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I'm Planning to switch between two power sources with a Arduino like below diagram.

what I'm going to do is monitor AC power failure with a sensor and then once AC power failed switch the Relay to Battery source.

My question is, Connecting two ground wires from DC adapter and Battery Ground will cause any trouble?

enter image description here

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  • \$\begingroup\$ What's wrong with the normal diode ORing that makes it unsuitable for your application? \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 10 '15 at 8:20
  • \$\begingroup\$ @IgnacioVazquez-Abrams I just heard about this :( \$\endgroup\$ – lasita Oct 10 '15 at 8:25
  • \$\begingroup\$ A problem with this circuit is that your load will be shut down, when switching between sources. There is better solutions like using a Diode. \$\endgroup\$ – HOPE Oct 10 '15 at 9:31
  • \$\begingroup\$ I notices that diode ORing works only if main supply is greater than backup supply, so if main is 12V and battery is 12V then Diode ORing will not work as expected though.. \$\endgroup\$ – lasita Oct 10 '15 at 11:17
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My question is, Connecting two ground wires from DC adapter and Battery Ground will cause any trouble?

No, this will not cause trouble providing the battery in question is a just a battery and not some other AC / DC power supply with maybe a dodgy earth. Even then it won't burn but might throw up a few operational problems but... unlikely.

Regards Ignacio's comment, it is advisable to have a diode if the DC from the AC supply is a bit bigger than the battery voltage - this would nearly always work without going to the complication of a relay but if the voltages are somewhat similar then it's hard to say whether the battery will remain powering the target circuit when AC is on.

Another thing - when the relay changes over, there is a small but finite time when you lose power so you should have enough capacitance across the load to prevent drop-out.

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  • \$\begingroup\$ Thanks for the quick answer as always :) could you please point me to a good article or explain how the diode OR works and what diodes to use ? \$\endgroup\$ – lasita Oct 10 '15 at 9:50
  • \$\begingroup\$ Sorry, I haven't read an article on that in possibly 30 years. \$\endgroup\$ – Andy aka Oct 10 '15 at 11:56
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Be sure to use a break-before-make relay (as most are, but good idea to check) otherwise there will be a momentary connection between the Adapter +ve and battery +ve, which may give problems depending on the design of the AC adapter or battery, e.g. of the adapter has a low source impedance and a higher voltage than the battery, then the momentary current pulse into the battery may cause battery damage.

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    \$\begingroup\$ But the only reason the switchover is happening is because of a mains failure, so it's unlikely the adapter's output will have much of anything coming out of it. However, there may be a problem with the battery momentarily feeding into the adapter, so your point's still valid. \$\endgroup\$ – EM Fields Oct 10 '15 at 10:08
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There's no need to monitor power failure with a sensor, you can just use an AC relay connected across the mains or a DC relay connected across the output of the AC/DC adapter. Wire the normally-open contact to the plus output of the AC/DC adapter, the normally-closed contact to battery plus, and the relay common to load plus.

Wire the AC/DC adapter minus, battery minus, and load minus together, and you're done.

A caveat: When the relay is switching and the common contact is flying from the NO to the NC contact, (or from NC to NO if the mains come back up while the thing is operating on batteries) there'll be a short time when it's on neither the battery nor the adapter, and there'll be no output to the load, so you need to make sure there's a BFC across the load in order to keep its supply voltage high enough and long enough to keep it (the load) happy during switching.

IgnacioVazquez-Abrams makes a valid point, and if you can live with the voltage drop across the [Schottky] diode OR, that might be the way to go since it'll be a very smooth transition from the AC/DC adapter to the battery and way, way cheaper than with a relay and a BFC.

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  • \$\begingroup\$ Thanks for the quick answer. looks like better way is using a diode OR. could you please point me to a good article or explain how the diode OR works? \$\endgroup\$ – lasita Oct 10 '15 at 9:48
  • \$\begingroup\$ @lasita: I can, but comments isn't the place. Post it as a question and I'm sure you'll get multiple answers. Well, at least one... ;) \$\endgroup\$ – EM Fields Oct 10 '15 at 10:00
  • \$\begingroup\$ question Posted :) \$\endgroup\$ – lasita Oct 10 '15 at 11:02

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