IC Power Dissipation Questions

Discussion in 'General Electronics Chat' started by atrumblood, May 28, 2012.

  1. atrumblood

    Thread Starter Member

    May 13, 2012
    59
    2
    I am trying to figure out the total Power dissipation is for a 555 IC and a 4017 Johnson Counter.

    The 555 is connected in Astable mode and the current is reading 150mA.




    My LM7805 is getting very HOT. Too hot to touch. When I measure the 4017 I am showing just from the VSS to common ground around 350mA and at 5V that is 1.7W. This doesn't sound right to me but I am having trouble locating the information in the datasheet that tells me how much current is typically used by that IC.

    I am using a resistor on the output clock because without it the lm7805 gets hotter faster.

    Attached is a picture of my breadboard.

    The LM7805's input is supplied by a 12V 500mA adapter.


    The datasheet is found here : http://pdf1.alldatasheet.com/datasheet-pdf/view/17686/PHILIPS/HEF4017BP.html


    Can anyone tell me where and or if I am going wrong about this?
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,026
    3,236
    Something is seriously wrong with your circuit. The 555 draws no more than 15mA with no load. What is the output load resistor for the 555?

    The 4017 only draws microamps (not milliamps) of current under low frequency operation (see Quiescent Device Current, Idd Max in the data sheet) so you are really off there. From your breadboard picture I don't see any connections to most of the 4017 inputs. That's a no-no. All inputs have to be tied to V+ or ground, as required by the circuit operation.

    Post a schematic.
     
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  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Looks to me like the 555 is plugged in backwards.
    On the 4017, connect 13 and 15 to GND. never leave unused CMOS inputs floating, even if the gate is unused.
     
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  4. atrumblood

    Thread Starter Member

    May 13, 2012
    59
    2
    crutschow: Maybe I am measuring in the wrong spot; in fact I think I was. Am I correct in measuring the Current usage of an IC by putting my meter in serial with the Vcc pin/ Gnd pin?

    If so then my 555 is only drawing ~8mA without a load, before I was measuring the current being sourced from the output pin. Like the newb that I am.

    However I still read ~350mA from the Vdd pin on the 4017.

    The load resistor is 220Ω

    Ron H: Now in regards to leaving no inputs without value (non floating). Does this apply to outputs as well. Such as Q0-Q9; or just Vcc, Gnd, C0, C1, and MR (master reset)?

    Also can you explain how the 555 appears to be connected backwards.

    It should be noted that I have both positive rails tied to each other and both ground rails tied to each other in case there was any confusion on my bread board.


    Attached is the schematic. (NOTE: the 4017 schematic symbol is not the same model I am using. I have connected all inputs to a value, none of the outputs are currently tied to anything)
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,447
    3,363
    Yes, do not measure the current at the output pin, only at the Vdd pins (orange wires on your breadboard).

    Disconnect the 555 from the 4017 and measure the Vdd supply current separately.

    If you are measuring 350mA drawn from the 4017 then something is definitely wrong.
    Make sure all unused inputs are connected to Vdd or GND.
    Never connect outputs to Vdd or GND. Leave all unused outputs unconnected.

    Disconnect power to your circuit. Make the changes stated above.
    If the 4017 is still drawing more than microamps then it is toast.
    CMOS circuits are susceptible to a phenomenon called CMOS Latch-up which will destroy the chip in a hurry.
     
    Last edited: May 29, 2012
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  6. atrumblood

    Thread Starter Member

    May 13, 2012
    59
    2
    MrChips: Thank you, as you instructed I have tied all inputs to a value (Vcc or Gnd) and still got 350mA. I took that chip out and inserted another of the exact model. This time since my meter is not sensitive enough I pick up no current reading. It seems that first chip I tried out was bad or I damaged it some how.

    Thanks again for your help and of course everyone one else who contributed.
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    See the attachment.
    It looks like the IC is plugged in backwards, and pin 1 doesn't seem to be in the hole (which is the wrong hole).
     
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