# Dual power supplies - which pulls first?

Hello, I'm working on a project that will keep my arduino outdoors. I am planning to run a 6 cell AA to the DC and have a solar setup from another project that runs to USB. My question for you experts is - if I have both plugged in, which will the unit pull from? One vs the other, or both? I'd like to have the solar as the primary (USB) and the 6 cell AA as backup (DC). Thanks for your help.

Depends on the board.

If it’s an Uno and both usb and the dc jack have power, the the dc input will take priority when it is greater than ~ 7.0 volts.

if I have both plugged in, which will the unit pull from?

The higher voltage source will try and put current into the lower voltage one. This is known as cross charging and is a bad situation. The exact situation is described here:- Superposition Theorem

To stop this you should put a series diode on each source before they are connected together. This will cause a voltage drop but if you use a schottky diode, this loss is minimised, perhaps to 0.2V.

Grumpy_Mike:
The higher voltage source will try and put current into the lower voltage one. This is known as cross charging and is a bad situation. The exact situation is described here:- Superposition Theorem

To stop this you should put a series diode on each source before they are connected together. This will cause a voltage drop but if you use a schottky diode, this loss is minimised, perhaps to 0.2V.

Right answer, wrong question. The question was USB jack or dc jack input.

The Uno handles this with its internal power source switching and diode on the dc input. You don’t need or want a diode on the USB jack input side. More importantly, the voltage connected to the usb jack needs to be a regulated 5 volts since this input bypasses the internal 5 volt regulator.

Place a low forward voltage Diode (Schottky) in series to where the Battery power connects to the same positive point at the USB positive wire.
Only when the USB power is off or lower than the battery will the battery supply current...

dynamicsallen:
Hello, I'm working on a project that will keep my arduino outdoors. I am planning to run a 6 cell AA to the DC and have a solar setup from another project that runs to USB. My question for you experts is - if I have both plugged in, which will the unit pull from? One vs the other, or both? I'd like to have the solar as the primary (USB) and the 6 cell AA as backup (DC). Thanks for your help.

One analogy is a car jack ... not a car theft, but a car jack for lifting up a car ....for changing tyres etc.

If you put two jacks next to each other... the one that happens to be at the higher level is expected to do the work. Ideally that is. If both just so happen to be at the same height (ideal case only), then both will share the work. Ideally that is.

For two voltage supplies connected in pararallel...... if one supply is at a higher voltage than the other........ and considering a case where the connecting wire (that connects together the the terminals) has relative small resistance only .... or ideally zero resistance, then having one regulated supply at one voltage, and the other supply at a different voltage ---- is going to cause a large current to flow. And power dissipation in the wire (resistance 'R') can be calculated (in one way) as (I^2).R. So the power dissipation can be relatively high ..... and things get hot. So to avoid this....as some people mentioned..... can use diodes at the output of each supply (oriented correctly).

If you connect power supplies in parallel several bad things can happen:

The higher voltage supply back-powers the other one and damages it (batteries or regulators)

The whole system oscillates madly or goes into shutdown or similar (typically two voltage
regulators fighting). Voltage regulators have lots of gain, and two connected together may end
up with a system with no phase or gain margin to prevent instability - figuring this out is
advanced stuff, no designer of regulators expects anyone to do this - they assume the load
is basically passive.

If you are lucky nothing bad happens - but to be safe always use diodes.

Absolutely right. In basic circuit theory.... two constant voltage sources in parallel with finite difference in source voltage...... is a conflicting condition.... and is a disallowed condition in circuit diagram drawing....due to definitions..... ie. keeps constant source voltage across its terminals... having the value specified. This obviously becomes a 'does not compute' situation if a voltage source having a different constant voltage is connected in parallel with the original voltage source.