Hi,

I have tried a circuit and I am not sure why this is not working if someone could help me with this.

This is the circuit.


The coil of a relay is connected in serial with a LED. First I only tried to connect the coil with the LED in serial. The result was the relay was not activated but the LED was lighted.

Then I tried to add a parallel circuit that goes directly to the ground. Now the relay works, but not the LED. This one I can understand that the current will flow through the less resistance path which is going directly to the ground.

My question is why the circuit with the relay coil in serial with the LED is not working?

Thanks in advance.

 

An LED uses a low voltage of 1.8V to 3.6V. It will instantly burn out if connected directly to 28V or in series with a low resistance relay coil and powered from 28V.
Therefore, your LED has a series resistor to limit its current which will also limits the series relay coil current.

Please post the datasheet and part number of your LED.

 

This is the LED I am using FL1M-8CW-4-G24V
And the relay I am using EV100AAANA

 

From what I understand there is already an internal resistance with the LED.

 

Your LED is rated at 24V then it has a resistor in series with it to limit its current to 20mA when powered from 24V.
Your relay coil needs up to 1500mA to turn on.

 

Oh I see, so if I understand correctly, the LED and coil connected in serial create a too big resistance which limit the current lower than the requirement for the relay to work.

The relay coil than act as a wire and does not activate the relay?

 

To correct this I could change the LED for one with a lower resistance?

Or add another relay DPDT that is controlled by a switch to power that relay and the LED?

 

Putting them in parallel should work.

 

This should work then?



Coil in parallel with the LED, both controlled by a switch.

 

Yes, that will work, but you may want to add a series resistor to the LED because 28 volts exceeds the LED maximum voltage.

 

you may want to add a series resistor to the LED because 28 volts exceeds the LED maximum voltage.

Since the LED is rated at 20mA max, you want the added resistor to be a least 4V / 20mA = 200Ω with a 28V supply.

 

The LED already has current limiting to 20mA when it is powered from 24V. With 28V, the current is 23.6mA that is only a little more current. Then the LED might burn out after 43 thousand hours instead of the rated 50 thousand hours.

 

Thanks for the help guys!