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I am repairing 'Siemens SITOP power 20' power supply. I have to draw its circuit to better understand it and a notice a bizarre design decision. Can you help me to understand it?

It has an arrester which one connection stands on the 'plus' of DC and another connection should be a 'minus'. That 'minus connection is goes through induction coil and connected to a 'plus' (!) of the capacitor. How does it possible?

On photos please note plus and minus from AC-DC bridge and how it is connected to arrester and capacitor.

(220V/380V AC as an input + one neutral, 3 thermistors and 3 varistors as protection layer, next is three-phase rectifier (diode bridge for 3 phase current) which converts AC to DC, next is a filter which 'plus' goes to arrestor)

1st side 2nd side reversed engineered so far circuit, have to do it on plain paper because of lack all required components in top list circuit software

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  • \$\begingroup\$ Welcome! Please draw a schematic and links to the datasheet of the arrester. \$\endgroup\$
    – winny
    Dec 17, 2023 at 13:33
  • \$\begingroup\$ I still working on a whole circuit, it isn't ready yet. Also Siemens's official circuit is not publicly available on the web (at least I haven't found it). Please check photos of related part of the schematic. I don't remember exact name of arrestor, but it contains this in name 'epcos 90' \$\endgroup\$
    – Gleichmut
    Dec 17, 2023 at 15:04
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    \$\begingroup\$ What you're calling an arrestor here looks like a gas discharge tube, which don't have polarity. With a schematic someone here could probably figure out the purpose of it, but as the question stands it's unlikely to be answered--you don't need a full circuit schematic, but if you can draw up a schematic of what you've reverse-engineered so far, that'd be very helpful. \$\endgroup\$
    – Hearth
    Dec 17, 2023 at 15:45
  • \$\begingroup\$ Part number(s) on the gas discharge tubes/varistors/arresters? \$\endgroup\$
    – winny
    Dec 17, 2023 at 16:20
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    \$\begingroup\$ A gas discharge tube is a type of surge arrestor. \$\endgroup\$
    – Hearth
    Dec 18, 2023 at 13:38

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It is a bit strange, but EPCOS arrestors marked as red have positive polarity - they are unipolar instead of bipolar. In this case plus from arrestor goes to a plus of capacitor which is fine.

By specification one of two arrestor's electrodes should be 'grounded', connected to the ground. It gives me an idea they are always bipolar. However, technically surge breakdown should be channelled to anything capable for it, so a capacitor will fit this use case as well.

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