Reading and understanding this schematic [Discussion]

Discussion in 'General Electronics Chat' started by Blue_Electronx, Jun 13, 2019 at 8:42 PM.

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  1. Blue_Electronx

    Thread Starter New Member

    Jun 10, 2019
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    See attachments Power1 and Power2. I know it's kind of weird, but I don't have a decent schematic capture software right now. I'm doing some reverse engineering and I'm trying to understand the functionality. The two schematics are located in different PC boards that are interconnected. As you can see, for example, the +27V point comes out from Power1 schematic and connects with Power2 schematic where it's feeding a LM317 regulator. I'm not showing the whole circuitry. There's another control circuit which processes a signal coming from a current transformer. Additional information: the input line to line voltage ranges from 220 to 480 VAC. The circuit is used as earth leakage protection. So based on the schematics:

    1) How are the +27 and -13 voltages generated? I'm kind of confused as it seems like in Power1 schematic the reference is NOT zero volts. I know there should be a trick with the zener diodes.
    2) How is the FET turned on? Is it through the +V point which connects with -13V through the SCR in Power2 schematic?
    3) At the drain of the FET, there is a freewheeling diode for the coil and a stack of three TVS diodes in series (not sure why three).
    4) At last, I want to understand better the setup at the source of the FET

    I'm actually not asking so much, just few hints that help me interpret the circuit in a better way.

    What improvements would you make to this circuit?
     
  2. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    Polarities are given but there is no reference point of zero volts given thato the voltages are referenced to. That makes the whole thing confusing even for me.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There does seem to be something missing.
    BTW, if needing a good schematic capture program, try (free) Kicad.
    Max.
     
  4. Blue_Electronx

    Thread Starter New Member

    Jun 10, 2019
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    Ok, I can try redoing the schematic. I will recheck the connections, but I feel confident I have drawn the circuit correctly.
     
  5. MisterBill2

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    Jan 23, 2018
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    For t starters, that line tagged "-13" is what would be called the "common" line, or even the "ground" in most instances. Then there is reference for all of the other voltages. And it makes sense to call it the common because it comes directly from the rectifier. One part that does not seem reasonable to me is the lower end connection of R3 in the first drawing, as well as the gate connection to M1. It seems that there is a fair amount missing from what is drawn. The SCR device show by itself does tend to clarify part of the first page, which is where it should be located. In addition the LM317 circuit is like no other application of that device that I have ever seen. So there is a lot missing from the drawings here that is quite relevant.
     
  6. Blue_Electronx

    Thread Starter New Member

    Jun 10, 2019
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    The circuit that I'm not showing is the control one. But I can easily see this control circuit is being powered by the 13.25V coming out of the LM317. The control circuits uses both references -13V, and the ground from the 13V zener cathode in the schematic 2. At the end of the day, I want to understand how the +27V and -13V are generated. I do not see more information about the power supply circuit than the one I have drawn.
     
  7. MisterBill2

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    Jan 23, 2018
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    The negative 13 volts, relative to the ground symbol shown on page 2, is created by the voltage drop across zener diode D3, with most of the current coming through the load, not shown, connected between the top of C1 and the return line, tagged as -13 volts. I would not call this a very good design given the dependence on a number of zener diode voltages. And the voltage rating of that 22nF capacitor just beyond the rectifier module is confusing. I understand that it is fro transient damping but it gives an incorrect impression at first inspection.
    I can see why you wanted help understanding this circuit, it is tricky.
     
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  8. Blue_Electronx

    Thread Starter New Member

    Jun 10, 2019
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    It seems like the zero volts reference in first schematic is brought down to -13V by the second circuit. So I have a non zero reference in the first circuit. The operating voltage of the coil attached to this circuit is 115 VDC. I still do not understand if that's the voltage the coil will have if the FET is turned on in that configuration.
     
  9. MisterBill2

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    It is not clear how the FET would be turned on and off because I see no signal lines connected to the FET.
     
  10. Blue_Electronx

    Thread Starter New Member

    Jun 10, 2019
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    Can this circuit be simulated using LTspice?
     
  11. MisterBill2

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    Jan 23, 2018
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    There is no "zero volts reference" in the first schematic, and that makes it a real challenge to understand the voltages, since by definition a voltage must be referenced to something., because a stated voltage must be referenced against some point. Certainly combining both of these drawings into one would make understanding what is happening a lot easier. The fact that nothing in the first sheet seems to be connected to the "ground" symbol is alsoconfusing.
     
  12. ScottWang

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