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LM393: Constantly sinking?

Part Number: LM393
Other Parts Discussed in Thread: LM5118, , TLV1702, LM2903B,

I have an application where I'm using the LM393 to compare a constant 12V source and a fluctuating 12V battery. I'm using an LM5118 board as a 12V source(powerboard+), and have also used my bench variable power supply in its place just to rule out any interference.

Battery voltage is fed into the positive pin of (one of) the comparators. The constant 12V source is fed into the negative serving as it's reference.

At the moment, the comparator seems to constantly be sinking no matter how the change in the Battery voltage.. All grounds between the 12V supply and battery are shared. I haven't found a short to ground.

This circuit is working in simulation and in my breadboard, I'm at a loss as to why it's not working here.

  • HI Calvin,

    You are exceeding the input range on both inputs. When both inputs are taken above the valid input range, the output will be low (sinking). So it is actually working as expected.

    Please see the LM393 family appnote:

     Application Design Guidelines for LM339, LM393, TL331 Family Comparators (Rev. A)

    The valid input range is 2V below V+, or 10V on a 12V supply.

    You will need to attenuate the input voltage to be below 10V, or, increase the supply voltage to 14V.

    In either case DO NOT DIRECTLY CONNECT THE BATTERY TO THE INPUT PIN!!! Please place at least 15k (minimum) resistor in series with the input to limit the current into the input pin.

    The LM393 is one of the few devices that will tolerate having 12V on the input with no supply voltage (I assume there can be times where VCC will be zero and the battery voltage is still applied to the input).

    I would have recommend the TLV1702 because it has a R-R input and can drop-in replace the LM393, but it cannot accept the input taken above VCC (diode clamp between the input and VCC).

    Will there be times that VCC will be "off" and the battery voltage still applied? Can you provide a complete schematic?

  • I see. I guess I can use voltage dividers to get the two inputs down to something below 10V. Input resistor for the battery is noted, thanks!

    There won't be any moments where VCC would be off and battery voltage would still be present. Ultimately, the battery feeds the upstream 5118, 12V regulator board.

    The intent of this circuit was if the battery voltage fellow below the output of the 5118 (12v ref/conditioning) board, to disconnect this boards load and save the battery from being drained during ignition off scenarios. The photo transistors actually support the load (U2, U3). The 3904 transistor just takes the load of turning on the LED in U2 off of the 393, and the diode-duo keeps the ignition override signal from backfeeding the comparator circuit.

    The end device is something that needs to always be on for convenience (device has slow boot times), but not kill the battery if sitting idle for long periods.

  • Calvin

    It sounds like you have a plan to get this working now.

    In your previous post, you mentioned that the simulation showed the circuit working without the resistor divider included.

    I believe that we have fixed this problem in the newer LM393B and LM2903B models found here.  If you want, you can try these out and hopefully that will solve the simulation problem.

    Chuck

    www.ti.com/.../LM393B

  • There has not been any new posts on this thread, so it will be closed.  If you have additional questions, please feel free to reply or open a new thread.

    thanks

    Chuck