FETs input capacitance. It does also help prevent ringing due to connection inductance, but that is often of secondary concern. With many gate drivers no external resistor is used in normal circumstances. If multiple FETs are driven in parallel, then a series resistor for each is essential to prevent unwanted interaction, including ringing, between the FETs. Usually the resistance is just a few ohms for those cases. Sometimes a series resistor is used to deliberately slow the switching transition of the FET in order to reduce radio frequency interference or help manage voltage spikes due to "stray" inductance in the circuit.

 

The FET is a voltage controlled device. The resistor doesn't resist voltage it resists current. So having even a 1 meg ohm resistor in series still won't block stray signals from reaching the gate of the FET.

I took a MOSFET and connected a very long wire to the gate. Using it in the resistance region I could put my hand near the wire (not touching it) and use the FET to increase the voltage to a small motor. Grabbing a hold on something grounded I could put my hand near the long wire (again, not touching it) and lower the voltage to the motor. FET's don't require current to control them they require voltage. So the series resistor protects the FET, as someone said already - the FET from ringing. If you don't understand that term you can google it too.

The resistor to ground pulls stray voltages to ground and does not allow the FET to turn on until you apply a deliberate control voltage to it. Because there's voltage and current then there is a voltage drop across the resistor, putting the signal voltage firmly on the gate and controlling the FET.

Hope this helps you understand a little better.

 

Small point!. I would not use term "ground" is this sense.I would prefer to call it "common". But thats just me.

 

Circuit 2 can work if the input labeled 5V is never floating.

As you've drawn the circuits, the MOSFET and resistors are extraneous and the MOSFET can be replaced with a wire.

 

Small point!. I would not use term "ground" is this sense.I would prefer to call it "common". But thats just me.

+1 for me.
The term Ground is used in N.A. for Earth GND as well as any circuit Common.
Using 'common' differentiates it from Earth version and avoids any need for extra clarification.
Max.

 

Circuit #1. Compensate the parasitic leakage current. It needs a place to go. It could be a uA or less for a FET. If the gate were left open, the leakage current could turn it on. It can also be used to help turn off the FET faster.

Circuit #2: Generally limits the gate current. Values of 200 ohms are common. There is gate capacitance too and the 200 ohm resistor isolates that.

Both circuits are normally combined.

OP amps because they are not ideal, also need places where Ib (The input bias current can drop across) Situations can arise where you have pA of current trying to drop across a few milliohms. That;s a no-no.