Relay circuit

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

  1. allenpitts

    Thread Starter Active 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

    Senior Member

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

    The common is B.
  6. allenpitts

    Thread Starter Active 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