Circuit for power source selection

I have two power source: main is about 6-7V, backup about 10-12V, I’d like to use main if it remains above a certain threshold (5.5V) and switch to backup if main is lower, how should I do?
If main were greater than backup I could use two diodes in series with the two sources, but this is not the case…
I think I should use a mosfet but I don’t know how, perhaps with a voltage divider…

Thanks for your help!

What is your load ?

Is this DC ?

Don’t use the word main .

Arduino and other stuff (sim808 shield, relays)

Yes, it is dc

Thanks for your reply

A simple SPDT (or DPDT) 5v coil relay powered by the 6v-7v supply could be used.

Many 5v relays will work up to 9v.


Feed o/p to a buck converter to get 5v.

Thanks a lot for your reply!
Let me understand: this is a relay, the coil is connected to the primary source, primary source is connected to NO contact and backup to NC; if the primary source is at 6V (at least) the relay is energized, NO closes and so primary source powers on the load, NC contact opens and backup source is excluded, if it drops below 6V NO opens and NC closes and backup source goes online, right?

Something like this:

Keep in mind, relay is pulling contacts at higher voltage than is releasing them, 7V on 5V off, I suggest to add voltage comparator to control relay.

Thanks again, you’re very kind! I think I can use a 6V relay because I want to feed arduino through V_in pin so I’ll use arduino internal buck converter (and other stuff can be fed also with >=6V), in your scheme I cannot understand why there is the diode D1…

The diode snubs the kickback voltage when 6v disappears.

Relays consume energy, relase heat and are not meant to stay energised for long hours…Personally, i d prefer a solid state solution.

Really? Where is that written? 40+ years ago I used to work on electromechanical systems. There was never any suggestion of that - some relays would stay energised for months or years, releasing only under fault conditions (for example).

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Oh, yes! Due to the inductance of the coil! Thanks again!

Perhaps i should have clarified that its not that they cannot stay energised for long hours but rather its better that the design is such that they dont have too. In addition the type of relays you are referring to are of different mechanical endurance than those OP is likely to use.

PS: In my 25 year experience in electrical utility high voltage transmission protection schemes, care is taken to ensure that relays spend most of their service life in their de-energised state.

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Thanks to you too!! And what would you suggest? I was thinking about an N-mosfet, perhaps in this way: the gate should be attached to 6-7v primary source, drain to backup source and load to source, when primary source on gate is above the threshold mosfet opens and backup is offline, when primary goes below the threshold mosfet closes and backup goes online.
My questions are:

  1. is there a mosfet that has a 6V threshold? If not, how can I obtain this threshold?
  2. how should I put primary source offline? Perhaps with another mosfet (this time P-mosfet)?

Thanks again…

have a look how arduino does voltage source switching between USB and socket

I really do not think we are dealing with electrical utility high voltage transmission protection schemes here, though one enthusiastic contributor did have such a proposal and was quite promptly seen off. :grinning:

If you are using lithium-ion batteries, you can use the the TP4056 battery circuit or IC to solve your use case

lol o know we are not.

i was just replying to previous message regarding past experience.

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I cannot use tp4056 because my battery pack (backup source) is 3s2p, so I’ll use two 3s bms, do you think the bms module can work as the tp4056?

I think that the part I have to look at is that in the red circle

There is an op amp which compares V_in with 3.3v (why? Where is this taken?) and controls a mosfet gate, but I cannot understand how V_in is used if it is above the threshold, perhaps is is taken where there is a sort of arrow up symbol and +5V?