Need a bit of help with MOSFETS. Am a bit flustered.

[Darksunrise957]

Hello,

I am planning on building a small project which will require some hardware this time. I need a circuit that can control several different components (motors, solenoids, a power inverter...) via Arduino. Looking around, it appears MOSFETS would be ideal over relays, being that (I believe) it requires less power, allows for pwm, is smaller, and switches significantly faster. The problem is, I have almost no experience with circuitry, each and every troubleshooting case I have found seems to be rather specific and complex, and the selection of components is absolutely insane, especially since I do not even know if IGBTs, NPN transistors, etc. are close enough to work, or are completely different.

I have found this page, the circuit on the right appears to be pretty much exactly what I want, if it works: https://bildr.org/2012/03/rfp30n06le-arduino/
I just don't have a clue as to what components to use in said circuit.
I will be using one 6v power source, and one 12v. (Batteries)
I am planning on finding one working circuit for each voltage, and replicating them for each of the components needed.

I thank you for your help in advance,
Dark

 

[dougy83]

The MOSFET they list (RFP30N06LE) is suitable to be driven from the Arduino, and is suitable for driving 30A loads on supplies up to 60V. You will need a heatsink if your load draws more than about 4A (this will cause the case to be 85°C, assuming 25°C ambient). Don't forget the diode for inductive loads (e.g. coils, motors), and the resistor from gate to GND to keep the MOSFET off by default.

 

[Darksunrise957]

Thank you!
Two things though: The pull down resistor would be the 10k Ohms as marked in the diagram, regardless of the MOSFET?
And the diode to prevent the backlash (paraphrasing, I forget the technical term...) after the gate stops recieving current; does it need to be any specific specification?

 

[Darksunrise957]

Oh, and since this will be in parallel, do I connect the grounds of both the 6v and 12v circuits to the arduino?

 

[dougy83]

Two things though: The pull down resistor would be the 10k Ohms as marked in the diagram, regardless of the MOSFET?
And the diode to prevent the backlash (paraphrasing, I forget the technical term...) after the gate stops recieving current; does it need to be any specific specification?

10k is fine for the pulldown; the value is not that important (2k - 50k would be fine). The diode is called a free-wheeling diode and prevents the back-EMF caused by the collapsing magnetic field in the coil. The diode peak current should be rated to be able to handle the peak current of the coil, and if you're using PWM, then the diode average current rating somewhere around the coil current.

Oh, and since this will be in parallel, do I connect the grounds of both the 6v and 12v circuits to the arduino?

Yep, all grounds common. You can use a star-grounding configuration if your loads pass high current.

 

[Darksunrise957]

Ah! Thank you VERY much!
Now, I am a heck of a lot more sure of what to get, and how to do this, thanks to you.
Have a nice day/evening/night/limbo, wherever you are!

 

[crutschow]

Note that there is an error in that Bldr.blog description of the circuit. He states the drain of the transistor goes to ground but it is the source (as shown in the picture) that goes to ground (negative) with the drain going to the load.

 

[Darksunrise957]

Yeah, that I figured out, and the diagram appears to be correct despite the typo. It would be illogical to connect the drain to ground, I would believe. Thank you for your advice, anyway. If I had not caught that, it could have caused a bit of a mess.

 

[crutschow]

Yeah, that I figured out, and the diagram appears to be correct despite the typo. It would be illogical to connect the drain to ground, I would believe. Thank you for your advice, anyway. If I had not caught that, it could have caused a bit of a mess.

If you did reverse the source and drain, the substrate diode would conduct and the load would be on all the time, independent of the gate voltage. It's unlikely though, that anything would be damaged.

About the only time a drain would be connected to ground is if you were using a P-MOSFET as a source-follower where the source would be connected to V+ through a resistor.

 

[Darksunrise957]

Hence how I figured that it would not make much sense to do. XD I'm SLOWLY picking some knowledge up. I will get the hang of making circuits yet!

 

[4pyros]

I am planning on finding one working circuit for each voltage, and replicating them for each of the components needed.

You could use much smaller MOSFETs for your smaller loads and lower voltage.
Assuming nothing heavy is going on the 6 volt supply you could use the same circuit with a smaller MOSFET.

 

[Darksunrise957]

Well, I would prefer to be better safe than sorry, not to mention that I may possibly need even more than the 6v and 12v I have planned. I will try out the circuit with the MOSFETs that are mentioned above, first. Also, I believe a one-size-fits-all MOSFET would be useful here, to make it easier to duplicate the circuit. I will see, though, if I have to change anything later, but thanks for the tip, nonetheless. I will keep it in consideration.