Importance of a Resistance in this connection

Thread Starter

Kadav

Joined May 11, 2018
130
Hello i am building this connection and i want to know what is the R3 in this set up

I would Imagine it's a pull down resistor , but in simulation ,when its not there it works also , so what it is it's importance..

Thanks .....
1596451289295.png
 

Irving

Joined Jan 30, 2016
948
Discharges the gate capacitance which your simulation may not model or visualise fully (depending on timing) and ensures a cleaner switch-off.

Also, that's a really bad MOSFET to use, what is your load?
 

jpanhalt

Joined Jan 18, 2008
10,095
R3 is a "turn off" resistor. Its purpose is to ensure the gate is discharged to be sure the mosfet is turned off. Often 1K or larger.
 

ericgibbs

Joined Jan 29, 2010
10,487
hi K,
A MOSFET has Gate capacitance, when the opto is switched On this Cap charges and the MOS is turned On.
In order to turn Off, the Gate capacitance requires a discharge path.
E
 

AlbertHall

Joined Jun 4, 2014
10,243
It's more than just the capacitance in this circuit.
The MOSFET gate is very high resistance and even when U1 is off there will still be some leakage current and therefore there will still be voltage on the MOSFET gate.
 

Thread Starter

Kadav

Joined May 11, 2018
130
I have 3 questions regarding this
1) How do you know the MOSFET has a capacitor
2) why do we need to put the capacitor there( at the gate )
3) why do we need to discharge the capacitor
 

Delta prime

Joined Nov 15, 2019
430
Hello there. :)for kicks and giggles why is the mosfet there?
Optocoupler output stages are not designed for high load currents. Maximum current levels for Darlington outputs are around 150 mA, and 300 mA is typical for SCR and TRIAC outputs. When high load currents are to be switched, the optocoupler output stage is used as a trigger circuit for a high power device.
R3 is there for voltage to develop across the resistor .
 

crutschow

Joined Mar 14, 2008
25,267
R3 is there for voltage to develop across the resistor .
The voltage is there whether there is a resistor or not.
1) How do you know the MOSFET has a capacitor
All MOSFET gates look like a capacitor.
2) why do we need to put the capacitor there( at the gate )
We don't put it there, it's intrinsic to the MOSFET gate.
3) why do we need to discharge the capacitor
You must do that to change the gate voltage and turn the MOSFET off and on.
You can't change the voltage on a capacitor without charging or discharging it.
 

Thread Starter

Kadav

Joined May 11, 2018
130
The voltage is there whether there is a resistor or not.
All MOSFET gates look like a capacitor.
We don't put it there, it's intrinsic to the MOSFET gate.
You must do that to change the gate voltage and turn the MOSFET off and on.
You can't change the voltage on a capacitor without charging or discharging it.
@crutschow Makes sens thanks
 

Thread Starter

Kadav

Joined May 11, 2018
130
Hello there. :)for kicks and giggles why is the mosfet there?
Optocoupler output stages are not designed for high load currents. Maximum current levels for Darlington outputs are around 150 mA, and 300 mA is typical for SCR and TRIAC outputs. When high load currents are to be switched, the optocoupler output stage is used as a trigger circuit for a high power device.
R3 is there for voltage to develop across the resistor .
@Delta prime i would think that the opto coupler acts like a trigger for the MOSFET , but high currents don't actually pass through them, is n't it the case with my diagram ?
 

Audioguru again

Joined Oct 21, 2019
1,767
A power Mosfet has a high gate to source capacitance because it is made with thousands of tiny Mosfets in parallel and each tiny Mosfet has some capacitance that is multiplied by the thousands of them. Many tiny Mosfets are used so that their parallel on-resistance is very low.
 

Thread Starter

Kadav

Joined May 11, 2018
130
How about if i remove the R2 what happens ? in my experience the motor runs faster because there is no resistance at the collector of the optocoupler , which in turn increases voltage at the gate of the MOSFET .
1596468565199.png 1596468631478.png
 

Irving

Joined Jan 30, 2016
948
That's true, because you're only putting about 8v on the gate with the 220ohm there, but the full 12v without it.

But it also shows you are not turning the MOSFET on hard enough AND its a rubbish MOSFET. That device has an ON-resistance of 4 - 5 ohms... not milliohms, OHMS!

1596471245657.png

A good MOSFET, say an IRLZ44, will have an ON-resistance of 25milliohms

1596471415169.png

The difference is you are probably losing 1 - 2 volts across the MOSFET, depending on the size of your motor. And that MOSFET is going to get very hot if you ask your motor to do any serious work.

A very good MOSFET eg IPP048N04N will have an ON-resistance of ~4milliohms

1596472043369.png
 
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Thread Starter

Kadav

Joined May 11, 2018
130
That's true, because you're only putting about 8v on the gate with the 220ohm there, but the full 12v without it.

But it also shows you are not turning the MOSFET on hard enough AND its a rubbish MOSFET. That device has an ON-resistance of 4 - 5 ohms... not milliohms, OHMS!

View attachment 213912

A good MOSFET, say an IRLZ44, will have an ON-resistance of 25milliohms

View attachment 213913

The difference is you are probably losing 1 - 2 volts across the MOSFET, depending on the size of your motor. And that MOSFET is going to get very hot if you ask your motor to do any serious work.

A very good MOSFET eg IPP048N04N will have an ON-resistance of ~4milliohms

View attachment 213914
@Irving

OK Thanks very much , by the way like the IRLZ44N ,can we find it on actual electronics board normal appliances ?
since it takes me days to order these small items..
Thanks for the info it's very clear
 

Thread Starter

Kadav

Joined May 11, 2018
130
Hello there. :)for kicks and giggles why is the mosfet there?
Optocoupler output stages are not designed for high load currents. Maximum current levels for Darlington outputs are around 150 mA, and 300 mA is typical for SCR and TRIAC outputs. When high load currents are to be switched, the optocoupler output stage is used as a trigger circuit for a high power device.
R3 is there for voltage to develop across the resistor .

As @Delta prime asked me i know , have the simillar question because i have to plug in a 10V 2A and at the MOSFET , is it safe to use that opto-coupler ?
 
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