# Multiple power supplies in home-made circuit

I'm planning to build an ebb-and-flow hydroponic system (nope, not for weed) and as my electronics skills are close to zero, I'm trying to put things together before soldering anything. I'm a decent software developer, so coding is not the issue here. I know it's ambitious, but well... I want to learn :)

I'm facing power problems and I'm not sure about my solution.

My set-up includes :
- a barebone arduino (5v, 70mA)
- a submersible pump (12v, 3W, 250mA), controlled by an arduino pin and a mosfet
- a RTC DS1307 (2mA)

I made these working together, using USB for the arduino, and a 12v battery for the pump. To achieve a definitive set-up, I plan to use a laptop charger (15v, around 4A) and to split its output to 2 home-made lm2576 circuits (12v and 5v, both rated for an overkill 3A).

Next step would be power LEDs... and I'm getting even less confident about it. These LEDs look like what I need : 3W, warm white (forward voltage : 3.5-4.5v max, 750mA max).

I know LEDs need constant current, and among many others, this circuit seems simple and efficient enough.

If I understand everything, 1 LED would require 4V, 2 LEDs in series : 8V, and so on... Current must stay at 750mA (max). I plan to use only one LED as I don't think my basil will require more, so I'll stick to 4V.

Here are all my search results, but I still don't know if these circuits will work together :

15v laptop charger
\ lm2576 (12v) --> pump
\ lm2576 (5v) --> ICs
\-> constant-current supply (0.5V dropout) --> 1 LED (4.5V max)

Arduino will turn the LED on and off through (I guess) another mosfet, just as the pump.

So there I am, humbly asking for a educated eye on these questions...

D

• Good job on the question. All looks OK to me except I am not clear on how you plan to drive the LED. The circuit you linked to is a linear circuit, so it will draw full LED current from the power supply, and dissipate any extra Voltage in a wasteful, heat producing way. So you can use it if you connect it to your 5V LM2576 regulator, but don't connect it to your laptop charger. Also, there is a chance that the pump will need more than 3A, briefly, during startup. Try it and see what happens. Make sure it is reliable before you leave it unattended. Sep 8, 2015 at 22:16
• Why not get a 12v supply from the start and skip one regulator? Sep 8, 2015 at 22:51
• Thanks @mkeith for your fast answer ! My schema isn't very clear (space-padding is disturbing), but yes, I planned to connect the LED driver to the 5v regulator :) It's good to know my guesses were not hat wrong... And thanks again for the warning about my pump ! Sep 8, 2015 at 23:03
• @Passerby : it's the only one I have :) Sep 8, 2015 at 23:04