General MOSFET Question

theinfamousbob

Member

• Mar 4, 2007

• #1

I have a project I'm working on, and am going to be using MOSFETs as motor drivers. The gate input will be variable (up to 5v) and I was wondering If I'll need a source-gate resistor or something similar...the motor supply voltage will be between 9 and 18v.

Thanks for the help!

 

dknguyen

Well-Known Member

Most Helpful Member

• Mar 4, 2007

• #2

Gate-source resistor? Well you would never want a resistor in the source since that sticks in series with your high current load which is pointless. So I assume what you really meant was gate resistor? You don't need one. They are there to slow down the speed that the FET switches on at.

 

jrz126

Active Member

• Mar 4, 2007

• #3

Isn't the gate resistor used for current limiting?

 

dknguyen

Well-Known Member

Most Helpful Member

• Mar 4, 2007

• #4

If by current limiting you mean to limit the transient gate current when switching in order to slow down the switching time, then yeah. But it won't limit the source-drain current.

 

Gayan Soyza

Active Member

• Mar 4, 2007

• #5

**broken link removed**
replace the NTC with a Variable resistor for your choise.

 

H

hjames

New Member

• Mar 4, 2007

• #6

Series gate resistors dampen oscillations between multiple mosfets (apparently ferrite beads work too)
https://www.microsemi.com/micnotes/APT0402.pdf

A gate-source resistor would be useful for turning the mosfet off faster - but that is meaningless when dealing with CMOS gate driver chips. However, if you expect the gate connections can be accidentally disconnected/mangled, a gate-source resistor right next to the mosfet wouldn't be a bad idea.

 

Roff

Well-Known Member

• Mar 4, 2007

• #7

theinfamousbob said:

I have a project I'm working on, and am going to be using MOSFETs as motor drivers. The gate input will be variable (up to 5v) and I was wondering If I'll need a source-gate resistor or something similar...the motor supply voltage will be between 9 and 18v.

Thanks for the help!

Click to expand...

Variable gate voltage for a motor driver? Not a good idea, unless you have tiny motors. If you want to control the speed, use pulse width modulation (PWM).

 

Oznog

Active Member

• Mar 4, 2007

• #8

5V will not fully turn on the gate of many MOSFETs. Check your spec sheet for the Id current for Vgs=5v. Some turn on very strong at 5v.

Gates are sensitive to overvoltage. For example, putting them on the +12v line and that line is also supplying a motor, the motor may create spikes in excess of +20V or whatever the gate's max is. It could damage the gate.

 

K

kchriste

New Member

Forum Supporter

• Mar 4, 2007

• #9

5V will not fully turn on the gate of many MOSFETs.

Click to expand...

True. I'll just add that theinfamousbob can buy some logic level power MOSFETs that will work great with 5V gate to source voltages. The IRLZ44, is a logic level MOSFET suitable for driving motors.

 

theinfamousbob

Member

• Mar 5, 2007

• #10

Thanks for all the help...it was just an idea I had to save our maze robot for a competition this Saturday, which I have not been working on and thus isn't ready at all.


The plan was to mux left and right sensors through an op-amp, then run 2 mosfets off of the output to the motors.

Thanks for your answers!

 

Sceadwian

Banned

• Mar 5, 2007

• #11

If you're worried about frying the gate on your mosfet, use an appropriatly rated zener.