HELP Transistor selection

Discussion in 'The Projects Forum' started by tigerapna, Mar 11, 2012.

  1. tigerapna

    Thread Starter Member

    Dec 14, 2011
    73
    1
    Hello,

    I am making a circuit which controls two relays and there is a timer to prevent the first relay from being on for more than an hour.

    I need some design critique and part selection help.

    Will this circuit work?

    Values for:

    R10, R11, Q2 and Q3.

    I have attached a jpeg image.

    THanks for looking and responding!
     
  2. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    Any plain old NPN transistor in a to-92 package will do if it can stand at least 35 volts. The resistors should give the transistors a tenth of the current that the relay coils require.
     
  3. DickCappels

    Moderator

    Aug 21, 2008
    2,650
    632
    I am sorry, I don't have time to review your circuit in detail right now, but I noticed that you show the relays connected directly from +24V to ground, and the transistor also connected between the +24 volts and ground. If the transistor ever tried to turn "on" fully, it would try to short the power supply.

    I suggest you get this corrected before building the breadboard.:)
     
  4. tigerapna

    Thread Starter Member

    Dec 14, 2011
    73
    1
    I totally missed connected the transistors. I realized it after I submitted it lol.
     
  5. tigerapna

    Thread Starter Member

    Dec 14, 2011
    73
    1
    i corrected the shorting transistors.
     
  6. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    Schem looks much better--use the ubiquitous 2N4401
     
  7. tigerapna

    Thread Starter Member

    Dec 14, 2011
    73
    1
    Thank you so much:).

    Is there anything specific I should be looking for when selecting an AND GATE IC?
     
  8. tigerapna

    Thread Starter Member

    Dec 14, 2011
    73
    1
    I noticed my relay one flicking off and on very fast before quitting.

    Can I not connect the two outputs of an AND gate to make an OR? Do I have to use and OR instead:confused:?

    My transistors also blew:confused:. How do I calculate Rb? Should I use two transistors to turn on the 24VDC Relay? (one NPN and one PNP)?

    I was using PN2222A transistors.
     
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