first of allI'm sorry for my english.

I'm starting with electrical circuits (this is my first circuit) and I'm trying to create a Power Supply for an Arduino with battery backup.

Searching I've found three ways:

  • A diode on every source to switch to the higher voltage source.
  • Using a relay to switch when the main fails
  • Using a Mosfet and a Transistor to swith when main fails

The first works while the main Power Supply has an higher voltage, but both circuits has similar voltages and maybe sometimes the backup circuit will have higher voltage.

The second works perfect, but the relay uses an high power (about 4 times the arduino comsumption) and I want to create a low power circuit.

The third seems to work but for now I don't have materials to test the circuit and I want to ask if is OK and/or can be better, before buy all the things i need.

Searching I've found some info and I've done a prototype on a program that looks like this:

Switchable DC/battery circuit

I've done some tests with the program and seems to work, but are not truly real (even the stabilizer has it's own power on that program...).

With this circuit I want to avoid the stabilizer comsumption while is on battery to last as log as possible, then I need to separate both circuits (D2).

This prototype is OK or I've to change something?. Someone knows a better way to do it?.

I take this opportunity to ask for a program to make better tests before build a circuit. I'm using LiveWire now.


  • \$\begingroup\$ What are your IN/OUT specs? for case 1) thresholds. These must include tolerances and switch point. \$\endgroup\$ Commented Dec 28, 2016 at 22:44
  • \$\begingroup\$ My arduino circuit needs 5v on input, so I need near 6V in both Inputs to get an output of about 5v after the diodes. A solution can be a DC input of >8v switched with diodes and a battery of 7.5v (5 AA batteries). Both diodes cathodes connected to a 5V regulator to get an stable 5v output, but I want to avoid that regulator consumption. Is not high, but i want the best battery duration and the regulator can drain about the 50% of the whole circuit consumption. \$\endgroup\$ Commented Dec 28, 2016 at 23:20
  • \$\begingroup\$ A MOSFET half bridge can be used instead of diodes, but you still have not define the range for each input to be enabled and priority criteria for low voltage. on either or both,. \$\endgroup\$ Commented Dec 29, 2016 at 5:49
  • \$\begingroup\$ I'm not fully sure about what do you mean (i'm very new). The priority should be high for DC and low for batteries. The batteries are only used when the DC fails. About the range: do you mean voltage range?. I'm planning to use 6v for batteries input (just 4*1.5v or 5*1.2v) and between 8v and 12v for DC input, limited to 6v using a regulator. The real output that I need is 5V. Anyway, I don't want it because should drain the battery faster, but maybe I'll need to use a zener or a regulator for both, because the diode drain is lower than expected. \$\endgroup\$ Commented Dec 29, 2016 at 19:07
  • \$\begingroup\$ Srry for double post, but i can't edit for now. I've seen the datasheet of the main IC and works between 4.5v and 5.5v for 16mhz and 2.7v and 5.5 for 8Mhz. Maybe 8Mhz is enough, but i preffer to be sure and use a range between 4.5v and 5.5v. \$\endgroup\$ Commented Dec 29, 2016 at 19:13

1 Answer 1


Finally I've found this circuit that have lower voltage, but the rechargeable battery is better:

P channel mosfet battery charging/power selection circuit help

Greetings and thanks!!


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