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Relay Assistance

Discussion in 'Sensors and Actuators' started by Luke Frost, Apr 6, 2018.

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  1. Luke Frost

    Luke Frost

    2
    0
    Apr 6, 2018
    Hi All,

    I'm really sorry to ask what I'm sure is a really stupid question however I'm really struggling to work it out. I have a basic working knowledge of electronics however relays are something that i have always struggled with.

    I have a 24v dc supply that is powering my project but i want to power a 12v dc led that when the power supply is active. i have a step down transformer to convert the 24v to 12v.

    So the question i have is what relay do i need. Am i correct in thinking that i need a 24v dc relay so the 24v input goes into the relay and activates the relay which then turns the 12v light on. Secondly with the actual does the output just switch say the positive like you would with a normal switch?

    I look forward to hearing from you.

    Thanks in advanced

    Luke
     
  2. kellys_eye

    kellys_eye

    4,279
    1,146
    Jun 25, 2010
    No extra relay required. Just fit the appropriate resistor for the LED to operate at 24V and connect it across the relay coil.

    If it is a 'true' 12V LED then it will already have a series resistor fitted (soldered to one pin?) that allows it to operate at 12V. You could find that resistors value and replace it with one of double the value or just fit another of the same value in series with it.
     
  3. hevans1944

    hevans1944 Hop - AC8NS

    4,321
    2,028
    Jun 21, 2012
    There is no such thing as a "step down transformer" that works with DC. You can purchase a "buck converter" that will accept 24 V DC as input and produce 12 V DC as output.

    LEDs can be powered from any DC voltage source that provides a greater voltage than the forward voltage drop across the LED. All that is needed is a current-limiting resistor to absorb what excess voltage there is and to provide a means of limiting the current to LED. This isn't very efficient for higher-wattage LEDs however. If your "12 v dc led" is a commercial off-the-shelf product that requires 12 V DC to operate, powering it from your 24 V DC supply by means of a buck converter is your best bet. No relays required.

    If you have a separate power supply for the LED that will provide 12 V DC, you can use a SPST relay to connect this power supply to the LED load, using your project 24 V DC supply to energize the coil of the relay. Most common relays are SPDT or DPDT, having a moving contact (armature) that alternately connects to the normally-closed relay contact(s) when the coil is de-energized, or to the normally-open contact(s) when the coil is energized. You would connect the armature terminal(s) and the normally-open terminal(s) of the relay between your 12 V DC supply and your 12 V DC LED load.

    There is no such thing as stupid questions. Plenty of stupid answers are available.
     
  4. Luke Frost

    Luke Frost

    2
    0
    Apr 6, 2018
    Hi Hevans

    Thanks for your very information and detailed reply. In regards to the transformer apologies I didn't realise it would be a buck converter that I am using as its taking a 24V DC input and outputting a 12V DC.

    The LED is a standard off the shelf one (https://www.maplin.co.uk/p/8mm-panel-mount-orange-with-plastic-bezel-led-n03bb). I do have a separate Power Supply in the form of the buck converter as there is also a 12V fan in the case so based on what your saying a SPDT Relay would be a good option. What do I need to be looking for in terms of obtaining this SPDT relay to ensure that it will work as from looking around there are so many different sorts. Would something like this be ok https://www.maplin.co.uk/p/15a240vac-spdt-miniature-relay-n07aw

    Luke
     
  5. kellys_eye

    kellys_eye

    4,279
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    Jun 25, 2010
    Connect your LED to the 24V supply via a 1kΩ resistor.

    As soon as the 24V supply is on (i.e. relay activated) the LED will light. No extra relay, no problem with the LED.
     
    hevans1944 likes this.
  6. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Yes, that relay would be okay if ordered with a 24 V DC coil, but you don't need the relay because your 24 V DC-to-12 V DC "bucking" power supply is connected directly to the output of your 24 V DC "project" power supply. When the "project" supply is ON, 12 V DC will be available from the "bucking" supply to operate your 12 V DC fan as well as your 12 V DC LED panel indicator. No relay is necessary.

    Or, as @kellys_eye has suggested in post #2 and later, in post #5, you can operate the 12 V DC LED panel indicator directly from the 24 V DC "project" power supply by just adding some more series resistance to the panel indicator, no relay is necessary.

    Please don't overthink this. Relays are used only when a control signal cannot supply sufficient current or voltage directly to a load, but said control signal is sufficient to operate the coil of a relay. The relay contacts would then take over to switch the proper current and voltage to the load from an appropriate power supply; in your case, the 12 V DC fan and the panel indicator powered from the bucking power converter. However, since the bucking power converter is being powered directly from the 24 V DC project power supply, no relay is necessary to connect its output to the fan and panel indicator.
     
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