oddly shaped resistor - Page 1
Forum section: electronics / beginners

author: Benjam
author_rank: Regular Contributor
timestamp: April 21, 2022, 09:39:14 pm
content: PN: WSL3637https://www.vishay.com/docs/30099/wsl3637.pdfTo be honest, this is the first time I see this kind of resistor, I'm just getting started and learning about circuits, I don't know why this resistor has 4 terminals, why does it have 4 terminals? What is its pathway? Is there any difference with ordinary resistors? If I want to use it, how should I use it? In which circuits is it used?

author: Cerebus
author_rank: Super Contributor
timestamp: April 21, 2022, 09:47:57 pm
content: Google "Kelvin Connection" and all will become clear.It's all to do with eliminating the effect of resistance in the conductors used to measure current indirectly by measuring the voltage across a current shunt resistor, such as these ones. The resistance of the measurement wires can become significant if they themselves are carrying current, so to avoid that one has one pair of wires to carry the current to be measured and another pair to carry the voltage, generated by the effect of the aforementioned current on the current shunt resistor, off to be measured over by a high impedance circuit so that the resistance of the measurement leads isn't itself significant.

author: ledtester
author_rank: Super Contributor
timestamp: April 21, 2022, 09:49:36 pm
content: It's used with "4-wire" resistance measurement technique to null out the voltage drop across the voltage test leads.Given that small resistance, the large tabs are probably to support a large current flow and the smaller ones for the voltage reading.

author: Benjam
author_rank: Regular Contributor
timestamp: April 21, 2022, 10:38:17 pm
content: Quote from: Cerebus on April 21, 2022, 09:47:57 pmThanks!

author: Benjam
author_rank: Regular Contributor
timestamp: April 21, 2022, 10:39:48 pm
content: Quote from: ledtester on April 21, 2022, 09:49:36 pmI have a new doubt, how do I distinguish which is pin 1, which is pin 2, which is pin 3, and which is pin 4. In its datasheet, I did not find a specific and clear description. The extra thing I want to know is that it seems to be quite large, is there a smaller package?

author: TimFox
author_rank: Super Contributor
timestamp: April 21, 2022, 10:56:22 pm
content: I assume that you pass the current from one wide terminal to the other wide terminal, and sense the voltage between one narrow terminal and the other narrow terminal.

author: Cerebus
author_rank: Super Contributor
timestamp: April 21, 2022, 11:17:00 pm
content: Quote from: Benjam on April 21, 2022, 10:39:48 pmWell look at the data sheet for part specific pin designations, but there is pretty much an industry standard for all parts that, looking from the top of the part, pin 1 is the one with some kind of mark or other designation that singles it out, and then pins are numbered counterclockwise from there.A quick look at the data sheet shows a pinout diagram with pins designated E1, E2, and I1, I2. It shouldn't be too hard to work out which are the current carrying pins (Ix) and the voltage measurement pins (Ex) from the clues in the letters chosen for the pin designations. As expected, the current carrying ones are larger, the voltage measurement ones smaller.Size will be related to power dissipation, which in turn goes hand in hand with current rating and resistance (P = I2R). These are rated at 3W dissipation, which is quite chunky for any surface mounted part, so the part is chunky. Calculate its area, and that 3W power rating comes out at (crudely) 30kW/m2. Toasty, it's gonna need alotof copper area to keep it cool.

author: thm_w
author_rank: Super Contributor
timestamp: April 21, 2022, 11:22:39 pm
content: Yeah as cerebus says, they show you.But I admit there are some that are skinny or wide sense types, and it can get confusing.

author: Ranayna
author_rank: Super Contributor
timestamp: April 22, 2022, 01:06:26 pm
content: This may be a stupid question, but seeing these 4 pin resistors...Would it not be just as possible to tap the measuring connection directly from the primary pads? Why do you actually need a 4 pin resistor to do a kelvin measurement?

author: Monkeh
author_rank: Super Contributor
timestamp: April 22, 2022, 01:16:34 pm
content: Quote from: Ranayna on April 22, 2022, 01:06:26 pmTo eliminate the copper and solder joint resistance from the measurement. When you're dealing with resistors specified in the single digit milliohms, this matters.

author: golden_labels
author_rank: Super Contributor
timestamp: April 23, 2022, 12:08:13 am
content: Also: to make sure you are tapping off very precisely chosen points.If you would connect directly to current-carrying pads, the measurement would depend on unintended voltage drop between known resistance and your probing points. Most precise measurements would also be affected bySeebeck effectin an unpredictable manner.With separate probe pads there is no current to cause noticeable voltage drop and the Seebeck effect is either negligible or accounted for.Summing up: it’s all about control.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 23, 2022, 12:15:11 am
content: Think I last used that family ca. 2014 in a precision lab supply. Pricey, but very nice. Used with AD8210 for high side current sensing.Most current sense resistors you see these days, suggest a Kelvin layout by connecting sense traces underneath the part; this is adequate down to modest levels. Usually in higher precision values (like these 1% and better), explicit terminals are given, so that the sensed voltage drop can be trimmed from the factory. Especially important for precision foil parts like these.You will almost never use these, as a beginner; they're much too expensive, more precise than you will need. But when you need exactly that, yep, quite nice.Tim

author: Benjam
author_rank: Regular Contributor
timestamp: April 23, 2022, 11:56:05 am
content: Quote from: Cerebus on April 21, 2022, 11:17:00 pm

author: Benjam
author_rank: Regular Contributor
timestamp: April 23, 2022, 11:56:44 am
content: Quote from: thm_w on April 21, 2022, 11:22:39 pmThank you for the enthusiastic drawing, it was great!!! Love you

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 24, 2022, 03:44:47 pm
content: Quote from: Ranayna on April 22, 2022, 01:06:26 pmThe answer to that question is more complictated then it seems at first sight.The copper pads (much wider then their length so proportionally maybe 1/4 "square" concerining resistance) and on top of that a generous blob of solder form a block of conductivity right at each end of the resistor.It is quite common to tap-off the sense lines directly from the center of the SMT pads, and then on the inside (so below the resistor).Quite some time ago, in one of the teardowns / reviews that Davey Jones did of some high-end Agilent / Keysight / Siglent equipment (Mabye a SMU, power supply or DC-Load) One of this type of resistor was used, but both the "L1 and "E2" connections were on a single pad, and the "L1" and "E1" connections were also on a single pad.And I still don't know whether this was by design or by coincidence.It may have been that it was first designed for a "regular" resistor, but that was not available in the required stability or tolerance.It could also have been a deliberate design. Such splits on the side of an SMT resistor are a design compromise. It could be that the copper pads and the thick solder blob does a better job at "equalisation" then the slots in the resistor with the sense wires on the side.

author: sahko123
author_rank: Frequent Contributor
timestamp: April 24, 2022, 11:51:44 pm
content: With sense pins you want to be as super close as possible to what you are measuring to make sure that there are as little error or offset as possible. Thats why you can sometimes see sense connectors on some power supplies.Because a large current going through a small resistance can generate a voltage difference. The traces leading up to the resistor have resistance as well and rather than tapping off of that you can tap off of the component directly and then you dont need to worry about the pcb trace's resistance.