help with 4012 NAND !?!

Discussion in 'General Electronics Chat' started by rnehrboss, Jan 27, 2010.

  1. rnehrboss

    Thread Starter New Member

    Jan 27, 2010
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    using 12 VDC input to 4 input NAND gate 4012 chip. Ouput works as expected, but only drops to 6 volts instead of 0 for output low? Any thoughts?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Show us a schematic of how you are doing this.

    Are you grounding the unused pins, and how are you feeding logic into the gate are using?

    If I was doing this it would be on a protoboard, with wires going between Vcc and Gnd. All unused inputs should be grounded.

    The little details matter.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Sounds like you are asking it to sink more current than it is capable of sinking.
     
  4. Wendy

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    Mar 24, 2008
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    Possible, my suspicion is some of the pins have not been taken to a clean logic state, so the chip is floating in places.

    For the OP: The base assumption I'm making is you are not loading the output of the logic gate, but using a DVM to measure this level directly. CMOS does not have much drive (almost none as a matter of fact), but they also have extremely high input impedance, which compensates nicely.

    If you need to drive something like an LED I recommend the following circuit.

    [​IMG]
     
  5. rnehrboss

    Thread Starter New Member

    Jan 27, 2010
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    Thanks guys.

    continuing:

    I switched to driving a IRL520 MOSFET. This works great. Solenoid engages when all inputs to the 4012 are tied to 12 volts.

    Problem is, the NAND gate is burning out (or something?) After a short time, pulling the inputs high (to 12vdc which is what VDD to the chip is) the ouput of the NAND no longer goes to 0 vdc, but 6-7vdc.
    I pull the inputs low through a 47k resistor, and pull them high, directly taking them to 12 volts. Any ideas?
     
  6. MikeML

    AAC Fanatic!

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    Are you driving just the gate of the IRL520 with the output of the 4012, or is there a resistor in there somewhere?
     
  7. Wendy

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    Mar 24, 2008
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    When driving an inductive load such as a relay or solenoid you MUST have a diode across the coil. This absorbs inductive spikes that will burn out voltage sensitive components such as the MOSFET or CMOS gate.

    Something like this (by way of example):

    [​IMG]

    Typically a 1N4001 diode will do, though if the coil pulls a lot of current you may want to increase the current requirement of the diode.
     
  8. rnehrboss

    Thread Starter New Member

    Jan 27, 2010
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    Thanks... still having the problem... even without the MOSFET

    RE No untied inputs. I assume this is only on one quad of the two quad input 4012 chip. I'm pulling all inputs low through 47k resistors, but one quad input I'm not using at all.
    I've disconnected the MOSFET and just measure the output of one of the quad input NAND gates. Doing this, a new chip "breaks" without any other circuitry.
    Could the problem be straigt to 12v high? Again, they are tied to ground through the resistors for the low state.
    Help!?!
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    We would like to help you.

    However, until you post a schematic of your circuit precisely as it currently exists, we won't make much progress, and you will probably remain frustrated.

    It's just too difficult to adequately convey in words what a schematic says in moments. Besides, you may discover the problem yourself, once you start the process of properly documenting the circuit.
     
  10. rnehrboss

    Thread Starter New Member

    Jan 27, 2010
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    Thanks again. I've attached my circuit diagram. The chips keep breaking, even if I don't have anything hooked to the output of the 4012.

    Again, any help is appreciated.[​IMG]
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
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    The inputs are all floating. That will cause rapid failure due to uncontrolled oscillation.

    What are those devices attached between the IC output and Vcc?
     
  12. Wendy

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    Mar 24, 2008
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    I mentioned that on the second post. You must ground any inputs that aren't in use, or if it will kill the logic functions that inputs to a high.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Along with tying any unused inputs to either ground or +V, you should have an 0.1uF capacitor across the IC's supply pins.

    Check the diode across the coil you're switching to make sure it's good. If it's blown, you will be zapping the 4012's.
     
  14. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    What is the complete part number of your 4012? It sounds like you maybe using a 5V part.
    When you save/post schematics, use .png or .gif instead of .jpg. The schematic will come out much clearer.
     
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