Design Issues

[ahmedragia21]

Hi All,

I came a cross a design problem which I need your suggestions in it.

I have 3 power sources, Main Supply which is 9-32DC, a battery pack 12VDC, and PoE Module, all are connecte to tripel output 12, 3.3, 5VDC voltage regurlators, and finally a microcontroller.

I would like to detect a power failure on the three of them, and also use one of them on power failure.

I thought about first the user must use the Main Supply first, then the uC use its ADC to check for the voltage on the battery pack, use a voltage divider to lower the voltage then measure the value and check if its lower than a value, however this method is also I want to use it for charging the batteries it self!.
The same scenario I will do with the main supply and the PoE, but the problem with the resistors that they must have high power, the current that can be drawn I assume its more than 9A. I'm also not sure which VRef should I feed the microcontroller with, it's very problematic issue too.
Do you guys havae an opinion or suggestion for better design ?

 

[alec_t]

You could use steering-diodes such that the highest available voltage among the 3 power sources is selected to power a source-monitoring circuit which then takes any necessary load-switching action.
An SMPS might be preferable to voltage-dropping resistors.

 

[ahmedragia21]

Thanks for your reply.
Would you please show me a simple schematic ?
I also didn't get how an SMPS would help while measuring the voltage using the ADC ? it will always give a constant voltage ?

 

[ahmedragia21]

Is this what you mean by the steering diodes ?

 

[alec_t]

I also didn't get how an SMPS would help while measuring the voltage

I wasn't suggesting the SMPS was for measuring. The SMPS could provide the regulated voltages instead of using dropper resistors (which I inferred you were using because you said "but the problem with the resistors that they must have high power").

Is this what you mean by the steering diodes ?

Yes.
Here's what I drew: