Conjoined Twin 555 Timer(s)?

Discussion in 'General Electronics Chat' started by luvv, Jan 11, 2013.

  1. luvv

    Thread Starter Member

    May 26, 2011
    186
    31
    Just some random thinking leading to random questions..

    What would happen if you where to physically stack two DIP chips parallel configured in astable mode.

    Would slight differences cause one to be "dominant" ?

    Would it double it's I/O - V/I tolerances?

    Would a black hole open up on the breadboard?

    I've seen pictures of ram done this way,just wondering is there any case of other chips being done similarly and to what effect.

    I would just attempt to force the smoke out of a couple timers just to satisfy my curiosity..but...

    Unfortunately I haven't any handy:(


    Thanks,
     
  2. BReeves

    Member

    Nov 24, 2012
    412
    64
    Can see no reason why anyone would ever want to do this, just add a transistor to the output if you need more power. The reason you see RAM wired this way is most of the leads are common to the processor and it's one way to increase the amount of RAM without having room on the circuit board for aditional chips.
     
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    RAM is wired like this because it is designed to connect like this, the outputs can be swithced off so only one ram chip is active at a time. With a 555 it won´t give you any advantage, just possible unstable operation.
     
  4. luvv

    Thread Starter Member

    May 26, 2011
    186
    31
    Wasn't really a question of possible benefits,more of question of what would be the outcome.

    I assumed "instability" but of what type and why?

    So far as RAM is concerned,it's the only example I had seen,but surely it isn't the only time it's been done.

    Parallel mosfets,voltage regulators,SCRs all come to mind,tho they lack the complexity of something like the 555.

    Suppose when asking a question about something that is never done it's best to do it and find out.

    Thanks for the input guys.
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Mostly when you connect the outputs together, one migt change level sooner than the other, and this would draw high current from the supply. The collapsing supply rail under the load could make the input cross the other trigger level and cause the whole circuit to oscillate.
     
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    The 555 changes state when the voltage on its threshold pin matches an internal voltage made by it's internal resistor network.

    That internal voltage will vary with different chips, so you would definitely get that problem where the two 555 timers have differing outputs and high fault currents if those outputs are connected together.
     
    luvv likes this.
  7. JingleJoe

    Member

    Jul 23, 2011
    185
    10
    To get around the output shorting problem you could connect the outputs through diodes, with that problem out of the way I can't think of any other advantage or difference in operation apart from lowered input impedance of some of the input pins... I get a feeling that the transistor (now two transistors) with collector to pin 7 might not be very happy when used in the standard astable circuit arrangement. I'm no expert in that field but I've seen two transistors connected like that to make some unusual circuits in which the transistors "fight" for current ... seems like just another thing which may lead to random oscillation.
    Damn, now I want to get a 556 and have a play with combined 555 circuits :D
     
    luvv likes this.
  8. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    The 555 switch to either pole of the battery, if the two outputs are in different states you have created a dead short through both chips.
     
  9. luvv

    Thread Starter Member

    May 26, 2011
    186
    31
    Guess the only way it could function as a astable is if every aspect of the chips where truly identical..

    Somewhere,these two chips exist..just not likely in my 1st order...

    Now I just need help building a precision device to compare thousands till I find them..

    Anyone have a schematic handy?




    Only kidding,thanks or the response guys.
     
Loading...