Help with PTC Overload Indicating Circuit

Discussion in 'The Projects Forum' started by jhausch, Jun 7, 2009.

  1. jhausch

    Thread Starter Active Member

    Jun 7, 2009
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    6
    What a great forum and website. I really love the fact that you use vBulletin, too.

    I would like some help with the attached schematic. I am self-taught and always learning, so please forgive any misuse of terminology and feel free to "set me straight" where needed.

    I am using 4 banks of PTCs for motor protection of 4 individual motors. The banks of PTC's will flow as much as .6A per bank when in their high-resistance state (i.e., "tripped on overload").

    The idea is when the PTCs are in their low resistance state, I am turning the motor and pulling up the base lead of the PNP transistor. To do this, I go through a diode (to prevent the banks of PTCs from back-feeding each other), go through a 3V zener diode (to get the voltage down to an acceptable level for the transistor), and then go through a 100 ohm resistor as a load.

    When the motor overloads a bank of PTCs, the motor holds the bank of PTCs in the tripped state, but now the base lead of the PNP transistor is pulled low through the 1k resistor. This saturates/switches the PNP transistor and illuminates the 12V led (it is a pre-packaged LED with the necessary circuitry embedded to run off of 12v).

    When the motor switch is opened, the PTC bank cools and the overload indicating LED goes out.

    FWIW: the transistors are the TO-220 package so they can take the load of much more than the LED.

    Some of my questions:

    Does this look correct?

    Do I need those first diodes to prevent the back feeding of the PTC banks or do the transistors provide that for me?

    Have I sized those resistors correctly? I think the 100 ohm resistors will have 30mA/.09W flowing through them when PTC's are in the low resistance state. When in the high resistance state since there is only .6A available, so it won't "make it through" the resistor allowing the base to be pulled low (and allowing the opening of the motor switch to "clear" the PTCs)

    I know there are easier ways to probably do this with a CMOS IC, but I am going for simple, noise immune, and robust.

    Thanks in advance for the assistance.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,650
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    Hello,

    In the current situation the status can not be checked.
    The transistors are always in conduction.
    Is the led one with an internal resistor?

    [​IMG]

    Greetings,
    Bertus
     
  3. jhausch

    Thread Starter Active Member

    Jun 7, 2009
    49
    6
    Yes, the LED is a pre-packaged unit set to work on 12V and provide sufficient load to a power source. I showed it on the diagram as a simple LED for the sake of clarity.

    The transistors are PNP so I thought by supplying 3V to the base I'd be "holding the switch open", then when the 3V goes away (PTCs in high resistance state), the base would be pulled low through the 1k resistor. If I remove the traces you've circled, would the base ever be pulled low or would it be "high" or "floating" only?
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    If the PTC is in "high resistance" state, the motor will pull the base down.

    Greetings,
    Bertus
     
  5. jhausch

    Thread Starter Active Member

    Jun 7, 2009
    49
    6
    Oh, that makes sense. Thanks.

    Do I need the 1k resistors and the regular diodes, or can those be eliminated?
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I think the diodes can be eliminated.
    The 100 Ohm resistor will be enough.
    The 1 K Ohm must be used.

    Greetings,
    Bertus
     
  7. jhausch

    Thread Starter Active Member

    Jun 7, 2009
    49
    6
    OK, here is the revision.

    So the collector-base junction should act like a diode?

    Edit - I can probably eliminate the 100 ohm resistor, too.
     
    Last edited: Jun 7, 2009
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Now when I see the schematic the 100 Ohms and the zener diodes can go too.

    [​IMG]

    The diodes accross the motors will protect the transistors against back EMF.

    Greetings,
    Bertus
     
  9. jhausch

    Thread Starter Active Member

    Jun 7, 2009
    49
    6
    Bertus,

    First of all, thank you very much for your assistance. You're being very helpful and I did not want to forget to thank you.

    Please help me understand how I can eliminate the zener diodes. I thought I had to limit the voltage to the transistor base? Does the 1k ohm resistor do this as effectively?

    I had not considered back EMF. Are their any special considerations for the sizing of that diode, or would the DigiKey 1N4007CT-ND do the job?
     
  10. bertus

    Administrator

    Apr 5, 2008
    15,650
    2,348
    Hello,

    As long as the voltage between the base and the emmittor is less than 0.7 Volts the transistor will not conduct.
    The 1K resistor will limit the current when the PTC gets high resistance.
    The current will flow through the emmitor-base, the 1 K resitor and the motor to ground.
    So the point of conduction is dependent on the voltage accross the PTC.
    When the voltage over the PTC is higher than 0.7 Volts, you can add diodes between the +12 Volts and the emittors of the transistors.

    The 1N4007 will be fine as back EMF diode.

    Greetings,
    Bertus
     
    Last edited: Jun 7, 2009
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