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This is input of a non-isolated buck offline switching power supply (MP175GS with absolute maximum input voltage of 700v). Input is 100-265Vac. TVS is SMBJ440CA (713v clamping 600W BiDir). Resistor's color coding indicates 51 Ohms.

Why there is a resistor used instead of a fuse? Is it replaceable with a 1206 1A fast acting fuse?

If not, how should I find the correct wattage of that 51ohm resistor?

TVS circuit resistor dimension circuit around the resistor of interest: schematic MP175 typical application: MP175 typical application

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    \$\begingroup\$ Could it be a Fusible resistor? See Color Code for Fusible or Flame Retardant Resistors: Data Sheets, Standards? for a summary. \$\endgroup\$ Commented Sep 28 at 20:19
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    \$\begingroup\$ @chesterGillon AFAIK, all Fusible resistors have 5 band color coding. So, probably no it isn't. \$\endgroup\$ Commented Sep 28 at 20:32
  • \$\begingroup\$ We are not the designers of that power supply, and we don't have any documentation for it except what you provide. Therefore, we cannot say if that is a fusible resistor or not, and if it can or cannot be replaced with a fuse. Some more reverse-engineering must happen before these questions can be answered. Is the resistor blown and you are repairing this circuit? Resistors do not blow up without a reason, so replacing the resistor may just make it burn up again, unless the fault downstream is fixed/removed. \$\endgroup\$ Commented Sep 28 at 20:59
  • \$\begingroup\$ @Justme In one device it is blown but I'm using another one to find out the replacement. (I've just added a sharper image of the resistor to my question) \$\endgroup\$ Commented Sep 28 at 21:11
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    \$\begingroup\$ @AKTanara I think I can guess how these two components are wired to anything, but you have not drawn any schematics to be sure. And the difference is, repair questions without sufficient knowledge on repaired device are off topic, component ID questions are OK, but people may be reluctant to answer as in case they are wrong, you end up repairing with wrong part that might be dangerous. \$\endgroup\$ Commented Sep 29 at 6:28

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Assuming the circuit is as described: most likely a combination of filtering, inrush limiting and fusing. Preferably, the resistor would have pulse ratings.

I would not recommend replacing it with a fuse.

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  • \$\begingroup\$ Thanks for the insight. I've added some extra information to my question. I'd appreciate if you took a look at it again. \$\endgroup\$ Commented Sep 29 at 8:16
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Why there is a resistor used instead of a fuse? Is it replaceable with a 1206 1A fast acting fuse?

You can find very similar looking fusible resistors (in an MELF package): -

enter image description here

Image from here. I suggest you measure its dimensions and start looking around to see what current rating it might be. Regarding the above, the fusible resistors on the left appear to be 5-band whereas the ones on the right appear to be 4-band.

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  • \$\begingroup\$ I am certain Fusible resistors have 5 band indicators. So it is not a fusible... \$\endgroup\$ Commented Sep 28 at 21:05
  • \$\begingroup\$ @AKTanara I am not certain so why you are? I did find MELF packaged resistors, and they were fusible, and had 5 bands because they required 5 bands for the resistance and tolerance, not because of being fusible. \$\endgroup\$ Commented Sep 28 at 21:17
  • \$\begingroup\$ I think they're fairly rare, but there are certainly fusible resistors with 4 bands. Page two of this datasheet (ttelectronics.com/TTElectronics/media/ProductFiles/Datasheet/…) states "...resistors are color coded with five bands...parts with 20% tolerance have no fourth band" though I doubt that's what you have. The TVS might turn on a larger crowbar device as shown on page 1 of this app note (littelfuse.com/assetdocs/…). Does the middle trace go anywhere else? \$\endgroup\$ Commented Sep 28 at 22:10
  • \$\begingroup\$ What about resistors "0 ohms", is a 0 ohm resistor cheaper than a fuse? (same protection level). \$\endgroup\$ Commented Sep 29 at 6:15
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    \$\begingroup\$ @Panamanian, Even though it isn't 0ohms (the color coding indicate 51Ohm-5% and measurement has also confirmed this value) I think that it is a simple resistor as fuse but it would be nice to find out the wattage considering the dimension: 6.25x2.5mm \$\endgroup\$ Commented Sep 29 at 6:20

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