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Output short protection circuit

  1. Aug 23, 2012 #1
    I am making a totem pole digital output of 1.5-23v. i was searching for a protection circuit for the output short to ground and +24v dc. please help me out by sending a diagram of the circuit if possible.
  2. jcsd
  3. Aug 23, 2012 #2
    Post the circuit you have first and we'll try to protect it. Not going to design the whole thing for you.
  4. Aug 23, 2012 #3
    this is the circuit with which i am working please replace the resistors
    R2 with 470ohm
    R3 with 120ohm
    R4 with 220ohm

    also the supply voltage is +24v and gnd.

    Attached Files:

  5. Aug 23, 2012 #4
    Somehow I can't open the image.
  6. Aug 23, 2012 #5
    uploaded a jpeg version.

    Attached Files:

  7. Aug 23, 2012 #6
    I can protect V3 easier. Put a resistor R5 between the emitter of V3 and V5. Put 3 diodes D1, D2 and D3 in series with cathode of the D1 connect to the base of V3. The anode of D3 to the output. This will serve as current limiting. If you have a short from output to the ground, then you draw excess current from V3. The excess current will create a voltage drop across R5. As soon as there is 0.7V drop across R5, the voltage from base of V3 through V5 will be over 3 diode drop and D1 to D3 will turn on and start pulling current from the base of V5. This will serve as current limit. The value of R5 can be set depending on the maximum current you want to allow V3 to source.

    It would be a little trickier to protect V4. You need to put extra transistor in. Say call the new NPN Q1. you put a resistor called R6 from emitter of V4 to ground so any current draw through V4 will create a voltage drop across R6. You put Q1 so the emitter connects to ground, base to junction between the emitter of V4 and R6. The collector of Q1 connects to the base of V4. So if enough current through V4 develops over 0.7V across R6, Q1 will turn on and pull base current from V4. This will serve to limit the current through V4. V2 should be save as R2 will limit current through Q1 to a few mA only.

    This is my idea after a few minutes of looking. Someone might have better and easier way. I'll think about this a little more to see whether I can come up with something better.
  8. Aug 23, 2012 #7
    thanks for your reply.. but still the problem exists.. the transistor v3 is not getting off when the output(23v) is shorted to ground. i am using the proteus software to simulate the circuit. i was also thinking in the direction to somehow switch off the transistor v3, but cant get anything..
  9. Aug 23, 2012 #8
    You only have +5V, where's 23V? I am not trying to switch the V3 off, I am trying to current limit it. As you short the output to ground, current increase in V3 which increase the voltage across R5 beyond 0.7V and the three diodes turn on and pull current from the base of V3 and thereby limit the current of V3.

    You better modify the schematic to show the +25V as you only have +5V and I just do accordingly.
  10. Aug 23, 2012 #9
    sorry for the inconvenience created i am sending the edited schematic of the circuit. by the way i had written in my 2nd thread about the voltage levels..

    Attached Files:

  11. Aug 23, 2012 #10
    If the supply is all 25V, my comment in post #8 stands. I am doing current limiting, not trying to turn off V3 when output is shorted to ground.

    My main concern is the way I do current limiting on the low side where I use extra transistor Q1. For a clear cut short to +25V, the circuit should work to current limit, but if just drawing excess current during operation and Q1 is at the threshold of turning on, the circuit becomes analog and oscillation might happen. I yet to think of a safe way to do the protection of V5.
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