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How to make a Voltage Detector on a PCB

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JeeDee78

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Hi there! I have a system that have 3 differents input that can supply the system when 2 of the other input are not there. I have a 48VDC, 120VAC and an input that can be between 120 to 240VAC. But since this 3 voltage can be at the entry of the system at the same time, I need a detector to see which voltage is present. On the PCB there is a detector for each voltage but this is a resistive divider. I remake the design of this supply and I would like to know if there is another way to detect the voltage without to consuming to much energy?
 
in all hoesty the simplest solution is usually the best, if you say you could have 240VAC present you will not have a problem with lack of energy
 
What are the AC mains connected to? I hope it is a transformer. Detecting the presence of AC will consume less power on the secondary side of the transformer. My choice would be opto-isolators with the open collector outputs wire-ORed together and pulled up to a supply rail. If any of the three supplies can turn on its associated LED then the output will be low.

In words the logic is "a low or a low or a low is a low" which is the same logic as a three input AND gate
 
The 240VAC comes from an alternator, 120VAC comes from the electric network and the 48VDC comes from batteries. On the AC lines I have a bridge rectifier. The voltage detector, its job is to light a LED to indicate

But I thank of it this morning, maybe i could use a pcb transformer (like 120VAC/6VAC-58mA and 240VAC/6VAC-58mA). It's small (up to 1 scare inch) and it will take less space than an optocoupler and a couple of resistors. The transformer cost 4.63$US, but the maximum power consumes by the detector will be 0.35VA instead of a couple of WATT. Is it a good solution? Is there a solution less costly?
 
Oops, I forgot some words in my reply!

The detector, its job is to light a LED to indicate that the power is present on the other supply source.

It's sure if the 240V is present, it will be it that will supply the system, but the 120VAC and the 48VDC can be present. If the 240VAC stop, the 120VAC or the 48VDC supply the system depending on which is there.
 
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