Digital Pot max amp question

Discussion in 'General Electronics Chat' started by Tobias, Oct 18, 2009.

  1. Tobias

    Thread Starter Active Member

    May 19, 2008
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    Attached is the spec sheet for the digital pot and my schematic.

    I am taking the output of three mechanical pots and based on certain conditions I am either adding or subtracting voltage from the mechanical pots to get a voltage output that represents a different position of the mechanical pots.

    My problem is I am smoking the digital pot. The max voltage on one leg of the digital pot is 3v. It fails when I input the max 3v into the digital pot. So I thought to myself, what the hell I will read the spec sheet for the pot, maybe I am exceeding the max amperage tolerance of the digital pot. Here is a line from the spec sheet
    AX–BX, AX–WX, BX–WX . . . . . . . . . . . . . . . . . . . . . . ±20 mA

    So if I have 3v, I need a 150 ohm current limiting resistor on the leg getting the 3v. So I lift up the pin and place a SMT resistor in between the SMT pad and the pin. It failed again.

    So am I reading the spec sheet correct? Maybe I am just so close to the max amperage I need to go with a higher value current limiting resistor.

    On the schematic the digital pot is on the bottom left corner

    Thanks
    Toby
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    What is the value of your voltage Vdd?

    hgmjr
     
  3. Tobias

    Thread Starter Active Member

    May 19, 2008
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    5v is the VDD
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    Is opamp OA2d being power by +5V and ground?

    hgmjr
     
  5. Tobias

    Thread Starter Active Member

    May 19, 2008
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    12v for the power.
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    That means that the output of the opamp can drive one end of the digital pot to a voltage greater than the digital pot's 5V power supply. Am I correct?

    hgmjr
     
  7. Tobias

    Thread Starter Active Member

    May 19, 2008
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    Take a look at pin 12 of the op-amp. It has 4.33 volts on it constantly. Pin 13 has the output of one of the mechanical pots with a voltage range of 4.33-1.33 volts. I am using the op-amp to output on pin 14 the difference between the two pins so pin 14 is 0 to 3v. This 0 to 3v is then sent to A1 of the digital pot.

    I used 12v to power the op-amp so my 3v wasn't even close to the rail value.
    Hope this helps
     
  8. hgmjr

    Moderator

    Jan 28, 2005
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    Perhaps the output of the opamp is spiking to a voltage greater than 5V for a very brief period. If it did, that would be enough to latch up the digital pot and destroy it.

    Do you have a 5V tranzsorb that you could put on the output of the opamp so that it would clamp it to 5V?

    hgmjr
     
  9. Tobias

    Thread Starter Active Member

    May 19, 2008
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    So R29 needs to be canned? I am pretty new to the op-amp deal. This project has been a hell of a learning experience. I am using the schematic in the following link as my baseline. When I test the outputs everything seems to be fine until I get to the full span of the mechanical pot.

    http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar6.html#c1
     
  10. Tobias

    Thread Starter Active Member

    May 19, 2008
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    Whats a tranzsor?

    Am I interpreting the spec sheet correctly and need the current limiting resistor? I am going to put a 1k resistor between the op-amp and the digital pot.
     
  11. hgmjr

    Moderator

    Jan 28, 2005
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    No, R29 is needed. I think the reference voltage on the positive terminal does need to be at 2.15V.

    hgmjr
     
  12. hgmjr

    Moderator

    Jan 28, 2005
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    A transorb is like a zener except that it can react to voltages higher than its voltage rating in nanoseconds.

    hgmjr
     
  13. Tobias

    Thread Starter Active Member

    May 19, 2008
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    Ill see if Radio Shack has them tomorrow or I will order some to try too
    Thanks

     
  14. hgmjr

    Moderator

    Jan 28, 2005
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    Take a look at part number SA5.0CA. I think Mouser or Digikey has it.

    hgmjr
     
  15. Tobias

    Thread Starter Active Member

    May 19, 2008
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    Thanks for your input. I have some ES2D switching rectifiers. Would that work?
     
  16. hgmjr

    Moderator

    Jan 28, 2005
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    Nope, those are just fast recovery diodes. A 5V transzorb is what I am suggesting. The name TransZorb is a trade name for a specific manufacturer's device. The generic name of the part is a "transient suppressor".

    hgmjr
     
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