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Question about a rectifier PCB of a welding power supply

Discussion in 'Power Electronics' started by BlackMelon, May 6, 2020.

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  1. BlackMelon


    Aug 7, 2012
    Please refer to the following file:


    I am converting the aforementioned PCB into a circuit. The relays are marked "RL" and highlighted by corresponding colors: yellow for the contacts and coil of the relay 2, for example. Connectors are represented by open circuits. However, I don't understand how the circuit circled by the red circle works.
    To my sense, it is a transient protection system, since it got TVS diodes (KE150CA). However, why do we need D5, since it will make the whole protection system work in only one direction? Also, I don't get the idea of using C5 here too.

    Should you provide resources for my further studying, I would be appreciated.

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Makes no sense at all to me.
    please verify your schematic. With a small change some logic creeps into the circuit:
    Withe the connection from R3 changed from D5/R4 to C5/RL2 this becomes a small non-isolated (!) low voltage supply:
    - D5 is a one-way rectifier
    - the TVS diodes drop most of the mains voltage
    - D6 limits the output voltage
    - C5 smoothes the peaks to achieve a more or less smooth DC output
    - RL2 discharges C5 in case of no load connected
    - R3 limits the output current to terminal COM3+
    - RL2 would connect the negative output to COM3-. It is probably NC because this connection is already available via the connection at the bottom of the schematic
  3. duke37


    Jan 9, 2011
    It seems to be a circuit to turn on the relay when the input voltage exceeds 300V. There is an electrolytic capacitor across the relay coil so it is DC and needs a DC supply via D5. Probably nothing to do with transient protection. Perhaps slow start.
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