Relay circuit

Discussion in 'General Electronics Chat' started by allenpitts, Mar 27, 2011.

  1. allenpitts

    Thread Starter Well-Known Member

    Feb 26, 2011
    Good afternoon All About Circuits,

    Can't believe I have to ask such a basic question but If I knew I
    wouldn't have to ask. The diagram below is from the manufacturer's
    spec sheet for a five volt relay. The frustrating thing is no where does it actually say how it works but from research and studying I believe that
    when power (nine volt battery ok?) is applied to A and C (postive to A and ground to C [Does it make any difference?]) a connection is made through B and E or B and D. True? If that is true under what circumstances does B connect to D and under what circumstances does B connect to E?


    Allen in Dallas
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    For the relay, B-E are the Normally Closed contacts (NC). They will be in contact with no power applied to the coil. B-D are the Normally Open contacts(NO). You have to power the coil to make that contact.

    The part number may be used in a search engine to find a data sheet -

    The coil voltage is a bit obscure, as the data sheet says it can be anything from 3 volts to 48. If the coil measures 70 ohms, it is the 5 volt coil. I would think the 12 would indicate 12 volts (just because it makes sense).

    Anyway, the coil is DC, and has no polarity. If your 9 volt battery doesn't do anything, then it may be a 12 volt coil. If it's 5, the current draw is too high for the battery, and it will only last for a few minutes. 15 ma is about as high a drain as you want on a 9 volt battery.
  3. SgtWookie


    Jul 17, 2007
    Actually, looking at that same datasheet, I take the 112D portion of the part number to mean:
    1 - Single pole (all of those type relays seem to be single pole)
    12 - 12V coil (it should measure about 400 Ohms, and draw ~30mA)
    D - Standard DC coil.
    Does your relay have a suffix code on the D?
    If not, it is a form "C" (one NO terminal, one NC terminal, and a common terminal.)
    If there is an "M", it has one form "A" (NO contacts; two terminals).
    If there is a "B", it has one form "B" (NC contacts; two terminals).

    It will take at least 9v to cause the relay to operate. You are trying to use a 9v battery, and unless it is an "industrial" type 9v battery that has 7 cells internally, it's just not going to have enough voltage to make the contacts switch.
  4. sharma@vivek

    New Member

    May 2, 2010
    Since B-E is already connected these terminals are referred as NORMALLY CLOSED contacts in short NC,. this is the situation of the relay when it is not energized, and here B-E is known as the NORMALLY OPEN contact also known as NO., Once if u apply a battery source across A-C the contacts switches to B-D from B-E . This type of relay is called as single pole double contact... Here B is ur pole..
  5. eblc1388

    AAC Fanatic!

    Nov 28, 2008
    B-E is the NC contact, B-D is the NO contact.

    The common is B.
  6. allenpitts

    Thread Starter Well-Known Member

    Feb 26, 2011
    Good afternoon All ABout Circuits,

    Thanks to BeenThere and Sgt Wookie. BeenThere's explanation of the normally closed between B and E and normally open between B and D was the key. I could prove the nomally closed B to E connection with a simple resistor/LED circuit. But Sgt Wookie's input was also important because I was using a nine volt battery across A and C
    to get a continuity between B and D with no luck. The idea that I needed more than nine volts to close the contact between B and D was essential. I hacked an old twelve volt wall wart and it worked like magic. The cool part was hearing the relay
    click when it released the contact. Thanks guys!

    Allen in Dallas