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Is this a normal resistor and what is its value

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Western

Member
I'm trying to work out the value of an smd resistor that measures strangely.

It is in series with the primary winding of a transformer ... Shurter 4300.5041 .. IF-0306 ... that is used as a communications tx.

The resistor is marked 027u ... or upside down nl20 ... and physical size is 3.5mm x 4.95mm.

Not sure why it would need to be so large physically, given its role in the primary of the comms transformer circuit.

Normally I'm very careful to measure components before I disturb too much ... but this time I didn't because the conformal coating is so thick and difficult to penetrate without lots of effort ... and now I'm regretting it.


Resistor R217.jpg Schurter IF-0306.jpg
 

Western

Member
Inrush limiting perhaps?
Quite possibly.


I'm wondering if it is an inductor and not a resistor. At one stage (on the board) it measured 6.9 milliohms ... and when off the board it meaured 5.9 ohms ... but 18uH !!!

On the board there were definitely no other components across it.


An earlier version of the board uses a similar looking device with some unusual markings ... see pix (for some reason this one has 2 comms channels)


Earlier version.jpg


EDIT: So on this board it looks like they are a PPTC Resettable Fuse or Polyswitch judging from the centre character. Possibly the same sort of device in the first picture.

Now I just need to identify a suitable replacement.
 
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Western

Member
It appears to be a PTC fuse, a Bourns MF-MSMF020 which is rated 0.2A
Thank you very much for the pointer and links.

I know I've seen quite a few in my time ... but never actually known what they were ... and never had a faulty one, so never had to think about it. Even this one probably wasn't faulty ... I still put my money on a dry joint on the transformer ... but now I know what it is I'll change it just to be sure.

I do wonder about the sense of having the manufacturing date info in the marking ... I'd prefer a bit more obvious part number or value.

One thing that kept me off track was seeing the 027 ... and thinking that was a preferred value ... when the 7 was actually the year of manufacture. Oh well, learning all the time.

Thanks for the help guys.
 

unclejed613

Well-Known Member
Most Helpful Member
that notched shape is usually some kind of fuse, so a poly fuse would make sense
 

Western

Member
that notched shape is usually some kind of fuse, so a poly fuse would make sense

I've been working on this stuff for years and had never clicked that the notches were significant. I obviously haven't had to chase any before. Like I said above, I learn something every day. :)
 

schmitt trigger

Well-Known Member
I've been working on this stuff for years and had never clicked that the notches were significant. I obviously haven't had to chase any before. Like I said above, I learn something every day. :)
Count me as another educated individual!
What would be the reason for those notches to be associated with SMT fuses?
 

unclejed613

Well-Known Member
Most Helpful Member
maybe it has something to do with the pick and place equipment. i've never seen resistors or caps with notches... it's probably listed in a standardization document somewhere.

actually, i found a close-up picture of a 1206 size fuse, and i think the reason for the notch is that it's made from PCB material, and the notches are actually plated vias. it's very similar to the connections you see on RF modules, where there are notched pads on the edge of the PCB.

smt-fuse.jpg
 

rjenkinsgb

Well-Known Member
Most Helpful Member
notches are actually plated vias
-- and / or the notch causes solder to flow into it by capillary action (or just the solder paste squeezing in to it), giving better current distribution / thermal connection through the thickness of the device ??
 

Western

Member
Well, the bits arrived this morning and I fitted one.

Interesting in that the datasheet says the resistance can be between 0.4 to 6.0 ohms (at 23C) ... so I measured one before I fitted it and it was 1.8 ohms. After I soldered it in, it was up around 7 ohms ... and eventually settled back down to about 6.5 ohms. I was gentle with it ... don't think I overheated it.

The other new ones measure 1.6 ohms.

You can see the end structure from the photo with the (half) vias that unclejed mentioned.

Another unusual point is that they are labled the same on both sides ... and that the marking is in copper ... like the solder pads either end.

In fact on the original, I tinned the label to see if it actually was copper.


Bourns_MF-MSMF020_PPTC.jpg
 

schmitt trigger

Well-Known Member
Interesting....copper. Means that the lowest contact resistance is required.

How do they prevent copper from oxidizing? Unprotected copper will stain very quickly. Obviously the component is not Sn-plated. Would it have some sort of organic protective layer?
 

unclejed613

Well-Known Member
Most Helpful Member
unless you get water on it which would cause corrosion, the tarnish on copper acts as a barrier to further oxidation, similar to the way aluminum behaves, but the oxidation takes place at a much slower rate.
 
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