Author Topic: Connecting two "same voltage" power sources  (Read 1567 times)

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Offline namlu

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Connecting two "same voltage" power sources
« on: September 14, 2014, 08:47:11 pm »
Hello.

My circuit can operate with 2 power sources: 12v dc from external dc source or 220v ac that goes to 12v ac to dc module. Output of the ac dc module is connected to the 12v dc input.

They are both 12 volts, but may be around 0.5 volt different from each other. If I connect both ac and dc at the same time, there would be 0.5v on non loaded wire.

How should I connect 12v dc and 12v ac-dc so I can connect both sources at same time? Should I put some passive component between them?

Thank you.


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Offline Araho

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Re: Connecting two "same voltage" power sources
« Reply #1 on: September 14, 2014, 08:56:17 pm »
Simplest would perhaps be to Diode-OR them? (See http://en.wikipedia.org/wiki/Diode-or_circuit)

That is, if you don't mind the voltage drop across the diode.
 

Offline sparx

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Re: Connecting two "same voltage" power sources
« Reply #2 on: September 14, 2014, 09:42:02 pm »
Could you use a relay (Normally closed) between 12V DC input and the point the ac-dc converter connects, when mains is used, the relay opens d/c the 12v external input? No drop then?
 

Offline Richard Crowley

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Re: Connecting two "same voltage" power sources
« Reply #3 on: September 14, 2014, 10:02:36 pm »
If we are to assume (since you did not explicitly state it) that you want both power sources available simultaneously (or automatically switch-over), most DC power input connectors feature a built-in switching contact that disables an internal source when the connector is engaged.

There are transistor switching methods similar to sparx' suggestion for the relay scheme.  Some Arduino boards featured this scheme where the board would automatically switch to the USB power source whenever available, etc.

Not nearly enough information about your application to really give you suitable suggestions.
 

Online Jeroen3

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Re: Connecting two "same voltage" power sources
« Reply #4 on: September 18, 2014, 09:56:19 am »
Linear technology has some powerpath or ideal diode IC's. Such as the LTC4412.
That might be helpful if you do not want to share load, or want more control over the active source.
 


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