# Mosfets and Resistors

Discussion in 'The Projects Forum' started by DannyKemp, Feb 2, 2015.

1. ### DannyKemp Thread Starter New Member

Jun 23, 2014
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Hello everyone, I am still fairly new to electronics but I am getting there. Now my biggest weak point is working out what components to use and why in a circuit. I want to build a simple Mosfet switch circuit using a N-Channel fet, How to work out the suitable resistors for the circuit attached below.

For example why do you need a resistor between the source and gate, and how do you work out the suitable one for this purposes?

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2. ### crutschow Expert

Mar 14, 2008
13,496
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The resistor is used to discharge the gate capacitance of the MOSFET to turn it off.
The size of the resistor is determined by the gate capacitance of the particular MOSFET you use and how fast you want it to turn off.

Note that the gate-source voltage (Vgs) to fully turn on the MOSFET as a switch is 10V for a typical MOSFET or 3V-5V for a logic-level type.
Do not confuse that voltage value with the Vgs threshold voltage which is much lower.

3. ### DannyKemp Thread Starter New Member

Jun 23, 2014
16
0
Oh I see thanks so much is there an equation to work out the best resistor for the job?

4. ### Dodgydave AAC Fanatic!

Jun 22, 2012
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770
try starting with a 10K gate and 1k gate/source.

5. ### DannyKemp Thread Starter New Member

Jun 23, 2014
16
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So would a 1k be ok for this mosfet: AUIRFS8409-7P ?

6. ### MrChips Moderator

Oct 2, 2009
12,635
3,453
Think what Dave meant is use a 10kΩ resistor between the MOSFET gate and source pins.
Then use a 1kΩ series resistor between the control signal and the MOSFET gate pin.

7. ### DannyKemp Thread Starter New Member

Jun 23, 2014
16
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oh I see I did think that was a bit high for the signal

8. ### MrChips Moderator

Oct 2, 2009
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Could you elaborate?

9. ### DannyKemp Thread Starter New Member

Jun 23, 2014
16
0
I ment using a 10k resistor for the gate.

10. ### MrChips Moderator

Oct 2, 2009
12,635
3,453
Why do you think 10kΩ is too high?
What would be the consequences of a high value resistor?

11. ### DannyKemp Thread Starter New Member

Jun 23, 2014
16
0
Well the circuits I've been looking at the gate resistor always seem to be a lot lower rated than the gate-source resistor. And I guess the value is all dependent on the mosfet rating, like I said I am fairly new to all this and one thing ive still got to get my head around is numerical values and reasoning in circuits. Which is why I was wondering if there's a basic calculation to work out the ideal resistor between the gate and source. I have included a link to the datasheet of the mosfet I was looking at if anyone could help explain to me the method for determining the correct resistor I would be very grateful.

Datasheet

12. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
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What voltage is the power source? 12V battery?

How many HP or what is the motor starting current? Motor running current? Stall current?

How often are you turning the motor on/off?

Is this a PWM application?

What is in the control box?

What voltage/waveform does the control box deliver to the gate of the FET? rise time? falltime? source current? sink current?

You see, the resistors come into play only when we know a lot more about what you are trying to do...

13. ### shortbus AAC Fanatic!

Sep 30, 2009
4,095
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As a second level newbie I don't understand this statement on the gate resistor. All the app notes say the gate resistor should be somewhere between 10 and 50 ohms not in the k ohm range. My understanding is the gate resistor is only needed to prevent gate 'ringing' at turn off. The higher the amps available at turn on is good, turns on the gate capacitance faster. Have I misinterpreted this? Many of the IRS mosfet data sheets show the resistor value in them for different voltage and current levels in the mosfets and they almost never go above 50 ohms for a gate resistor.

14. ### wayneh Expert

Sep 9, 2010
12,382
3,241
It's about speed. If you are using a MOSFET as a switch at 1Hz or below, there's no need for a massive gate current. A higher value gate resistor can help avoid surges elsewhere in your circuit. The MOSFET will dissipate heat during slow switching, but this is not a huge problem if the slow switching is a small percentage of the cycle.

But if you need high frequency switching, you need enough to gate current to ensure that the gate spends only a small fraction of the cycle time charging and discharging. You can calculate the switching time using the RC time constant for the gate resistor R and the gate's capacitance C.

A proper gate drive should source and sink the current needed to switch the gate. I think of the gate-source resistor as a failsafe only for when the gate control goes "open". It only has to work once, not each cycle, so it doesn't need a large current, just enough to shut off the MOSFET fast enough that it doesn't overheat from a slow switch.

15. ### crutschow Expert

Mar 14, 2008
13,496
3,373
How fast do you want the switch to operate?
How often will it be turned on and off?

16. ### ronv AAC Fanatic!

Nov 12, 2008
3,398
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First, your schematic doesn't look correct. The more positive voltage should go to the drain and the source to ground. Then you can put the load in the drain side.
The gate resistor and the driver determine the speed at which the FET will switch. For example if you have a gate driver capable of 1 amp and 12 ohm resistor in series with the gate on your FET and a 12 volt power supply driving it you can get 1 amp of gate current. If you then look at the data sheet you can find total gate charge - in your case 340NC. This number is the time (in nano seconds) it will take the FET to turn on and off with 1 amp of gate current. So in the case above the FET would switch in 340 ns. In real life it will be a little slower. So if you use 1k you would have 12 ma and it would take 28 usec. to switch. (340/.012) X ns.
The gate to source resistor keeps the FET from turning on with noise if it is not connected to the driver.
The gate resistor also lowers the Q of the inductance in the gate source driver loop to reduce ringing which can damage the FET or driver if it gets to big. I don't know how to calculate it I always just use what they have in the data sheet or larger. The ringing is usually only a problem if the gate pulse has fast rise and fall times or if the FET is a ways away from the driver. Always try to keep the line from drives to gate and source back to driver short.

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