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

Thread Starter

Alihamidi20

Joined Apr 29, 2021
6
Screen Shot 2021-05-05 at 7.56.50 PM.png

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:

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.
 

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
 

Thread Starter

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.
 

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
 

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).
 

Attachments

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).
 
Top