Sharing a 12V power supply


I have a working but early stage project that controls solenoid valves. The solenoids are isolated via TIP120’s. At the moment I am using 2 separate power supplies. The first is a 12V supply for the solenoids and the second is a 9V supply for the Arduino Nano.

Can I use the 12V supply for the Arduino as well as the solenoids, make a connection from the power in straight to the Arduino vin, or should I add some kind of protection between power supply and Arduino?

That's a slightly tough question.

Presuming the 12V supply is regulated - and if the answer is "no" then you probably should not use it for the Arduino - and you are not powering anything else from the Arduino, you can probably get away with it.

And if the 12V supply is regulated, you can simply add three series power diodes to drop it down to about 10 and reduce heat in the Arduino regulator.

the problem is when you intend to add other devices that get 5V from the Arduino, in which case it may be preferable to add a switch-mode regulator module to derive a heftier 5V supply from your 12V.

Thanks for the reply.

The 12v power supply is a mains adaptor. I assume this is regulated but don't actually know and there is no information on the item or its packaging.

I can feed the 12V in to a small power supply circuit (based on the LM7805) and then to Arduino. Would this be suitable? I would like to reduce the number of power supplies. If the above is not workable what solution would you suggest?

7805 is hugely inefficient: the current is the same on both sides so with 12 to 5v you are wasting 7i watts as heat. Rather take Paul__B's advice and go for a switch-mode one.

yes you can put 12 volts in but that is real max as the power transistor on the uno will get rather hot, depending on how much is running on 5 volts. easier is to have a separate 5 volts psu as this is more efficient and no problems with sparks. when you have a short on your 12 volt you can see it as the uno keeps on working.

You say the solenoids are isolated. When you share power and ground that won't be so.

Thanks for the help and I will keep with 2 separate power supplies for now.

Eventually I will put this in to a box and would still like a way to have a single power input. Can anybody point me to what I should be looking in to.