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Thread: How does this overpower/short circuit protection circuit works?

  1. #1

    How does this overpower/short circuit protection circuit works?

    Hi all,
    All is in the title, I've joint the schematic.
    How to implement short circuit protection if the driving transformer have no extra winding for this function (only 3 pins in input) and without using pins 15 and 16 of the IC?
    Many thanks
    Attached Images Attached Images

  2. #2
    It looks like it is using flux imbalance or leakage to detect an over-current condition that energizes a733 which in turn kicks on c945 latching up the two transistors and lifting pin 4 above maximum dtc voltage?

  3. #3
    Quote Originally Posted by Easyamp View Post
    .... lifting pin 4 above maximum dtc voltage?
    Yes, the goal is to pull pin 4 up. But how ?
    What is the roles of diode and capacitor in the base of a733?
    When there's a short circuit the voltage at the capacitor increases or decreases?
    Why we don't invert the diode and connect it's cathode to the base of the npn transistor c945?

  4. #4
    diysmps Senior Member Silvio's Avatar
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    Maybe you can opt for a current transformer in the center of the half bridge, Then feed the rectified output of the current trafo to control the pulse width via the soft start pin. This will become a current fold back control. One other option is to use the over current opamp on the TL494 to control the output current via a sense resistor on the output. I do not see any particular reason why not using it. What about output voltage regulation using pin 1 and 2? What kind of PSU are you making after all?

  5. #5
    Quote Originally Posted by Silvio View Post
    1- you can opt for a current transformer
    2- use the over current opamp on the TL494
    3- What kind of PSU are you making after all?
    1- I want the simplest solution.
    2- you mean pins 15 and 16?
    3- I'm moding a PC ATX PS to get regulated voltage using pins 1 and 2 and constant current using pins 15 and 16 and a sense resistor.
    Because of the constant current option we don't need a short circuit protection, but I want to add this function in case of a short in the sense resistor, who knows !
    How to check if the short circuit protection is working without burning the power supply?

  6. #6
    diysmps Senior Member Silvio's Avatar
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    Follow this circuit it works I tried it you will have variable voltage and variable current. You can make fixed resistors instead of pots if you do not want it variable. Just omit the 4.7uf cap tied to pin 3
    modification close up.jpg

    you can also follow this link http://320volt.com/en/akim-voltaj-ay...nagi-modifiye/
    Last edited by Silvio; 07-12-2017 at 08:09 PM.

  7. #7
    You could build a simple latch and monitor the current on the primary ground return(s)
    any transistors should work, although some tweaking maybe required.

    latch.jpg

  8. #8
    Quote Originally Posted by Silvio View Post
    Follow this circuit it works I tried it you will have variable voltage and variable current. You can make fixed resistors instead of pots if you do not want it variable. Just omit the 4.7uf cap tied to pin 3
    modification close up.jpg

    you can also follow this link http://320volt.com/en/akim-voltaj-ay...nagi-modifiye/
    Quote Originally Posted by Easyamp View Post
    You could build a simple latch and monitor the current on the primary ground return(s)
    any transistors should work, although some tweaking maybe required.

    latch.jpg
    OK thanks a lot for the link and the circuits.
    I've already do the mods as shown in 320V circuit and it works, but if I fix the current for a given load and change to other one the current change a little !!

  9. #9
    diysmps Senior Member Silvio's Avatar
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    by how much does it changes? It could be that the sense resistor is getting hot and changing value. What current limit do you need?

  10. #10
    Yes, perhaps it's due to the sense resistor !!
    The change is small, about 100 mA.
    I need a range of 0-10A

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