I have this circuit and I need help to understand why it doesn't work please

Thread Starter

slevesque

Joined Jan 31, 2023
43
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.
1684932901796.png

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.
 

Audioguru again

Joined Oct 21, 2019
6,671
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.
 

Audioguru again

Joined Oct 21, 2019
6,671
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.
 

Thread Starter

slevesque

Joined Jan 31, 2023
43
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?
 

Thread Starter

slevesque

Joined Jan 31, 2023
43
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?
 
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