The idea you describe can work, but it is very crude to remove the power from the coil by shorting the battery. At the very least you should insert a resistor to limit the current somewhat, as is you rely on the internal resistance of the battery, which can be OK for a 9V radio battery, but disastrous for 12V car battery.
The more common way to achieve roughly the same is to use a 'relay' with a Normally Closed (NC) contact, so you can open the circuit instead of shorting the battery. This is in fact how the common electro-mechanical doorbell works.
There are three main issues that come to my mind:
- Wire resistance: you already took it into account.
- Wire inductance: you already took it into account, too (more on this later).
- Transmission line effects: these will affect your circuit if the wires have a length which is comparable or greater than the minimum wavelength of the "signal".
About point 3: since you are not concerned with signal integrity (your "signal" is the power rail to the relay) you only need to worry if your switching times are too quick (some energy could be reflected back from the line toward your transistor ad fry it). If you switch the MOSET relatively slowly the frequency content of the "step" (a ramp, actually) won't hit that limit and you won't have problems, apart from higher power dissipation in the MOSFET during switching, but given the extremely low duty cycle of the system it is of little concern here probably.
Anyway LTspice has two different models that can represent transmission lines: a lossy one and a non-lossy one. Excerpts from the online guide:
T. Lossless Transmission Line
Symbol Name: TLINE
Syntax: Txxx L+ L- R+ R- Zo= Td=
L+ and L- are the nodes at one port. R+ and R- are the nodes for the
other port. Zo is the characteristic impedance. The length of the line
is given by the propagation delay Td.
This element models only one propagation mode. If all four nodes are
distinct in the actual circuit, then two modes may be excited. To
simulate such a situation, two transmission-line elements are
required. See the schematic file
.\examples\Educational\TransmissionLineInverter.asc to see an example
simulating both modes of a length of coax.
and:
O. Lossy Transmission Line
Symbol Name: LTLIN
Syntax: Oxxx L+ L- R+ R-
Example:
O1 in 0 out 0 MyLossyTline .model MyLossyTline LTRA(len=1 R=10 L=1u
C=10n)
This is a single-conductor lossy transmission line. N1 and N2 are the
nodes at port 1. N3 and N4 are the nodes at port 2. A model card is
required to define the electrical characteristics of this circuit
element.
Model parameters for Lossy Transmission Lines
[...table with all parameters omitted...]
Point 2 is more problematic, especially when switching the relay OFF: you could have an inductive kickback that destroys your MOSFET due to the wire inductance. Note that the diode across the relay won't protect you in this case. Thus a protection Zener at the switching transistor output (between drain and ground, cathode connected to drain) may be necessary to dampen that inductive kickback.
An article on the subject is here (not directly related to your specific case, though).
Best Answer
It should be near Zero. Specifics will depend on the specific for your switch. The first tact switch I found on google, the Omron B3F series shows Contact resistance of 100 milliOhms (0.1 Ohms) initially. It is also only rated for 1 - 50 mA at 5 to 24VDC. It lasts a few million presses when used with 100 grams of force.
This is subjective. There are Two considerations.
Resistance of the contacts does not matter much in input sensing application where current flow is minimal. But it matters when being used for switching a large sensitive load.
Or when for some reason being used to switch in an analog input where the resistance is being measured. If the switch has built up more resistance than is accounted for, it will interfere with measurements. 1 to 100Ω may not matter when the measurement is looking for an order of magnitude higher (10k resistance), but if its looking for 1k resistance and your switch is now measuring 1k, that can lead to false positives.