AGID
Posts: 36
Joined: Tue Mar 17, 2020 1:02 pm

powering up peripherals from different power supply

Tue Jun 30, 2020 7:11 pm

I have a project with pi3 b+ and i have some power hungery peripherals like fans and servos so i decided to power them from external 5v 1a power supply but at the same time i want to control them through raspberrypi 3 GPIO is that safe? and do i need to connect the external power supply ground with the rasbperry ground or is it not nescessary ? and is there any cautions i should put in my consideration ?

Thanks

Heater
Posts: 15945
Joined: Tue Jul 17, 2012 3:02 pm

Re: powering up peripherals from different power supply

Tue Jun 30, 2020 7:17 pm

Yes, you need to connect the grounds together. Other wise GPIO control signals have no reference zero to work against.

Be sue that 5v cannot get back into your Pi GPIO pins. That rather depends on what devices you are conneting.

Some external 5v devices may not work with the 3.3v logic levels of the Pi GPIO outputs.
Memory in C++ is a leaky abstraction .

CX32
Posts: 8
Joined: Thu May 28, 2020 5:40 am

Re: powering up peripherals from different power supply

Tue Jun 30, 2020 7:19 pm

Generally its a good idea to have a common ground of separate power supplies whose circuits interface with each other.

drgeoff
Posts: 10766
Joined: Wed Jan 25, 2012 6:39 pm

Re: powering up peripherals from different power supply

Tue Jun 30, 2020 7:19 pm

AGID wrote:
Tue Jun 30, 2020 7:11 pm
I have a project with pi3 b+ and i have some power hungery peripherals like fans and servos so i decided to power them from external 5v 1a power supply but at the same time i want to control them through raspberrypi 3 GPIO is that safe? and do i need to connect the external power supply ground with the rasbperry ground or is it not nescessary ? and is there any cautions i should put in my consideration ?

Thanks
Safe? Depends on the peripheral.

Connect the grounds? Usually essential unless using optocoupler, relay etc.

stevend
Posts: 267
Joined: Fri Oct 11, 2013 12:28 pm

Re: powering up peripherals from different power supply

Tue Jun 30, 2020 7:38 pm

First, let's be clear that in this context when people say "ground" they mean zero volts or "common". As drgeoff says, if you aren't using optoisolators or similar, yes, you must connect the commons together.

You will need to consider what happens during power-up and power down, when there could be up to 5 volts difference between pins of the Pi and the peripherals.

For a non-isolated circuit, you need to protect the Pi's I/O such that when the voltage on a peripheral's pin is greater than the voltage currently on the 3.3V supply pins (put that way to allow for power up/down) the current is no greater than a few mA. A 2.2K to 10K series resistor is a simple way of current limiting, providing it doesn't affect circuit operation.

You also need to consider what happens to the peripherals in the time between them being powered, and the Pi's outputs being configured and set to the correct levels. Will you get spurious outputs? Does it matter? If so, you need some sort of reset circuit.

AGID
Posts: 36
Joined: Tue Mar 17, 2020 1:02 pm

Re: powering up peripherals from different power supply

Tue Jun 30, 2020 7:55 pm

drgeoff wrote:
Tue Jun 30, 2020 7:19 pm
AGID wrote:
Tue Jun 30, 2020 7:11 pm
I have a project with pi3 b+ and i have some power hungery peripherals like fans and servos so i decided to power them from external 5v 1a power supply but at the same time i want to control them through raspberrypi 3 GPIO is that safe? and do i need to connect the external power supply ground with the rasbperry ground or is it not nescessary ? and is there any cautions i should put in my consideration ?

Thanks
Safe? Depends on the peripheral.

Connect the grounds? Usually essential unless using optocoupler, relay etc.
The pripherals that will be powered from external power supply is two fans 5v 0.2mA and one servo s90 and LCD with i2c, MQ135 and the rest of sensors are not power hunger like PIR module and DHT11 Module etc. there are no relays or optocoupler.. but the fans , lcd, servo and MQ135 will be controlled by GPIOs of rasp. i am gonna use the MB102 breadboard power supply connected to 12v 2amps adaptor as external power supply for fansm servo, lcd and MQ135.

notcing that i am using this connection for the fans:

Image

thats mainly it

Return to “General discussion”