I am trying to determine exactly what this part is. It is next to a current sensing transformer and is connected across the output of the transformer. There are no markings on the board nor on the part itself.
The specs on the transformer call for a 1.4 ohm resistor across the output of the transformer, but this doesn’t look like that to me.
The item in question is the one on the left of the picture.
Hello Bart,
I am sorry. I do not think it is a resistor either. I guess I am going to see if someone else knows this one. To me this could be a capacitor. Do you have a part number on the current sensing transformer? maybe there is something on the data sheet.
Hello Bart,
I’ll second @Verna_1353 statement. This looks like a capacitor.
By chance, is there any writing on the side?
Also, out of curiosity, what is the purpose of the PCB?
Best Wishes,
APDahlen
No, there is absolutely no marking on either the board or the component. The manufacturer of the transformer specifies that there be a resistor across the pins of the transformer, but the device on the board really doesn’t seem to be a resistor.
The device is for an occupancy detector for a model railroad. The manufacturer of this board is not making them any more and is reluctant to divulge the design of it. I have tried some of the currently available detectors, but none work as reliably as this design. So, I thought I would try to reverse engineer it and make enough for my needs. I don’t have any interest in making them commercially available.
I have located all of the required components except this one item.
Understood, Bart.
That being the case, and assuming the board is functional, recommend you unsolder the device and test with a multimeter. If it is a:
capacitor: You may get a direct reading as many multimeters have the ability to measure capacitance.
resistor: The multimeter will read the direct value.
inductor: we are in trouble, as it is difficult to tell an inductor from a low value resistor. Few multimeters have this measurement capability. An LCD meter would be required or the ability to measure time constants with a function generator and oscilloscope.
other: While I can’t say for certain, the picture / age / application is unlikely to include custom components.
Please let us know the multimeter readings. Also, if you are in the process of reverse engineering the device, you could post a schematic and we may be able to better identify the component.
Sincerely,
Aaron
Thank you for your reply. I think, during my conversation with the manufacturer, that it is not a custom component. I do have a multi meter (A Klein MM600) that shows a setting for capacitors. If I can find the documentation for it, I’ll see if I can understand how to use that feature.
I’m in the process of creating a schematic, however I wonder if I should post it to the public. Legally I mean.
The transformer is a CoilCraft CS1050L. As I said, their data sheet shows a resistor across the output. I wonder if this device is, in fact, a inductor meant to provide a variable resistance based on the frequency of the output signal.
Think I’ll do a search of inductors on Digikey. Perhaps I can find a picture of a device that looks similar. At least I’d know what it is.
I’ll be back with what I find.
Bart
So I pulled the device off the board and, lo and behold, there are some numbers on it! My meter says it’s a capacitor and I get the same reading with the leads both ways.
The numbers on it say 104k and 100n. My meter says 105n
Shouldn’t a capacitor say something about Fs? I set the meter to Ohms, and it reads 0 both ways. So now, I’m confused.
Bart
Yeah, I know that. I can get it at Mouser! 
I can also get it from CoilCraft for a Dollar less!
Guess where I’ll be buying it?
With the usual caveat that IANAL and some oddball jurisdictions may have weird laws.
As far as I know, reverse engineering a circuit and publishing the details including schematics is 100% legal.
The two types of IP (Intellectual Property) law that apply to electronic circuits are:
Hello Bart,
With regards to the capacitor, we can focus on the 104k designation. Some capacitors are like resistors in that we use a code. In this case 10 with 4 training zeros. that’s 10 with 0000 or 100,000 pF. This is very close to your meter’s measurement of 100 nF. The k designation is likely a 10% tolerance designation.
I have some doubts about the 100n value. Redundant markings don’t seem appropriate. Perhaps this is a voltage rating with a code I am not familiar with. Hopefully others will be able to offer more assistance. In the interim, you may be able to determine the voltage rating by examining the surrounding components.
Your capacitor is likely in this bin:
You can down select by physical size and voltage with the understanding that higher voltage capacitors may be substituted for lower voltage components.
Best Wishes,
APDahlen
Hi bart,
That’s most certainly a capacitor, and as @APDahlen suggests, it’s almost certainly a 0.1uF (aka 100nF, aka 100,000pF) ceramic cap. One would select a voltage rating of at least twice the voltage it will ever see across its two leads.
