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74 chips hogging current

Discussion in 'General Electronics' started by jza, Jan 31, 2004.

  1. jza

    jza Guest

    I wired four 74 series TTL chips to a standard 7805 regulator circuit. I'm
    having a problem where every chip takes huge amount of current (35 mA or so).
    The 7805 starts of course to heat and the output voltage raises over time until
    it's near the input voltage (strange, I thought 7805s could supply even 1 A
    without problems like that?). What's going on, shouldn't a TTL chip normally
    consume only something like 2 mA? :) I first thought it was cause of some kind
    of short-circuit but it isn't.
     
  2. Quark Ng

    Quark Ng Guest


    I agree that 74 series chips should use very little current. I would
    first do the following.

    Take 3 AA batteries and wire them in series. That'll give you 1.5 * 3
    = 4.5 volts.

    Power up your TTL chips with the batteries. That is, by-pass your
    7805 chip.

    If the current is still high, then the problem is either bad logic
    chips OR the power and ground pins were mis-wired OR you have two
    outputs fighting each other. BE CAREFUL - do not let the batteries
    heat up too much.

    If the current is low, then you may have a bad 7805 chip. I think
    this may be the case because normally over-current would cause the
    output voltage to fall - not rise.

    Anyways, please do the above experiment and tell us what you get.
    Cheers!

    -Quark
     
  3. 35 mA a huge amount of current for TTL? Are you kidding? It takes 1.6 ma
    just to pull one input low. Data sheet for a 74161 counter shows 60-100 ma
    for the part, under conditions that aren't worst-case. What do the data sheets
    for your chips say? Almost certainly can get them at ti.com.

    Depending on the voltage dropped across the 7805 and the heatsinking provided,
    it can under some conditions supply 1A. Don't confuse best-case with guaranteed
    numbers. I agree the 7805 may be damaged, since the voltage shouldn't rise like
    that. Have you checked for oscillations due to inadequate bypassing?

    Dave Kinsell
     
  4. jza

    jza Guest

    Take 3 AA batteries and wire them in series. That'll give you 1.5 * 3
    Done. Well, I used a single 4.5V battery but I guess it does the same
    thing.
    The current is still about 35 mA. By the way, the chip I'm testing
    with is a 74154 (a 4-to-16 demux). I just connected Vcc and GND.
    Before I also tried whether it was the floating inputs, but wiring
    them didn't make any difference.
    I've actually tried with two different 7805s - I guess the first one
    burned already. :)

    One thing to mention. The power transformer is a second hand 9V DC
    supply which had strangely reverse plus and minus (white is actually
    plus and red is minus). It is however supposed to be +9V, not -9V.

    How sensitive is the regulator with the values of the condensers
    around it? I used something like 0.1 uF and 0.3 uF.
     
  5. If you would look at the data sheet, typical current draw is 34 ma with
    a max of 56. Perhaps everything is working just like it's supposed to.
     
  6. The Captain

    The Captain Guest

    74 TTL is a current eater. A simple solution would be to use either
    CD4000 equivalents or CMOS 74 series chips.

    One reason for the original development of CMOS logic chips was the
    huge current drain of TTL.

    Cap
     
  7. jza

    jza Guest

    You are probably right. Well, I replaced the 7805 with a heavier version and
    changed the power supply. Nothing burns now!
     
  8. Externet

    Externet Guest

    Hello.
    Replace your 74xx chips with 74LSxx chips.
    Miguel
     
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