After discovering why calculations being difficult when using I(Rxx) applied to B type device.
The random placement to a schematic of a resistor using rotate means current could be positive or negative depending on the rotation. I know it now, but a polarity dot to a resistor could be a hint or improvement. Thanks.
This is a topic that has been raised multiple times; almost everyone experiences this confusion when they first start learning SPICE simulation. Mike did not include polarity notation in LTspice for resistor, and this practice has carried forward. Personally, I think adding a ‘+’ sign or a dot to a resistor just makes the schematic more cluttered. My own practice is to either place a new resistor (directing current from top to bottom) or rotate it once (directing current from left to right); I believe advanced users all follow similar practices.
In addition, the lack of notation on resistors ensures that beginners go through the ‘painful’ process of learning how SPICE defines current direction, especially since this generation learn SPICE through GUI instead of netlist.
This forum serves the purpose of community support. All feature requests or bug reports should be emailed to Mike; his email address can be found under Help > About Qspice.
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Custom symbols with polarity notation
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After you get more and more advance, you will then find yourself needing to create custom block, then custom library.
Anyway, even after dealing with circuit and sim for 12years, I still often forget if entering or leaving the dot is the positive or negative direction. At the end I always check again.
*Another confession, I don’t remember current in diode flows from A to K or K to A.
For two-terminal devices is that +ve current flows from the first node to the second node. In SPICE, the first node of a diode is the Anode, and the second is the Cathode.
For three-terminal devices, +ve current always flows into the device through that node. For a MOSFET, Id(M1) is the current flowing into the FET, and Is(M1) is also defined as the current flowing into the FET; as a result, their signs are opposite.
Inserting a 0V voltage source for current measurement is a reliable method if you cannot tell which node is the first and which is the second, as a voltage source symbol must have + and - in its symbol. If I remember correctly, there is no simple way in LTspice to identify the specific nodes of a resistor. But there is an arrow pointing current direction if hover over the device in probing. Well, but for Qspice, no such option available.