LM 723 Circuit to operate a relay witout a pass transistor

Discussion in 'The Projects Forum' started by cevval, May 17, 2015.

  1. cevval

    Thread Starter Member

    Dec 6, 2011
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    Hello, everybody.
    Please see attached circuit diagram. This circuit is for putting off a load via relay contacts when the powering voltage drops a certain adjustable level.
    My questions:
    1) It seems possible to switch on a 12V relay when connected without a zener diode. But it is latching and not possible to switch off the relay without disconnecting the input power. Is there a way to operate a 12V relay witout using a pass transitor when connected directly to VO of LM 723?
    2) Second way: When a zener diode is used in series with relay coil, a 5V relay is seems to work as required. Is this possible and reliable.
    Thanks to everybody who interested on my thread.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Here is what I came up with for a 12V relay. First, I reversed the relay logic. It seems you want the relay drawing current from the battery while there is still plenty of charge. When the battery discharges below ~12.9V (adjust as needed), you want to drop load and drop the relay as well to prevent further discharging the battery. This circuit, the current into the 723 drops to ~7ma after the relay turns off.

    lm723.gif
     
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  3. cevval

    Thread Starter Member

    Dec 6, 2011
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    Hello, MikeML,
    First of all, thank you for your interest on my subject.
    I am trying to operate your circuit proposal for a couple of days but not successful yet. However, I made the following changes;
    1) I am using a metal can package of LM 723 with 10 pins and different pin assignments.
    2) As transistor, I am using 2N2907 which I have available now instead of 2N3906,
    3) Of course, variable resistors for R1 and R3 which have special values. By the way, how do you calculate the values of R1 and R3 to set a lower voltage value? Because, I would like to adjust voltage lower than 12,9V, say 11,8V. How can I adjust at your circuit diagram.
     
  4. MikeML

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    Oct 2, 2009
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    R1/R2 set the voltage. R3 can be anything close. Here I reconfigured it so you can set the trip point with a pot. Green trace is pot=1KΩ, gold trace is pot = 3KΩ, with 250Ω steps in-between. You will have to download the data sheet to do the pin conversion from the metal can to the dip version for which I have the spice model.

    test.gif
     
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  5. cevval

    Thread Starter Member

    Dec 6, 2011
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    Hi,MikeML again,
    I could not achieve to operate a circuit as you proposed to me as above. I tried several times on bread board as well as as soldered circuit on a perfboard and with LM 723 plastic package as well, but no success. I was so discouraged that, I had to do some other things in my life in the meantime until today.
    But attached circuit diagram is working very well if I have to use a 12V relay with a pass transistor.
    I am not an professional electronic guy. I am only a retired mech. eng. with great electronic interest and I learned everything about electronics from internet with the contribution of expert electronic guys as yourself.
    My questions from you are as follows about attached circuit:
    1) Why do we need a diode (D1, 1N4148) on relay power feed line? Is it absolutely necessary? I added this because there is one in your circuit.
    2) Any comments on my circuit, like current draw, etc.
    Thanks a lot for your interest on on my thread.
    LM 723 Circuit_Ask1.PNG
     
  6. MikeML

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    You don't need D1 in post #5.

    I put D2 in the emitter of Q2 in post #4 to guarantee that Q2 shut off.

    What happened when you built the circuit of post #4? Was the relay always pulled-in, or did it never pull-in?
     
    Last edited: Jun 28, 2015
  7. #12

    Expert

    Nov 30, 2010
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    Just a note about post #5: ZD1 is optional. It is inside the LM723 plastic DIP package, but I don't see why it is required in this circuit. R4 seems like too much resistance. You need at least 1.7 ma for the base of Q1
     
  8. cevval

    Thread Starter Member

    Dec 6, 2011
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    LM 723 is a metal package type and the datasheet says (Note 7) this.
     
  9. #12

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    Note 7: For metal can applications where VZ is required, an external 6.2V zener diode should be connected in series with VOUT.

    So why is the 6.2 volt zener required for your circuit?
    I say you can just put the right size resistor in for R4 and be done with it.
    About 5.6k should work.
     
  10. cevval

    Thread Starter Member

    Dec 6, 2011
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    With same configuation as in Post #4, relay never pulled in, being the results as below: with R1= 2,2K being a trimpot.
    1) When Vin= 11,76V
    V10= 10,16 V to 5,21V as R1 value changed.
    V5= 7,13 V
    V6= 7,14V
    V4= 7,18 to 6,92 V as R1 value changed.
    When transistor PN2222A used; Relay always pulled in.
    2) When Vin= 12,23V
    V10= 5,26V to 6 V as R1 value changed.
    V5= 7,14 V
    V6= 7,14V
    V4= 7,14 V as R1 value changed.
    and relay never pulled in.
     
  11. cevval

    Thread Starter Member

    Dec 6, 2011
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    Without zener, relay is always staying pulled in and not responding to stop even voltage is dropped.
    OK. I will consider your proposal for R4. Thanks.
     
  12. cevval

    Thread Starter Member

    Dec 6, 2011
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    Hello again,
    I faced hysteresis problem while operating the circuit shown on attached diagram. I mean relay is flickering when the voltage set value is reached. At voltage levels where away from set value either lower or higher, the relay is completely off or on without flickering. I assume that a differential voltage value should assigned to avoid this problem, but how? This is my question. Please, help.
     
  13. #12

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    How much does the battery voltage recover when the load is switched off? How much of this inaccuracy is acceptable in this application? You need to decide on how much voltage you want the decision point to change and use the output of pin 6 to send some current to someplace so it alters the set point.

    First approximation: To change the set point by 1/2 volt, 47K from pin 6 to the wiper of the pot. You will have to readjust the set point when the battery has good voltage. When the battery gets too low, the output of the chip will drop towards zero and the current through the 47k resistor will stop holding the set point of the wiper as high.
     
    Last edited: Oct 4, 2015
  14. #12

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    Here are drawings of the LM723 in TO-5 and P-Dip. If you open them in MS-Paint you can use them to draw circuits with the right pin numbers.
     
  15. grahamed

    Member

    Jul 23, 2012
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    Hi

    Two points -

    1) I see no hysteresis anywhere (then again maybe I am going blind).

    2) without Vz the base current of the transistor (in the on state) would be approx. 10V/R4. If we assume gain of 10 (the transistor is saturated after all) it needs approx 2mA so let R4 be 5k. The low voltage is 2V so 5k would give maybe (2-0.7)/5k = 0mA26, but what if the gain were in fact 50?
     
  16. MikeML

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    Oct 2, 2009
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    Since you are using the version of the '723 without the built-in Zener, you require the external one as I show here.

    I have added some positive feedback to create Hysteresis. The lower R7; the wider the separation between cut-in and cut-out.

    Testh.gif

    You put back the wrong-way relay logic that helps discharge the battery after it is already discharged. I showed you how to avoid that way back in post #4 using a PNP transistor in-place of the NPN.
     
    Last edited: Oct 5, 2015
  17. #12

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    Why is the zener diode necessary?
     
  18. grahamed

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    Jul 23, 2012
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    So the output can go down to zero.....
     
  19. MikeML

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    Oct 2, 2009
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    Because in the simulation I just posted in #16, the voltage at Vout swings from 11.5V to 6V, which would leave the 2222 always on...
     
    Last edited: Oct 5, 2015
  20. #12

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    I'm going to have to study this some more. I shipped thousands of linear regulators wherein the LM723 did not refuse to output below 6 volts. Apparently I need my beliefs adjusted.
     
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