signal decoupled but power coupled

Hi,
I wanted to decouple the signal of the Arduino from the motor part of the circuit.
However, to spare the extra power source I plan to power the Arduino with the power supply of the motor.
When you look at the schematic you see a red and a blue line going to the Arduino terminals.
With these I decoupled the ‘trigger part’ to drive the MOSFET (IRF3205) but a have a galvanic connection to power the Arduino. Does this make any sense ?
Regards, AgeBee

The schematic looks fine to me except you can't power the Arduino directly from 12v, that would have to go through the Arduino's barrel jack (which then goes to the onboard voltage converter).

Connecting the 12V directly to the 5v Vin of the Arduino will fry your Arduino immediately.

You will need to ensure that the motor doesn't introduce noise on the power lines - decouple as necessary before supplying the Arduino jack which can only deal with a certain level of power line noise.

Also, why have you got both a 1K and a 680R resistor like that? The 680R seems very low for a pull-down resistor. Have you checked this bit works like that? I've found that MOSFETs want a high driving voltage to work efficiently (that is, with a low 'on' resistance between drain and source). Test it and see if it works OK.

That circuit assumes a logic level MOSFET, the IRF3205 is not logic level so it won't work.

Change the 680 ohm resistor to 4k7 and it will work.

Standard MOSFETs want 10V to 12V of gate drive, logic-level work with only 4.5V

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MarkT:
Standard MOSFETs want 10V to 12V of gate drive, logic-level work with only 4.5V

Where “work” means “switch fully on with the lowest possible resistance”.

I wanted to decouple the signal of the Arduino from the motor part of the circuit.

This is a good idea.

However, to spare the extra power source I plan to power the Arduino with the power supply of the motor.

Not a good idea.

When you look at the schematic you see a red and a blue line going to the Arduino terminals.

This is why it's not a god idea ... these two power rail wires provide a conductive path for EMI interference.

With these I decoupled the 'trigger part' to drive the MOSFET (IRF3205) but a have a galvanic connection to power the Arduino. Does this make any sense ?

Isolation of the power rails is most important ... This can only be acheived with a separate power supply and removing the red and blue wires.

Thanks all for your replies.

I build up the circuit as described in my schematic. However, it did not work because it seems that I got a trigger from wherever which causes the motor to dither. I tried it with the 4.7 k as suggested by MarkT. However, I still got the same result.

When I separated the power from Arduino galvanically from the power of the motor it all worked fine. So it is like dlloyd said: "Isolation of the power rails is most important".

You could start by decoupling the 12V supply with 100 to 1000uF.

Decoupling in electronics doesn't mean isolate (which is what your first post said). Decoupling a supply means coupling the supply rail to ground with a capacitor to short out noise and load surges.

This means the disturbance from one load has less effect on other parts of the circuit (hence why this is called decoupling)

So try some large capacitance across the 12V supply - use electrolytic capacitor rated at 16V or more and connected the right way round.

MarkT's suggestion would smooth the source of the problem (the inductive motor load) and likely allow use of your original circuit, in which case the extra supply and opto-isolator isn't required. Which ever method you choose, I think the capacitor should be part of the solution.

The location of the capacitor is important. In this case, it needs to be directly at the Arduino terminals.

Also critical is the configuration of the power wires, and particularly the grounds. Input devices to the Arduino must have their grounds connected only to the Arduino, and all wires to particular components must be bundled closely together with no alternate connection pathways beside that bundle.