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Figuring out how a Relay circuit functions

  1. Jun 8, 2012 #1
    I'm trying to figure out how a relay functions in a circuit and I'm having trouble figuring why it has the effect it does in a circuit. I set up a simple circuit (see picture) using a 5 pin [ouaz-ss 105D] relay where the LED on the bottom lights up while the push button is not pressed. When the push button is pressed, the relay switches over to the LED on the top.

    Now, I understand how a relay works for the most part (I drew a schematic of the circuit I made but PLEASE correct me if there are any mistakes in my diagram) but here's my dilemma. When I remove the relay from my circuit (physically pluck it off the circuit board), both LEDS light up. As you can see in my schematic, the relay seems to be in parallel with the two LEDS which leads me to the following question:

    Why wouldn't the current still travel across both LEDS when the relay is still part of the circuit and light up both LEDS? Shouldn't current travel across both branches (through both 1KΩ resistors and then the LEDS) in this parallel circuit? Is there something significant about having one of these branches grounded (via the relay) which stops the flow of current through the other branch?

    Any help would be very much appreciated!

    Attached Files:

  2. jcsd
  3. Jun 8, 2012 #2


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    Staff: Mentor

    So it is performing all that, or not?
    Which LED shows reduced brightness?

    How many ohms is your relay coil?
  4. Jun 8, 2012 #3


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    Science Advisor
    Gold Member

    I am going to assume that the common terminal on the set of contacts on your relay is hooked to the negative side of the 5 volt source that powers the LEDs. I cannot really know for sure. You are going about it the wrong way in my opinion. Fully understand how the circuit works without the relay first. Both LEDs should light up. I would expect that. When the relay is put in, one of the LEDs will always be shorted out. Current flows through something, not across it. Get to know ohms law and kirchoff. If you understand those, your circuit will make sense.
  5. Jun 8, 2012 #4
    Okay, I went back and reviewed how relays are wired and I realized that I had incorrectly wired my circuit. The LEDS should not have been grounded but rather should have been in series with the relay via the NC and NO contacts. Thanks for bringing up the common terminal because that part was what ended up confusing me the most.

    Also, now when I pluck the relay off the breadboard, none of the LEDs light up. Just the way it should be.

    Thank you very much for all your help.
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