Can you help me understand this relay?

Discussion in 'General Electronics Chat' started by solexious, Jul 17, 2008.

  1. solexious

    Thread Starter Active Member

    Nov 20, 2007
    55
    0
    Hello all,

    I'm trying to work out this: http://www.maplin.co.uk/Media/PDFs/fx88.pdf relay, they only had 2 left and I need both so i'm a bit hesitant of trying it with out checking im right first.

    As far as i can tell when the coil activates it closes the switch between 1/14 and 7/8. And to activate the coil i connect 6 to 5v and 2 to ground?

    And with the coil I can just pass 5v, I dont have to connect a resistor or capaster?

    Thank you

    Sol
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    You have correctly deciphered the pin outs. Well done.

    Now the part number needs a closer look. If the second-to-last-two digits in the part number are "05," then you do indeed use 5 Volts. If those two are "12," you need 12V, and if they are "24," you need 24V.

    You won't need a capacitor or resistor, but if the last digit in the part number is "1," you will need a diode across pins 2 and 6. If that last digit is "3," or "4," you won't need a diode.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Correct.

    No! You would either fry the internal diode, blow your power supply's fuse, or roast tracks right off your PCB! :eek:

    Pin 6 to ground, pin 2 and/or 13 to +5v.

    Correct. The coil has an internal resistance of 500 Ohms. It will require 10mA current.
     
  4. solexious

    Thread Starter Active Member

    Nov 20, 2007
    55
    0
    Thanks.

    Its 05 so the 5v version, but its 1 so doesnt come with a diode, do I have to add one myself? And if so how come?

    Thanks for the help :)

    sol
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Ignore what I said earlier about pin outs. The Marine is correct.
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    The diode protects against something called "inductive kick." This is a phenomenon which occurs any time we try to stop current from going through a coil. It can develop nasty big voltage spikes that eat components and spit out acrid smelling fumes. The diode simply shorts out the voltage spikes. Connect the anode of the diode to pin 6 and the cathode of the diode to pin 2.
     
  7. solexious

    Thread Starter Active Member

    Nov 20, 2007
    55
    0
    Great :) how do i work out what diode I need for the job?

    Sol
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, you should then add a diode to your schematic. Place it externally the same way that it shows in the schematic for the relay you're buying. That way when you have to replace it down the road, you can use a similar part whether it has the diode or not. And if you keep this thing (whatever you're building) you will eventually have to replace the relay.

    When current flows through a coil, it builds a magnetic field around the coil. When current suddenly stops flowing through the coil the magnetic field collapses, which tries to keep the current flowing in the coil. This causes the voltage polarity across the coil to invert, and the peak voltage can be VERY high, high enough to destroy CMOS and MOSFET devices in the neighborhood.

    Having a diode in parallel across the coil provides a current path for this reverse polarity voltage; they're sometimes called "flywheel diodes". The maximum reverse voltage is then limited to how quickly the diode can turn on, and what it's forward voltage is. For small devices like your relay, a 1N914 or 1N4148 switching diode are typically used. Any fast switching diode would work. Avoid slow rectifier type diodes.
     
  9. solexious

    Thread Starter Active Member

    Nov 20, 2007
    55
    0
    Amazing help guys, Will run down and get a couple tomorrow.

    Thank you for your help

    Sol
     
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