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Why are relays used in circuits?

  1. Jan 27, 2016 #1
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    I'm honestly just not getting this. What is the purpose of a relay in a circuit? What I understand so far is, a lower current somehow activates a larger one. I know how they work and their parts, but I'm not so sure on their purpose. How does a small current all of a sudden activate a larger one? Where do the added amps come from? Why is a larger current even needed from a relay?

    An explanation is greatly appreciated, thanks.
  2. jcsd
  3. Jan 27, 2016 #2


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    It is just a mechanical switch: you can allow current flow by connecting wires. There are no "added amps", you have two completely independent circuits. One can open and close a switch in the other circuit.
    Typically you have a low-power control application switching on something that needs much more power.
  4. Jan 27, 2016 #3
    A relay could also be solid state, a transistor is essentially the same thing.

    OP- think of it like a dam full of water. By pulling a lever, you use a small amount of force to open the floodgates and release the water, which generates a LOT more force. Your small force activated a larger one. The force you applied on the lever did not suddenly be one much stronger, instead the stronger force was independent of your movement, which only allowed it to flow. Just substitute force for current in my analogy: a small current "opens the gate" and allows a larger current to flow. The two currents are generated seperately.
  5. Jan 27, 2016 #4
    I invented this when I was 8 or 10 years old. With a Meccano electromagnet. Proudly showed it to my father.

    I was very disappointed when he said: "Ah, nice, you made a relay."
  6. Jan 27, 2016 #5


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    there's nothing "all of a sudden" about it --- it's just one circuit activating another

    The power supply being used

    It's all about being able to safely and conveniently switch loads with high current requirements

    3 classic examples from the auto electrical ( vehicle/car electrical systems)

    1) the headlights
    2 horns
    3) starter motor

    -- they all draw lots of current from the car battery. But you don' t want all the current going all the way from the battery up to the steering column
    where the switches for those things are generally located. You would need lots of heavy duty cable and switches to be able to handle that current

    instead, light gauge cable and switches are used that just carry low current and they operate relays closer to the lights, horn and starter motor
    This keeps costs and voltage drops to a minimum

    here's a very simple example from every day electronics

    Arduino Relay-Motor Sw.GIF

    The Arduino micro controller doesn't have the capability of switching high current loads
    so we use it to switch a transistor on that in turn turns on a relay and allows the larger current required by the motor to flow

  7. Jan 27, 2016 #6


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    Here's a substation circuit breaker (like a relay) where a "small" current opens the switches to a "large" current supply. You really don't want the large current circuit mix with the small current circuit.
  8. Jan 28, 2016 #7
    Why do we need a small current to activate a larger one? Is a relay just like a switch, but one that operates automatically if its threshold current is reached?

    Where is the larger current generated?
  9. Jan 28, 2016 #8


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    Could be automatically or manually; but that depends on how the relay is wired into the circuit. Start a motor that mechanically opens a switch, turn the key on your auto causing the starter motor to engage, ... The relay as a component on a circuit board generally is just a small solenoid that closed a switch. In the majority of cases, there is electrical isolation between the two parts e.g. coil, contacts.
  10. Jan 28, 2016 #9
    Dave answered your questions, I just gave an analogy to understand a relay.
  11. Jan 29, 2016 #10


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    @Metals: if you had a doorbell push switch and you wanted to operate a large electromagnet with a car battery by using the switch directly in the circuit, the switch would just frazzle up, the first time you pressed it. If you used a beefy relay, you could activate that with a fraction of an Amp from the switch and the Relay could then switch the current to the electromagnet from the battery.
    Read this a few times and try to make sense of it. Don't expect every answer you get to be in preceisely the form you wanted. (Have you looked at any "Relay Circuits" on Google Images?)
    Last edited by a moderator: Jan 29, 2016
  12. Jan 29, 2016 #11
    Apologies for not responding to this. How does a relay only take a small current, then let out a much larger one? I just don't understand how that works.

    I'm afraid I'm not completely understanding some of the circuits. Could you give me the parts of the circuit mentioned above in order? I assume it begins with the car battery.
  13. Jan 29, 2016 #12
    It is nothing more then an electrically operated switch. When you operate a switch by hand on the wall (not a relay) you are not supplying any electricity - when you move the switch, the electrical contacts complete the circuit from the utility to the light bulb ( it needs to be a complete loop = circuit) - and the lights turn on.

    In a relay - an electromagnet moves the switch, to turn on the higher power (usually) circuit.

    Thinking of it as lower "current" is not 100% right all of the time - it is really that one electrical circuit - activates an electromagnetic coil, that activates a switch in another circuit.
  14. Jan 29, 2016 #13


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    I ask again - Have you looked elsewhere (which is what I would do if I could be bothered) with a Google search for Relay Circuits. We don't spoon feed people on PF and it would be nice to see a sign of some effort on your part.
  15. Jan 29, 2016 #14

    jim hardy

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    Thought experiment

    1. Make a robot that stands next to your kitchen light switch and obediently turns the kitchen light on when you tell it to.
    That's a voice actuated relay.
    2. Make a bigger robot that stands next to the main light switch at Yankee Stadium and throws that huge switch to turn on all the field lights when you tell it to.
    That's a bigger voice actuated relay.

    Now wire a flashlight battery and small switch to both of your your robots and tell them to act whenever they feel the 1.5 volts from the flashlight battery. That's an electrical operated relay, a small current controlling a large one.

    Your foot on the gas pedal of your car controls perhaps 175 horsepower with one toe-power. Same general idea.
  16. Jan 30, 2016 #15


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    Thanks to all contributors. The topic has now been well covered, so we can close this thread.
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