2 Different Supply Voltages on PCB

[jones12]

Hey guys,

Im designing a simple PCB with an ATmega328P to switch some fans on and off through a MOSFET.
I need 5v for the IC and 12V for the fans, not huge power from rough calculations about 1A on the 12V side and roughly 1A on the 5V side.

I was planning on using a 12V SMPS into a 7805, and tapping off the 12V incoming but I've been told that's not a good idea as it will get too hot.
What are my other options? Hoping to keep this as compact as possible.



Cheers guys!

[killcode]

First of all, how sure are you that you need 1A on the 5v rail? That's unlikely if its only powering the micro and a few leds etc.

If you do need 1A at 5v (note that is 5W) you will have the 7805 burning 7W! as waste heat (12-5*1A) = big heatsink.
In that case you should use a switching regulator - something like a LM2577/LM2673 or any of integrated switch regs. That can be squeezed into about 25x30mm (plus heatsink).

Can you tell us exactly what is powered off the 5v rail (atmega,...,...) ?

[jones12]

Thanks Killcode, Yeah the 1A is a generous calculation.

5V rail powering ATmega328P at 16MHz, 16x2 LCD (approximately 300mA from the datasheet), couple of 3mm LED's(36mA), DHT 11 Sensor(1mA) and supplying the gate for a IRF 540N.
I think I read the ATmega power consumption wrong this morning

Should I still use a switching Reg?

LCD Datasheet:
https://www.engineersgarage.com/sites/default/files/LCD%2016x2.pdf)

[Rerouter]

Not the cheapest option, But i have been running these in one the worst electrical environment out there without issue (council buses that are regularly jump started and face load dumps)

I used the C series, but you should get by fine with the E Series

C Series https://www.recom-power.com/pdf/Innoline/R-78Cxx-1.0.pdf
E Series https://www.recom-power.com/pdf/Innoline/R-78Exx-1.0.pdf

Depending on your supplier, there about $5 a pop for the E series. 

[jones12]

This looks like exactly what I am after!

Cheers!