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This question already has an answer here:

Couldn't find the answer on the internet. The leads of VZ1 component are directly connected to mains.

enter image description here

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marked as duplicate by DerStrom8, Tom Carpenter, PeterJ, Voltage Spike, ThreePhaseEel Dec 29 '17 at 2:48

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  • \$\begingroup\$ The above link suggests it's a zener diode, but I'm not sure in what case you would want a zener (or TVS) directly across mains. Are you absolutely sure that's how it's connected? \$\endgroup\$ – DerStrom8 Dec 28 '17 at 0:09
  • \$\begingroup\$ @DerStrom8, yes. If you don't believe me, I will post the image of another side on your request \$\endgroup\$ – Qeeet Dec 28 '17 at 0:10
  • \$\begingroup\$ @DerStrom8, also there is no other holes for possible 'one board multiple designs', that is I'm pretty sure there are always mains in all designs. Also mains connector is not polarized. \$\endgroup\$ – Qeeet Dec 28 '17 at 0:13
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    \$\begingroup\$ @Qeeet - The fact that L2 (a choke?) has been replaced by links, and other components aren't fitted, suggests a "cost-reduced" design, where components that improve reliability / longevity / EMI etc. but which are not strictly required, are removed. It might be interesting if you add a photo of the reverse side of the board, and also reverse-engineer the schematic and provide that, to show how those missing components would be included in the circuit (if they were fitted), but I've answered the specific question you asked about VZ1 in this specific context. \$\endgroup\$ – SamGibson Dec 28 '17 at 0:30
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The leads of VZ1 component are directly connected to mains.

Therefore in this context, the component which makes most sense is the varistor (often a Metal Oxide Varistor or MOV). [A normal Zener Diode, for which a designer might also use the VZ component designator, would not be connected across the mains.]

Varistors are often used to suppress high-voltage transients and are directly connected across a device's mains supply input in that case.

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  • \$\begingroup\$ Thank you. Do you know what R1A means? Is that some kind of resistor? Looks like pcb doesn't have R1B \$\endgroup\$ – Qeeet Dec 28 '17 at 0:47
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    \$\begingroup\$ @Qeeet - "Do you know what R1A means? Is that some kind of resistor?" The use of letters after a component designator, is not well-standardised so I can only guess. My initial hypothesis would be that, yes, it's some kind of resistor. However it would require the rest of the (missing) input circuit to be reverse-engineered, to help explain why R1A would be used, with those extra components fitted. It might be that there is a capacitor (CX1?) which, if fitted, would be discharged by R1A, to prevent someone who unplugged the device and then touched the pins, from getting a "jolt". \$\endgroup\$ – SamGibson Dec 28 '17 at 1:01
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    \$\begingroup\$ CX1, CX2 and CY1 are probably X and Y capacitors, special capacitors designed for AC line inputs. It looks like R1A goes under CX1 probably to discharge the capacitor as you suggested. \$\endgroup\$ – Ken Shirriff Dec 28 '17 at 4:36
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About the only Z type part that belongs across the power line is a bidirectional MOV. It ought to be after the fuse

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