From the CS1050L datasheet, it shows the following schematic (I added the cap):
I presume that the cap is connected as I show - however, you should verify that it is indeed connected between the two leads of the current sensor. The purpose of placing a cap there would be to form a low-pass filter, which would simply knock out a bit of high frequency noise across your sense resistor. You should be able to find the sense resistor near the current sensor as well, with a value probably somewhere between 1 and 5 Ohms.
David, thank you so much! It now looks like I’m winning this battle!
Yes, the cap is connected as you show.
I am looking at Digikey # 445-180563-1-ND as the voltage is way above what I would expect.
My concern is that, going by the picture on the web site, the part I selected looks WAY smaller than the one I have. Is it possible that over the years, this part has become smaller (like all the other components)? Surface mount comes to mind.
Hello Bart,
Yes, you are correct in that the components have become smaller over the years.
Great! Looks like I have a complete list of components! I’m now going to create a Gerber file in KiCad, place an order for boards and see how well this works!
I haven’t had this much fun in years! Thanks to all who have responded to my pretty basic questions. Seriously! A GREAT BIG THANK YOU!
Bart
Hi guys, Bart, be careful.
If you did not find a resistor than you have also discovered the culprit that caused the destruction of the CT.
That big component is a current transformer (CT) and they do not like open circuit at all. They burn out just like in the picture.
So, that recommended low value resistor 1R4 is exactly the value required to load the CT.
Without it…smoke from the CT!
Regarding the “mysterious component”: It definitely is a capacitor and 104 is a 100% confirmation i.e 0.1 microfarads (uF).
Regarding size: In the picture is a polyester cap.
You might have ended up with ceramic caps - they are much smaller and lower voltage so be careful.
Find your equivalent somewhere here:
**Regarding capacitor voltage: The CT itself very, very low voltage output under normal conditions, so 250Vdc is a huge overkill, unless for worst case design (As when CT goes open circuit due to missing resistor!)
BTW 1: Believe me, I had as much fun helping investigating your mystery.
BTW 2: I have only been on this forum for exactly 45 minutes now!!
Thanks.
Garys, I’m impressed! You spent a whole lot less time than I did. Thank you so much for your time!
There’s is a 1k resistor across that CT along with the cap. The item you suggested looks like what is on my board. And it’s only 10 cents more that the one I had chosen!
I’m curious though, Both of those caps are non-polarized. Aside from the voltage, would it really matter which one I used?
Bart
I actually meant I JUST JOINED THE FORUM 45 minutes ago, came across your problem and could not resist to help…and my response was of the hat mate, no problem. I love this.
Anyway:
1: Regarding voltage: No HI VOLTAGE voltage across the CT if everything normal.
2: Regarding type: For any repair it is always best to replace with original part (for we have no idea what that designer had in mind), and thus it is not about the money now is it?
3: Regarding cap polarity: A CT works on AC ONLY so you must not put a polarity sensitive cap in there. Like an electrolytic. Smoke again!
Conclusion: And so that leaves us with a 1K wrong resistor mystery, a beautiful conundrum for you. How did it work with the wrong resistor? Like I said- A CT don’t go up in smoke for no reason except when something like a wrong burden resistor causes it. And your CT is smoked!! 
I bet there is nice story behind the resistor like 1k mistaken for 1R - not that it’s easy to do cause its a nice fat resistor.
Enjoy further.
Hi bart,
Regarding the difference between ceramic and film type capacitors, in this particular application I doubt it makes a huge difference. Some types of ceramic capacitors have a tendency to lower their capacitance somewhat as a DC voltage is applied. They also can pick up physical vibration and transmit it as electrical noise. This is more of an issue for surface mount versions, as they are more solidly attached to the board. The lead wires of a through-hole part tend to reduce this effect. In this case, I don’t think either one of these issues are very significant. That being said, as long as cost is not a significantly different, which seems to be the case here, using a film cap seems a good choice.
I do worry about the resistor, however. You stated that you measured the resistor as 1k Ohms. That is way off from what the datasheet specifies (1.4 Ohms nominal). Are you sure you are looking at the right one, and if so, is that measurement accurate? Note that you can’t usually measure the value in-circuit, as the other components will affect the reading. The colored lines on it (assuming that’s the type it uses) will also tell you its value.
Looking very closely on the board, the capacitor and a resistor are clearly installed across the connectors of the current transformer. The resistor has 4 bands: Brown, Black, red and gold. I read that as 1 for Brown, 0 for Black, Red for 100 and Gold for tolerance. so, 1 0 00 or 1 k. Am I wrong?
The Digikey calculator comes up with 1k
Bart
