Hello friends!

I have a board which requires an external signal input. The external signal can vary in voltage and could be either AC or DC.

Is there a way to trigger a relay when any signal is detected? Possible to use some kind of current sensor to trigger the relay?

Thank you,
All the best!

 

If you specify the voltage limits (a.c. and d.c) when you want the relay to be energised (and the maximum voltages expected) – then I would think someone will be able to advise how such a circuit could be realised.

 

Voltage limits will be 24-48v AC/DC and sometimes 120v AC/DC.

120v is not necessary as it will be rare to run into a 120v signal. But would be nice.

Thanks!

 

Voltage limits will be 24-48v AC/DC and sometimes 120v AC/DC.

120v is not necessary as it will be rare to run into a 120v signal. But would be nice.

Thanks!

That is lot different from “any signal”. Do you see how that is so much easier than driving the relay from 1mV?

Next question: is there an additional power source, or must the power for the relay come from that input voltage? If the latter, how much current is available from the input?

 

That is lot different from “any signal”. Do you see how that is so much easier than driving the relay from 1mV?

Next question: is there an additional power source, or must the power for the relay come from that input voltage? If the latter, how much current is available from the input?

Yes

Yes, the board will power the relay. The external signal comes from a different power source.

The external signal will provide ~100ma.

Thanks!!

 

the board will power the relay.

So what voltage is available to power the added circuit to drive the relay?
Does the circuit need to drive the relay coil directly?

 

Hello friends!

I have a board which requires an external signal input. The external signal can vary in voltage and could be either AC or DC.

Is there a way to trigger a relay when any signal is detected? Possible to use some kind of current sensor to trigger the relay?

Thank you,
All the best!

AC/DC---->Bridge Rectifier---->CCR---->Opto---->DC Relay driver---->Relay

 

So what voltage is available to power the added circuit to drive the relay?
Does the circuit need to drive the relay coil directly?

The board will provide 5vdc. And no the circuit shouldn’t drive the relay directly.

AC/DC---->Bridge Rectifier---->CCR---->Opto---->DC Relay driver---->Relay

What is a CCR? I couldn’t find any info. And what kind of relay driver do you have in mind?

Thank you!

 

The board will provide 5vdc. And no the circuit shouldn’t drive the relay directly.

To continue our game of 20 questions--
So what exactly is the volage and current of the signal that controls the relay?

 

To continue our game of 20 questions--
So what exactly is the volage and current of the signal that controls the relay?

Here are all the details:

I have a 5v relay with it’s own 5v power supply.

I have an external signal which varies in voltage ranging from 24-120v AC or DC, providing ~100ma of current.

I need the external signal to actuate my 5v relay. The relay and the signal should be isolated. I can’t use the external signal to drive the relay directly.

Hope that clears things up.
Thank you all!

 

Hope that clears things up.

Nope.
Still doesn't answer my question about what is the signal volage and current needed to activate the relay.
Do you not know that?

 

The board will provide 5vdc. And no the circuit shouldn’t drive the relay directly.
What is a CCR? I couldn’t find any info. And what kind of relay driver do you have in mind?

Thank you!

CCR=Constant Current Regulator

Bridge Rectifier will convert AC to DC, or just pass DC. The CCR will limit current thru the LED of an optocoupler.
The Optocoupler will turn on a transistor that will drive the relay coil.

 

Nope.
Still doesn't answer my question about what is the signal volage and current needed to activate the relay.
Do you not know that?

The board interfaces with different devices. So the signal voltage will vary.

CCR=Constant Current Regulator

Bridge Rectifier will convert AC to DC, or just pass DC. The CCR will limit current thru the LED of an optocoupler.
The Optocoupler will turn on a transistor that will drive the relay coil.

This one sounds good!

Thank you so much!!!

 

he board interfaces with different devices. So the signal voltage will vary.

Okay, we have a failure to communicate.
I fully understand what the signal voltage is to the interface circuit.
My question is, what output voltage and current does the interface circuit need to generate to operate the relay circuit?

 

CCR=Constant Current Regulator

Bridge Rectifier will convert AC to DC, or just pass DC. The CCR will limit current thru the LED of an optocoupler.
The Optocoupler will turn on a transistor that will drive the relay coil.

It is the right idea, I think isolation is good here, but I don’t think you will easily find a constant current source that can handle 170V peak voltage. You will likely have to design your own.

 

It is the right idea, I think isolation is good here, but I don’t think you will easily find a constant current source that can handle 170V peak voltage. You will likely have to design your own.

OnSemi makes them (attached).

 

Okay, we have a failure to communicate.
I fully understand what the signal voltage is to the interface circuit.
My question is, what output voltage and current does the interface circuit need to generate to operate the relay circuit?

The relay circuit has it’s own 5v power supply with relay. It doesn’t need to generate anything.

OnSemi makes them (attached).

Thank you for this! Although it says it’s 45v max in the datasheet. I don’t think I will find a semi that can handle 24-120v range. I think I will have to exclude 120v signals from the board.

Thank you!

 

The relay circuit has it’s own 5v power supply with relay. It doesn’t need to generate anything.




Thank you for this! Although it says it’s 45v max in the datasheet. I don’t think I will find a semi that can handle 24-120v range. I think I will have to exclude 120v signals from the board.

Thank you!

It will work with the correct configuration

 

The relay circuit has it’s own 5v power supply with relay. It doesn’t need to generate anything.

Okay, I'm about ready to give up.
One last try--
I know the relay has it's own power supply but there is a signal that turns it on (ignore the interface circuit), correct?
So , what is the voltage and current required for that signal?

 

The relay circuit has it’s own 5v power supply with relay. It doesn’t need to generate anything.




Thank you for this! Although it says it’s 45v max in the datasheet. I don’t think I will find a semi that can handle 24-120v range. I think I will have to exclude 120v signals from the board.

Thank you!

Crutchow is asking how the relay is currently being activated. He is referring to the control signal not the power supply or the relay itself. There might not be a control signal like with car door locks as the circuit is activated for a short time.

An easy solution is to use a peak detector circuit with a comparator and optocoupler. Voltage over the threshold will turn on the output. Use voltage dividers at the input to make sure IC's do not get damaged by overvoltage. Use an RC circuit or 555 timer to provide delay or lockout functions if desired. If you set up the voltage dividers properly, you will be able to trigger from a variety input voltages.

Peak detectors work by charging a capacitor to the supply voltage. Since capacitors don't pass DC, the highest voltage appearing on the line will be temporarily stored in the capacitor as a voltage available at the output. This voltage is then compared against the preset values provided by the voltage divider(s).