voltage level detector

betwixt said:

I think I can tell you with some certainty. The circuit as drawn will not work with any AC or DC input because the supply is shorted out!

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p72 said:

"I would like to set Relay in "default mode" for DC supply. That means if I applied AC voltage to the Relay, it should switch Relay position/contacts and goes into further stage.For DC voltage, I am thinking to put diode before relay, which will block DC and will not allow to pass it through the Relay. As a result, Relay will stay in its original (default) position and DC will pass without entering into Relay".


Hi p72,
If you use Diode in place of AC Connection it will pass only single cycle as full wave can not be used it will pass the DC also:?:
As per your connection viz, Normally closed terminal of relay will be always connected to the DC & Normally open terminal connected to the AC:?:
Actually this circuit don't need diode,as when AC supply is present,it will change the Relay state & route for AC path & when there will be no Ac signal it will not change the path as Normally connected terminal will remain activated as a Default state DC path will be activated.:smile:
Pls correct if i am wrong.....:?:

Thanks,:smile:

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SACHIN C said:

p72 said:

"I would like to set Relay in "default mode" for DC supply. That means if I applied AC voltage to the Relay, it should switch Relay position/contacts and goes into further stage.For DC voltage, I am thinking to put diode before relay, which will block DC and will not allow to pass it through the Relay. As a result, Relay will stay in its original (default) position and DC will pass without entering into Relay".


Hi p72,
If you use Diode in place of AC Connection it will pass only single cycle as full wave can not be used it will pass the DC also:?:
As per your connection viz, Normally closed terminal of relay will be always connected to the DC & Normally open terminal connected to the AC:?:
Actually this circuit don't need diode,as when AC supply is present,it will change the Relay state & route for AC path & when there will be no Ac signal it will not change the path as Normally connected terminal will remain activated as a Default state DC path will be activated.:smile:
Pls correct if i am wrong.....:?:

Thanks,:smile:

Click to expand...

Hi Sachin,
Can you draw circuit diagram??

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betwixt said:

I'm assuming your input can be AC or DC on the same wires, otherwise rectify the AC, scale it with a potential divider and compare it with the DC scaled with a second potential divider. Even if you have two separate inputs, the circuit will work to check for AC presence.

The schematic works by utilizing the reactance of the capacitor to allow AC to reach the opto-coupler and also the capacitor's property of blocking DC. When DC is applied, apart from a very short pulse of current as the capacitor charges, no current flows into the opto-coupler LED so the output side goes to a logic high level.

When AC is applied, the capacitor allows current to flow, the diode conducts on one half cycle and the LED inside the opto-coupler on the other. The output side of the opto-coupler will go low when the LED lights and the capacitor across it will be discharged, taking the output to logic low. It will actually be a low level pulsing signal at the output because the capacitor will try to recharge when the opto-coupler is turned off but with suitable time constant it should stay below logic low level.

The bonus to the circuit is it gives safety isolation between the voltage you are checking and whatever follows it.

Brian.

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