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### Topic: Circuit which switches depending on power (Read 571 times)previous topic - next topic

#### sreedevk

##### Mar 30, 2013, 04:00 am
Suppose I have two power sources. Please suggest a circuit such that when the power delivered by the master source goes down, it has to switch to the slave source. Thanks

#1
##### Mar 30, 2013, 04:23 am
Have you tried Google?  I searched 'backup power switching circuit' and had over 4.5 million hits.

#### retrolefty

#2
##### Mar 30, 2013, 05:08 am
It can be as simple as using two diodes to form a wired-or gate such that the higher voltage source will supply all the current draw from the load and if it is turned off or fails, all the current flow will come from the other voltage source via the other diode. No physical switching requires, it's silent and automatic current switching at the cost of two diodes rated for the maximum current flow in question.

Lefty

#### tocpcs

#3
##### Mar 30, 2013, 05:37 am
What's the switching time of such a circuit?
It sounds good..

#### retrolefty

#4
##### Mar 30, 2013, 05:48 am

What's the switching time of such a circuit?
It sounds good..

Instantaneous, and it is a 'bumpless transfer'. Wire the two diode cathodes leads together and that then wires to the load to be powered. Wire each diode anode lead to it's independent voltage source positive terminal. Both voltage sources must have their negative terminals wired together and then wired to the load's ground return.

Which ever voltage source is +.6 volt higher or more then the other will be the one to normally power the load.

Lefty

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