# find the perfect values of RG and RGS of a transistor MOSFET

#### Alihamidi20

Joined Apr 29, 2021
6

Hello everyone.
i have a problem here!!!
i made this circuit to controle the Motor with the transistor MOSFET IRFZ44N.
the caracteristics of the MOTOR is (Imax = 200 mA, Vmax = 24 V ).
Vcc =24 V and the output of the 74LS02 is 5V and 0,4 mA maximum current.
my problem is that i couldn't calculate the resistances RG and RGS becaus i don't know how to .
i know them roles but i dont kdow how to calculate them based on Cdg and Cs
if anyone can help?????

Mod edit.
Is this homework?
Wendy.

Last edited by a moderator:

#### dl324

Joined Mar 30, 2015
12,871
my problem is that i couldn't calculate the resistances RG and RGS becaus i don't know how to .
There are no perfect values.

Is this school work?

#### Alihamidi20

Joined Apr 29, 2021
6
There are no perfect values.

Is this school work?i
i know there is no perfect values but how can i calculate them theoretically i mean based on the datasheet of the IRFZ44N

#### Wolframore

Joined Jan 21, 2019
2,273
Is this homework?

are you using PWM or is it simple on and off?

#### Alihamidi20

Joined Apr 29, 2021
6
Is this homework?

are you using PWM or is it simple on and off?
yes it's like the PWM but that's not what i'm asking for

#### Wolframore

Joined Jan 21, 2019
2,273

Please answer whether this is homework, we are not able to help otherwise

#### Papabravo

Joined Feb 24, 2006
16,103
Not only are there no perfect values, there is no unique solution as you have posed the problem. You have two unknows, you need two independent equations which need to be satisfied in order for a unique solution to exist.

#### dl324

Joined Mar 30, 2015
12,871
i know there is no perfect values
Then why did you ask for something that you know doesn't exist?

#### crutschow

Joined Mar 14, 2008
27,184
i know them roles but i dont kdow how to calculate them based on Cdg and Cs
What do you think the resistor roles are?
For the normal roles of those resistors there are no calculated values.

#### Alihamidi20

Joined Apr 29, 2021
6

Please answer whether this is homework, we are not able to help otherwise
you can consider like the 74ls02 has always 5 V in the output

#### Wolframore

Joined Jan 21, 2019
2,273
then it really doesn't matter if your system doesn't change and there are no speed requirements, calculate for efficiency and make sure it's not susceptible to ESD - false triggers.

Understand the voltage divider you've drawn. I would not use that MOSFET

#### Alihamidi20

Joined Apr 29, 2021
6
What do you think the resistor roles are?
For the normal roles of those resistors there are no calculated values.
but if i want to fix the current on RG i have to know the value of this resistor to fix that current under 0,4mA

#### Alihamidi20

Joined Apr 29, 2021
6
then it really doesn't matter if your system doesn't change and there are no speed requirements, calculate for efficiency and make sure it's not susceptible to ESD - false triggers.
but in my case i have to fix the current value demanded by the transistor so the 74ls02 can afford it

#### Papabravo

Joined Feb 24, 2006
16,103
A 74LS02 cannot source very much current and it certainly won't be able to source ANY current at +5 Volts. Read the datasheet and tell me ho much current it can source at what voltage and realize how impossible your goal actually is.

#### dl324

Joined Mar 30, 2015
12,871
For those of us who don't have the specs for IRFZ44 memorized, the threshold voltage is 2-4V.

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#### dl324

Joined Mar 30, 2015
12,871
Vcc =24 V and the output of the 74LS02 is 5V and 0,4 mA maximum current.
Why didn't you put the supply voltage on the schematic?

LSTTL outputs don't swing rail to rail:

#### BobTPH

Joined Jun 5, 2013
3,308
MOSFET gates draw current for only a short period at turn on.

They also need a higher voltage than your TTL gate can provide. The IRFZ44N need 10V to turn on fully. You need a logic level MOSFET, like the IRLZ44N.

Bob

#### BobTPH

Joined Jun 5, 2013
3,308
For those of us who don't have the specs for IRFZ44 memorized, the threshold voltage is 2-4V.
Which is irrelevant unless he is switching only 250 uA..

The threshold voltage is not the voltage at which the MOSFET is switched on.

Bob

#### Audioguru again

Joined Oct 21, 2019
3,173
you can consider like the 74ls02 has always 5 V in the output
Absolutely not! Obviously you did not see the datasheets.
The minimum output high of an old TTL 74LS02 is only 2.5V.
Some IRFZ44 Mosfets are barely turned on when the gate is 4V so are turned off with less voltage.
Use a IRLZ44 Mosfet instead. (look at its datasheet).

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#### dl324

Joined Mar 30, 2015
12,871
Which is irrelevant unless he is switching only 250 uA..

The threshold voltage is not the voltage at which the MOSFET is switched on.
It's very relevant. Driving the gate from LSTTL (VOH 2.7V worst case) isn't even guaranteed to start turning the MOSFET on (Vgs(th) 4V worst case).