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Different Voltages to Same Load...?

fatman57

May 27, 2013
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Please see image below...

diffVoltagesSameLine.jpg


What would happen if the load was resistive?
What would happen if the load was inductive?
Is this type of design found anywhere in the real world?

Much obliged for any help!
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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First, I assume your +24V and +12V markings are supposed to be on the anodes (left sides) of the diodes. It doesn't make sense for them to be on the cathodes, because the cathodes are connected together and therefore can't be at different voltages.

If that's the case, then the higher voltage will feed the load through the top diode. The bottom diode will always be reverse-biased and will have no effect.

Resistive vs. inductive load isn't relevant unless current is being switched somewhere, and that's just a DC circuit.

An arrangement like that would be used to power a single load from whichever input voltage is highest. It's sometimes called a "Diode OR" arrangement. You wouldn't use it if the input voltages are always 24V and 12V but it would be used if the input voltages could vary or might come from different sources. For example you could use that circuit to combine the voltage from a solar cell with the voltage from a storage battery, so whichever is higher at any given time is used to power the load at that time.
 

Harald Kapp

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Only the source with the higher voltage will supply power to the load. The diode to the source with lower voltage will be off. This is independent of the type of load.
Such a design is found "in the real world" when it comes to supplying power from different sources, e.g. a battery powered device with a wall wart option. However, using two diodes is the simplest option, not the most efficient in terms of power. More sophisticated solution will use MOSFETs and a controller to switch between sources.
 

fatman57

May 27, 2013
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Hi - thanks for the replies (and corrections - you can tell I don't do this every day) - have corrected cathode and anode issue as to where voltage source comes from!

Very cool, so basically the higher voltage will always win...interesting that a fallback can be implemented so easily if one power source fails...

Can anyone elaborate quickly how this would be done with a MOSFET....and why?

Just to throw a spanner in the works...what if both sources are the same voltage (and each from a different source - is this a state of Equilibrium [zero bias] - I presume that in the real world voltage fluctuations between the two will mean one will always win...)?

Thanks to Chris for saying why the higher will always win ("bottom diode will always be reverse-biased" - or is there more to it all). :) Thanks to Harold for a concise explanation! Sorry for all the edits but I'm just getting my head round all this ;)

Thanks!
 
Last edited:

fatman57

May 27, 2013
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Much obliged Harald - will get some reading in!

Regards Dan.
 
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