5v relay grounding

[Robodanjal]

I'm using my Pi to control a mains relay through a GPIO output and I have stumbled across something that I just need to check with people who are more familar with the Pi than I am.

Originally I was using the +5v (DC+), Gnd (DC-), and a single 3.3v GPIO pin to trigger the relay, which was all working fine. As I added more components to the project that I am working on I found that I was running out of usable power and it was causing issues at certain points during my program. As I have a 5v AC/DC adaptor around I decided to utilise it to power the relay and ake some of the strain off of the Pi.

Now I have the + and - to their respective points and the relay powers up correctly. To use a 3.3v to trigger the relay I would need to ground it back to the Pi; is it okay to connect the GPIO ground to the same point as the DC- on the AC/DC adaptor on the relay?

[scotty101]

Yes. Not a problem. Just keep the 5v away from your pi

[drgeoff]

Not clear how you have things wired up but I do have some worries.

1. What is the relevance of the 5 volt supply? If the relay is wired between a GPIO and +5v you risk damaging the RPi. The GPIO are not 5 volt tolerant.

2. If the GPIO is connected to the relay, the GPIO may be handling more current than it can safely do so. (RPi at risk of damage, not your safety.)

3. If the GPIO is connected to the relay and the relay is the old-fashioned coil type, the back emf when current in the coil is switched off can damage the GPIO.

There are many tutorials which show how to use a transistor, two resistors and a diode between a GPIO and a relay coil.

[Robodanjal]

1. What is the relevance of the 5 volt supply? If the relay is wired between a GPIO and +5v you risk damaging the RPi. The GPIO are not 5 volt tolerant.

The 5v DC power supply is for 240v mains relay which requires 5v to power and a Hi or Low input to switch states. I'm not sending 5+ into the GPIO, at least not with purpose. This is exactly why I am asking if there is any risk in sharing the GPIO ground return with the negative of the 5v DC supply as otherwise the GPIO cannot act correctly as a switching signal.

2. If the GPIO is connected to the relay, the GPIO may be handling more current than it can safely do so. (RPi at risk of damage, not your safety.)

GPIO is the switching input for the relay, it's not connected to the 5v+ nor to the 240v+.

3. If the GPIO is connected to the relay and the relay is the old-fashioned coil type, the back emf when current in the coil is switched off can damage the GPIO.

The relay is purpose built for use with micro-computers to control mains power.

I'm at work so here is my quick diagram detailing my setup.

https://drive.google.com/file/d/0ByGKVy ... sp=sharing

[Robodanjal]

Yes. Not a problem. Just keep the 5v away from your pi

Thanks for your reply.

[drgeoff]

1. The critical point is the relay. What is inside it on the connection that goes to the RPi GPIO?

If the relay has a coil whose other end goes to 5 volts or if there is a pull-up resistor to 5 volts, then you will be putting 5 volts on the GPIO when the GPIO is set high.

2. Irrespective of the above, it will never work if you do not connect the -ve of the 5 volt supply to the RPi -ve (ground).

[KLL]

pls also read this: http://www.douglaskrantz.com/Flyback_Diode.html

[drgeoff]

pls also read this: http://www.douglaskrantz.com/Flyback_Diode.html

I wonder what percentage of noobs reading that are going to connect the diode the wrong way. That article uses a pnp transistor and a negative supply.

[Robodanjal]

1. The critical point is the relay. What is inside it on the connection that goes to the RPi GPIO?

If the relay has a coil whose other end goes to 5 volts or if there is a pull-up resistor to 5 volts, then you will be putting 5 volts on the GPIO when the GPIO is set high.

2. Irrespective of the above, it will never work if you do not connect the -ve of the 5 volt supply to the RPi -ve (ground).

Thanks for your input. When I posted this question I was at work and didn't have access to my Pi so I was curious.

When I got home I took some readings and found that I got exactly the same results regardless of if I powered the relay using
5v from the Pi or 5v from the AC/DC mains supply.

[drgeoff]

Whether the 5 volts comes from the RPi or from somewhere else is not the issue. The issue is whether 5 volts from anywhere can get to the GPIO. I have already explained how that can happen.

As you seem to be unwilling or unable to give any information about the relay you are using, I cannot help you further.

[Robodanjal]

Whether the 5 volts comes from the RPi or from somewhere else is not the issue. The issue is whether 5 volts from anywhere can get to the GPIO. I have already explained how that can happen.

As you seem to be unwilling or unable to give any information about the relay you are using, I cannot help you further.

Not sure if you missed my above post but I have found the answer to my question, I no longer require any assistance on this matter.

[drgeoff]

You may have a working system this time. But you do not understand why. Next time you try something similar, but not exactly the same, you may be less lucky and have a non-working system and a dead RPi.

[Robodanjal]

But you do not understand why.

If you say so.

I feel you may have taken my line "got exactly the same results" as "it worked the same". What I meant is that my metered readings produced near identical results, allowing me to conclude that the external supply was not in-fact producing any harmful effects on the RPi.

Have a great weekend!

[mahjongg]

you may want to read this thread! : viewtopic.php?p=805079#p805079

and yes, you should wire the grounds together.

[Robodanjal]

you may want to read this thread! : viewtopic.php?p=805079#p805079

and yes, you should wire the grounds together.

Thanks Mahjongg.