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.

 

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?

 

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

 

Is this homework?

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

 

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

 

Your answer matters, apparently you don't understand...

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

 

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.

 

i know there is no perfect values

Then why did you ask for something that you know doesn't exist?

 

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.

 

Your answer matters, apparently you don't understand...

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

 

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

 

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

 

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

 

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 how much current it can source at what voltage and realize how impossible your goal actually is.

 

For those of us who don't have the specs for IRFZ44 memorized, the threshold voltage is 2-4V.

 

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:

 

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

 

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

 

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).

 

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).