Hi all,

I've got a MOSFET configured to run in full saturation (i.e. digital switch mode) to be controlled with an arduino pin output. The MOSFET controls a Solenoid of 1A at 12v, so I have to run it off its own supply. When this circuit was in breadboard stage, everything was operational, but when I transferred it to the PCB the solenoid doesn't turn off - it stays high. No shorts, even tested the MOSFET without the base pin by breaking the connection to the arduino. Nothing. It seems to be bypassing the MOSFET entirely and staying high - I'm really quite confused why... Here is the circuit diagram









To be clear, the 180 ohm resistor is broken that should short the solenoid with the ground, so that isn't causing the fault. Also, I'm using an IRF530 MOSFET not the one mentioned on the PCB (yes, I have matched the pins with wires instead). Can you guys spot any errors which I may have overlooked? Thanks so much.

 

I can't follow your layout. Start by checking the MOSFET is not in backwards.

 

I've got a MOSFET configured to run in full saturation (i.e. digital switch mode)

You mean triode. It's unfortunate that the powers that be decided to use saturation to mean what we call active mode for BJTs.

Here is the circuit diagram

Your schematic is difficult to read because you don't follow the normal flow.

Measure the voltages on the MOSFET and post.

 

Your schematic shows the inverting and non-inverting inputs of IC3 shorted.
With a supply of 5V the maximum output voltage from IC3 could be as low as 3V. Is that enough to switch the micro input reliably in the possible presence of noise?
Q3 collector and emitter are shorted.
There is no hysteresis for IC3 so the relay will probably chatter.
If pin 24 of the micro is an input it will be floating when SW2 opens.
Are any of the unconnected pins of the micro floating inputs?

 

Figured it out - the TO220 package is screwed down onto the PCB, and as luck would have it the 7805 is also a TO220 package... Which meant that the negative rail was also the copper at the bottom on the outside, causing the solenoid to bypass the control of the Arduino entirely...

 

You can get hardware intended to mount those devices but leave them electrically isolated.