After reading several papers regarding the kelvin connection and the difference between 4 wires and 2 wires connection. I end-up by choosing to use a 2 pads current sense resistor for my PCB. The problem I am encountering right now is that when trying to use a kelvin connection for a 0603 shunt resistor, the tracks are too big to be connected inside the resistor, as it can be seen below :
I tried to modify the nettie to achieve smaller dimensions but still in vain, the problem isn’t with the nettie, it’s within the dimensions of the resistor itself.
Ps: If you want to suggest to go for a bigger shunt resistor, it will be an overkill for my circuit. With 0.1 Ohms and (0.5A)
The datasheet recommends a shunt resistor of 0.01 ohm, which gives a voltage drop of 5 mV (with 500 mA and 4.2 V). However, the ADC operates with a range of -125 mV to 125 mV. That’s why I opted for a 0.1 ohm resistor, the voltage drop will be 50 mV, which makes the ADC more sensitive to voltage variations. I don’t want to go for a bigger resistor because the actual rated power of this resistor is 25 mW, which justifies the choice of a 0603 (0.1 ohm) resistor with a rated power of 100 mW. If I go for a bigger resistor it will has the power rating of 250mW which can be a overkill for my circuit.
Yes. Use a bigger resistor.
Power handling is not the only factor to determine resistor size. If you have difficulty fitting the sense wires because of the minimum track and clearance that your PCB manufacturer can handle then you have another good reason to use bigger resistors.
Other options are to move the net-ties. You can put them on the side of the resistor, with probably a small accuracy penalty. you can also modify the net-ties into an L shape, so the sense wires don’t have to go under the resistor at all.
But overall, using a bigger resistor is probably the best choice. It is just a few square mm after all.
What design is so small that making one resistor 0.5mm longer overkill your design?
If in amateur design I would need to use 0603 and be sure that resistance of soldered connection don’t adds to my measuring resistor resistance I would consider using 2 thin, short wires to connect to resistor pads from its top.
That’s like saying “give me a smaller glass - I’m only going to drink a sip.”
But, if you want to use a tiny resistor, have the sense connections exit the side of the pads.
I guess you’ll also have to replace a lot of resistors with 0402, 0201 and 01005 sizes then because a lot of other resistors will be grossly overrated too.
Resistors are normally packed on spools of 3000 pieces. Each spool of 0.25W resistors has therefore a power of 750W. If you need 50 different resistor values and buy a spool of each, the total is 37500W. What do you think, will such power kill your cabinet with drawers for components?
Kelvin connection only makes sense for very low Ohm values.My designs use 1 to 3 mOhm
The smallest package you can easily buy these is 2512. A quick search on RS finds plenty much bigger.
Additionally, overrating resistors reduces their temperature (As does connecting them to more copper which acts as a heat sink). As a result resistance changes due to the temperature coefficient will be smaller, and thus result in a more accurate current measurement. Or, a less expensive resistor with a bigger temperature coefficient can be used for the same measurement accuracy / stability.
How is this a problem? There is no rule saying that the power dissipation capability of the resistor cannot greatly exceed its power dissipation in the circuit. It sounds to me like you have a good reason to use a larger resistor, unless you do not have the pcb space for it.
I have been designing power supplies since 1980.
Another option is to split the pads to put two on each end. But I am not sure that that will help anything. With larger chip resistors, I have seen designs using three pads on each end.
However I think these schemes may be imprecise due to results which may vary with the way the solder flows. I think that simply connecting two tracks to each pad might be the best option for a small SMT resistor.
The whole topic started by this user about the use of a shunt resistor was never about KiCad and has been done to death already . . . there are already 2 posts closed on this topic I suggest we make it a 3rd.
Lets keep this forum about helping KiCad users who need help using KiCad . . . not Electronics 101.
I would prefer to go for bigger resistors in that case, I prefer to keep the sensing current tracks under the resistor to be close enough to the resistor pads for better sensing current.
Not the design itself, replacing a resistor that dissipates 2.5mW with a resistor that has a power rating of 250mW is like using *100 times the resistor power rating! isn’t overkilling ?
