I've a question about gate resistor selection as it pertains to driving Fairchild's FCP9N60N MOSFET from an Arduino digital out.
Datasheet: Intelligent Power and Sensing Technologies | onsemi
The MOSFET will be used to drive a solenoid with ~12 Ohm lookin resistance, rated for 12V (it's an automotive fuel injector).
a) Is a resistor required between transistor gate and Digital out? If so, why and what value?
What I've learned so far:
A resistor is a good idea in case the MOSFET fails. In that event current sink from the Arduino to ground will then be limited by the gate resistor. A gate resistor will however slow down T_turnon for the MOSFET, calculated as a simple RC circuit for the gate charge and V_turnon for the MOSFET. Are there other considerations?
b) What are the recommendations for isolating Arduino ground from Solenoid ground?
I understand when driving motors (noisy) optical or mechanical isolation is recommended but what is expected for a meager 1A 12V dc solenoid?
Is a resistor required between transistor gate and Digital out?
A FET gate looks like a small capacitor, and a discharged capacitor looks like a short circuit when you first apply voltage to it. So a resistor is to protect the output pin of the arduino from over current. Too big and it turns on slower, to small and it does not limit the current to below the maximum of 40mA.
Just a few tens of ohms should be enough but in practice for just a solenoidal anything up to 1K.
I can't find a reference now, but I recall reading the atmega can sink 40ma as a digital output. I expect that should be plenty to prevent the gate from floating and thus no need for the shunt.
That MOSFET you have selected is not a logic level MOSFET and requires +10vdc on the gate to fully saturate on. So a digital output HIGH may not turn on the MOSFET enough to supply full voltage to the solenoid.
A series resistor is as Mike specified. I usually also recommend a 10k or higher resistor from the gate to ground, Because when using switching MOSFETs it is usually a different voltage source then the Arduino's and if you ever had the case where the Arduino was powered off but the external voltage was still on the device could turn on by itself.
The external voltage source and the Arduino must share a common ground connection for the MOSFET to work. The only way around that, if you really wish to fully isolate the two systems is to activate the mosfet via a opto-isolator.
Alright, so I think I found an appropriate MOSFET: FQP4N20L
The inline and shunt resistors are now designed in, thanks again.
Re; isolation. I understand the Arduino and MOSFET ground need to be common. The question was whether it is necessary to isolate them given that the driven load is a low V dc solenoid.
Re; isolation. I understand the Arduino and MOSFET ground need to be common. The question was whether it is necessary to isolate them given that the driven load is a low V dc solenoid.
Only testing will tell. Inductive kickback from a solenoid can be a bear sometimes, but a suppresion diode across the solenoid's terminals helps a lot. Also extra filtering caps here and there also can tame it down some.
A lot depends on the frequency that you will be activating/deactivation the solenoid. If you have no problems then fine, if you do then goto: plan B and utilize an opto-isolator and remove the ground connection between the Arduino and the solenoid circuit.
Since last posting to this thread I've built and tested the circuit. I'd love to post a schematic, but all downloads have so far been nothing but sources of frustration (kicad, eagle, fritzing).
So until I go through the learning curve here it is in .txt format.
Connections:
Fuel Injector terminal 1 connected to 12Vdc source. Terminal 2 connected to MOSFET Drain.
MOSFET Source connected to ground.
TVS diode connected between MOSFET Drain and Source pins - Cathode connected to Drain
MOSFET gate connected to Arduino digital output via 220 Ohm series resistor.
MOSFET gate shunted to ground via 10k Ohm resistor.
Arduino ground and MOSFET Source are common.
Attach heat sink to MOSFET and electrically isolate it (the heat sink tab on the FQP4N20L is electrically connected to the Drain pin). About 1 Watt of heat dissipation is required for 100% duty cycle.
It was your "Motors 1" tutorial writeup that raised these questions for me in the first place just an hour ago.