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I am looking at using this small power supply device PSK-3D-5 to provide 5 V for a simple logic circuit from a wall outlet. I see this resistor (R1) on the AC line and was wondering if anyone could help me understand its function, and how critical it is to be included. I would also like to know how critical the varistor is to be included.

Is it a decidedly dumb idea to just wire AC in and connect DC to your project without any of the accessories shown?

PSK-3D-XX datasheet example circuit

Image source: CUI - PSK-3D Series Datasheet

Safety note; I am using a fuse to be safe as recommended in the drawing.

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  • \$\begingroup\$ It will be some manner of (inrush) current limiting resistor. It's more common to use NTC thermistors for that though. \$\endgroup\$
    – Lundin
    Commented Jun 14, 2023 at 12:34
  • \$\begingroup\$ Austin Fox - Hi, Please remember the site rule which requires that when a post includes content (e.g. text, image, photo etc.) copied or adapted from elsewhere, that copied content must be correctly referenced. As a minimum, the source webpage or PDF etc. should be linked (I noticed you mentioned the datasheet in the (normally invisible) HTML alt text - thanks for that). In order to help, I found what I believe to be the source PDF link in this case and added it for you. For the future, please remember it's your responsibility to do that :) Thanks. \$\endgroup\$
    – SamGibson
    Commented Jun 14, 2023 at 12:34
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    \$\begingroup\$ I think R1 is there to help you not having to replace that fuse too often. The varistor is there to handle voltage spikes so you don't have to replace the power supply too often. The fuse is there to ensure you don't have to replace your house too often. \$\endgroup\$
    – KristoferA
    Commented Jun 14, 2023 at 12:35
  • \$\begingroup\$ @KristoferA Rather, it's a cheap way to prevent the fuse from blowing due to inrush current when caps are charged etc. It's always kind of a delicate design to balance fuse values vs inrush current. With ESR of caps also adding to the equation. \$\endgroup\$
    – Lundin
    Commented Jun 14, 2023 at 12:37
  • \$\begingroup\$ After reviewing the comments it does seem like a simple 5v "brick" supply will be far more simple and cheap than reinventing the wheel here. My project does not demand on board power conversion. The responses have been very educational, thank you all. \$\endgroup\$
    – Austin Fox
    Commented Jun 14, 2023 at 13:08

2 Answers 2

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Let's start with MOV (varistor). This component is used to provide surge immunity. Surges are electrical spikes of high value but a very smaller duration. For ex - a 1500 V spike lasting for 6 microseconds. These can damage the circuit. MOV absorbs these spikes and doesn't let them travel downstream (ideally). MOVs tend to fail after they have absorbed a certain amount of energy and the failure mechanism is a short. To prevent things from blowing up, a fuse is required in series, otherwise, a shorted MOV will short line and neutral causing an MCB to trip or some other problems. I suggest that you use this component.

Now that MOV is decided, you must use a fuse. Can't avoid it.

R1 is to reduce inrush current when you power on the circuit. There will be bulk capacitors inside the power supply module that will get charged. In the charging process, they will pull a lot of inrush current lasting for a few milliseconds. R1 reduces the inrush value.

Sometimes, R1 and fuse can be combined. You can look for fusible resistors online. They are fuses with some resistance value. They can do inrush current limiting as well as protect the circuit when MOV fails. The fusible resistor must come in the place of the fuse (not in the place of R1).

Coming to your last question, it is not decidedly dumb I'd say. You are buying a product that promises you certain things. If it meets your requirements, go for it. At an application level, products like these can help you build things faster. However, the responsibility always lies on your shoulders to understand your own requirements well and then select the right component for that use case.

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  • \$\begingroup\$ Your response was very helpful and thorough, thank you! I \$\endgroup\$
    – Austin Fox
    Commented Jun 14, 2023 at 12:58
  • \$\begingroup\$ @AustinFox - happy to help. I am glad that you learnt something. :) \$\endgroup\$ Commented Jun 15, 2023 at 13:45
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It’s an inrush current limiter due to the smoothing capacitor behind the rectifier on the primary side.

For low power supplies, the static loss is acceptable to get a lower cost.

For higher power or where efficiency is more important, an NTC resistor is one step up.

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