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I got a problem with my Raspberry Pi setup.

I'm currently using an arduino to control an IRL520N to switch GND on and off on the rasperry and that works great, or it did...

My setup also includes an LCD screen with VGA input. The screen is powered with 12v. The PSU is an laptop brick, 20v and from there I've used two buck converters to bring it down to 5v and 12v.

The 5v goes to the arduino and Raspberry and the 12v to the screen.

I was using an powered hdmi to vga adapter for the screen and it worked for a couple of weeks until it started to get really hot and broke. So I replaced it with another one and thats when the problem started.

So now when I'm trying to turn the raspberry off (switching the GND of with the mosfet) it's still on. The red light is glowing.

I've traced it all down to the vga adapter and it seems like the ground from the adapter is making a full circuit so the raspberry stays on.

And now I dont really know what the best aproach would be to fix this problem. I would rather not have to reprogram the arduino but maybe I could fit another mosfet so switch off the +5 aswell? Or if I can isolate the 12v gnd somehow.

Any suggestions would be appriciated!

Thanks in advance

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    \$\begingroup\$ Switching ground on a device is almost always the wrong thing to do. \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 23 '17 at 17:17
  • \$\begingroup\$ Yeah, now I've learned that the hard way. This is my first big project thou so I'm still learning. I got some TIP120's that I was thinking I should use when I first started the project but the electronic store I usually buy my components at recommended the IRL520N MOSFET I'm using \$\endgroup\$ – user1728582 Aug 23 '17 at 17:21
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    \$\begingroup\$ Oh, I'd definitely use a MOSFET over a BJT for this, but I'd do it high-side. \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 23 '17 at 17:22
  • \$\begingroup\$ Do you have an suggestion to what MOSFET I could use? \$\endgroup\$ – user1728582 Aug 23 '17 at 17:27
  • \$\begingroup\$ @user1728582 - you might look at USB power switch chips as they are a little simpler to apply. Intended usage is for enabling power to downstream USB devices, but they come rated for a range of currents including well beyond the USB spec, and usually have a fairly wide voltage range. \$\endgroup\$ – Chris Stratton Jan 14 '18 at 19:05
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Your description is vague to say the least.

However, turning off parts of a system can be a real chore, whether that be high side or low side. IO pins connected from the unpowered part to the powered part can act as power connections with unfortunate and often unpredictable results.

A rather more intelligent power reduction system is usually required. You would likely be better putting the pi to sleep rather than unplugging it though. In fact these difficulties are why modern devices have such modes in the first place.

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  • \$\begingroup\$ I've tried to describe it as good as I can. :P PSU -> two buck converters 5v & 12v. 5v going to an arduino that when a switch is on turns on an mosfet that then turns on gnd to the raspberry pi. When switch is off it turns of the gnd to the raspberry. The 12v goes to an LCD that connects to the raspberry with an HDMI to VGA adapter. Thats where the problem lies I think. So maybe I should turn off the 12v with the arduino aswell to the screen also shuts down? \$\endgroup\$ – user1728582 Aug 23 '17 at 17:29
  • \$\begingroup\$ @user1728582 yes I know, it's just without detailed schematics it is hard to comment specifics. You would likely be better putting the pi to sleep rather than unplugging it though. In fact the difficulties are why these devices have such modes in the first place. \$\endgroup\$ – Trevor_G Aug 23 '17 at 17:38
  • \$\begingroup\$ Ok, thanks for the input. I'll try to play around with putting the raspberry to sleep and waking it up. That seems like and good solution. Thanks for the information! \$\endgroup\$ – user1728582 Aug 23 '17 at 18:06
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    \$\begingroup\$ @Trevor - the pi's heritage is unfortunately to a set-top box, not mobile. You can't really put it to sleep, you have to cut the power feeding it. \$\endgroup\$ – Chris Stratton Aug 23 '17 at 18:08
  • \$\begingroup\$ But I'm guessing it should work to wake it up from a shutdown by shorting the reset pins. That might work in my setup. \$\endgroup\$ – user1728582 Aug 23 '17 at 18:15

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