Hi all, I need some help: I have a 12V DC - 4A external supply and I hve to power a water pump (12V - 1.5A) a relay (5V) that let the pump start, and an Arduino Nano (via VIN pin).
I thought to use L7808 to power on the Arduino and L7805 to power on the relay.
The schema should be like the one below.
Could this work fine? Does someone have suggestions to improve this circuit?
Thanks a lot!
Ciao,
Giuseppe
Looks plausible.
How are you switching the relay on/off from the Arduino?
a7
A 5v relay should work at 8v.
If it's in fact a relay module it might want 5 volts.
a7
We need more info from the OP.
Thank you for the reply, alto777.
The relay is actually a two-relays module (and I use one of them). It's an opto-isolated module. I switch it on/off with a digital pin. I supply 5v to the JD-VCC and GND pins (I remove the jumper), and both 5V + digital from Arduino to VCC and IN1.
The module looks like the image below.
Thank you again.
Giuseppe
Thank you LarryD.
If this is true I can avoid the 5V part with L7805 and use only 7808.
I try to find some info about this.
Thanks again.
Ciao,
Giuseppe
Uhm... what is OP?
I apologize, I'm a newby...
Thank you.
Ciao,
Giuseppe
You are the OP (original poster).
I believe this is the schematic of your module.
Connect Arduino +5v to J5-2
Connect +8v to J5-1.
If you wish, add four 1N4007 diodes in series with 8v to knock it down to 5.2v
Damn! Can't see the image, it seems like "image broken"...
Now I see it! Thanks a lot Larry!!!
No way!
Please don't go powering Arduinox via the "Vin" pin for serious projects! 
The Nano runs on 5 V and you need 5 V to power the relays. Get a proper switchmode "buck" regulator to derive the 5 V from your 12 V and use it to power the relays and the Nano via the "5V" pin (and ground of course).
There are some tricks to wiring those relay boards.
"D1 " and "D2" on your original diagram are entirely unnecessary.
@geppox
You are showing the 7808's input connected improperly (depicts its IN and GND connected to your "12V -".
Nice catch!
a7
Thank you @runaway_pancake , it was my fault in designing.
In line of L7808 comes from 12V +.
Thamks a lot!
Ciao,
Giuseppe
Thank you @Paul_B , I thought that I could power the Nano with VIN, otherwise what is it for? uhm...
And about d1 and d2, I think you are right, some say that are useless, others that is better to have them. I think that I'll remove them.
Regards.
Ciao,
Giuseppe
You CAN, but the regulator has a very limited output, and does not have a heatsink! You are running a voltage regulator anyway, why not change it to a 5V regulaor and power your board through the 5V pin?
I do agree that using a switch mode regulator would be much better, as they are so much more efficient.
All clear!
Thanks @missdrew , many thanks to you!
Regards.
Ciao,
Giuseppe
It's a decoration. Essentially an ornament. 
OK, so the regulator on the Arduino UNO/ Nano/ Pro Mini/ Mega2560/ Leonardo/ Pro Micro has very little heatsink, so will not pass very much current (depending on the input voltage and thus, how much voltage it has to drop) before it overheats and (hopefully reversibly) shuts down.
It is essentially little more than a novelty provided in the very beginning of the Arduino project when "9V" power packs were common and this was a practical way to power a lone Arduino board for initial demonstration purposes. And even then it was limited because an unloaded 9 V transformer-rectifier-capacitor supply would generally provide over 12 V which the regulator could barely handle.
Nowadays, 5 V regulated switchmode packs are arguably the most readily available in the form of "Phone chargers" and switchmode "buck" regulators to regulate down from 12 V or other available voltages are cheap on eBay so these can be fed into the USB connector or (more appropriately) 5 V pin to provide adequate power for most applications. Unfortunately, many tutorials or "instructables" are seriously outdated or misleading and have not been updated to reflect the contemporary situation.
2 Likes
Great!
@Paul_B , I thank you very much for this exhaustive explanation, very very interesting.
Thanks a lot!
Regards.
Ciao,
Giuseppe