Spice model of Current Sense Resistors? - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: January 03, 2020, 06:00:28 pm
content: I am following one of Dave's great video's on 1A current sources:He is using a 4 pin resistor but I can't find any spice library that have those types in its component list. I have looked and installed KiCad, LTspice, OrCAD Trial, Tina-TI and Micro-cap, I have also searched on google but do only get sites where the component is described, no spice model.Do any of you have some advice on how to find a spice-model of one?EDIT: Or could I just make the sense pins as tracks instead?

author: rstofer
author_rank: Super Contributor
timestamp: January 03, 2020, 06:20:39 pm
content: As pointed out around 8:19, if you don't have a 4 terminal resistor, use a Kelvin style connection where the voltage sense wires do not carry load current. Any resistor you model in LTspice can have a Kelvin connection but since wires don't have resistance in LTspice, I don't know that it matters. You could, I suppose, model the wire resistance and then it would matter how you made the Kelvin connection.Kelvin connections have been common on bench DMMs forever.In any case, simulation isn't hardware. Even if LTspice tells you the circuit is perfect, until it is actually running, it isn't real.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 03, 2020, 06:47:08 pm
content: Easy enough to construct a model given a layout, but without a layout one can only guess at the strays (extra terminal resistance, ESL).Tim

author: FriedMule
author_rank: Frequent Contributor
timestamp: January 03, 2020, 08:55:32 pm
content: Thanks for your replies!As a noob do I know that a circuit do not work for real before it's real, but on the other hand do i also not know if a combination of components do even work as I hope. Therefore my simulation before constructing. :-)Yes Kelvin clamps is a fantastic tool to use, it's also a kind of what the resistor in Dave's circuit do work as. But am I wrong in thinking that pin 1 and 2 has common connection and 3 and 4 also has common connection?If so, would I imagine you could just connect tracks to a single point on each side of the resistor, and in that way make it work as a kelvin track.If I am asking about anything you have already written, please bare with me, there were some of the text I may not have understood completely.

author: magic
author_rank: Super Contributor
timestamp: January 03, 2020, 10:42:24 pm
content: The point of those 4 terminal resistors is that the place where you sense voltage drop is inside the resistor and everything is factory calibrated and guaranteed to meet the spec. You don't need to worry if solder resistance brings you out of 1% tolerance, what might be the thermal drift of the reistor's external leads and so on.For basic testing in SPICE just use a standard 2 terminal ideal resistor. You may add some additional resistance outside the Kelvin connection to see if the rest of the circuit can deal with voltage loss in PCB traces, cables etc.

author: rstofer
author_rank: Super Contributor
timestamp: January 03, 2020, 11:00:00 pm
content: In the control world, 50 mV shunts are common. Full current, whether a 1A shunt or a 500A shunt all result in a 50 mV drop at maximum current.Note how the Kelvin connection is set up. The small screws are for the voltage sense wires, the large terminals for the current carrying conductors.https://www.amazon.com/AMMETER-SHUNT-500-AMP-MILLIVOLT/dp/B005BHPG6KIs there some voltage drop in the terminal block between the current terminal and the sense terminal? Sure, but it won't be much.For LTspice purposes, I would ignore the issue.

author: FriedMule
author_rank: Frequent Contributor
timestamp: January 04, 2020, 07:49:04 pm
content: Quote from: magic on January 03, 2020, 10:42:24 pmThanks, so as I understand it, for the purpose of testing if I have made a "component smoker" or not, I can just make something that sort of function as te target design, and when I then later want to build, I can then use real components and test by a real scope and so on?My simulator tells me that I have made a