feedbackresistanceresistors
Are there any problems that can be caused by using resistors of large resistances (in the order of megaohms)?
I'm designing a feedback network that is just a voltage divider, and I want the feedback to drain as little current as possible from the circuit. The only thing that matters is the ratio between the resistors. So my question is: is there any reason why one would pick, for example, resistors of 1 and 10 Ohms instead of 1 and 10 MOhms?
At first order approximation, you are right in that Rg could be replace by a short and you'd get the same thing, at least in the steady state. However, the dynamics in response to Vdd changes would be different. MOSFETs have a fairly significant effective gate to drain capacitance, which forms a low pass filter together with Rg.
However, I suspect the real reason for high Rg is that this is just a small piece of a larger circuit. The intent is to bias M1 at some reasonable operating point, then feed in small changes to that bias point onto the gate. In the wider circuit, there is probably a capacitor connected to the gate, with the other side of the capacitor being driven by a signal that this transistor is supposed to amplify. Rg is therefore set high enough to be large relative to the impedance of the capacitor over the frequency range of interest. That means Rg supplies the DC bias value, and the AC component comes from the signal thru the capacitor.
Firstly, does it really need to measure resistance? As far as I can tell, it will be a case of a contact either being open circuit or short circuit - nothing inbetween?
The advantage of microcontrollers, Arduinos and so on is they can perform operations in very quick succession, such that they appear simultaneous to the human eye.
So make the Arduino test one part at a time, in quick succession. Thus it will appear as if it has tested all 3 simultaneously.
I cannot work out what your fencing body "cord" does exactly. If you give more information on how it works, perhaps someone can design the circuit for you.
Also, what you say about the mis-wiring problem is not clear: do you want the circuit to some how know if you have mis-wired it? I can't see that being possible.
Edit:
I think I know roughly what you want now. At the least, I can give you a circuit which you can adapt.
The principle will be as per Spehro Pefhany's comment: use 6 pins from the Arduino- three digital to drive and three analog to read. So this is how that would look:
simulate this circuit – Schematic created using CircuitLab
Ok, it looks like CircuitLab is giving me errors. I will come back to that circuit when it's back up. In the meantime, this is how your pseudocode for the above would look:
The above will firstly allow you to confirm that the plug is connected the right way. (If the first A is connected to the other A). Then it will let you measure the resistance between all three wires. If you want, you could expand the above to test between all three wires on both ends. The above circuit has some redundancy, you might have noticed. With the above pseudocode, you wouldn't need DO4, D4, R1, A1. So if for example you wanted to test resistane/continuity between 6 wires, you would just need 5 digital ouputs, 5 digital inputs, 5 resistors and 5 diodes.
So you do not need to read the resistances simultaneously. Steps 1-8 above can be completed within a very tiny fraction of a second - in the order of 100 microseconds. (one ten-thousandth of a second).
Best Answer
There are many drawbacks to both low and high values alike.
The ideal values will fall in between very large and very small for most applications.
A larger resistor of same type will, for example, create more noise (by itself and through small induced noise currents) than a smaller one, though that may not always be important to you.
A smaller resistor will drain more current and create more losses, as you have surmised yourself.
A larger resistor will create a higher error with the same leakage current. If your feedback pin in the middle of your resistors leaks 1 μA when the resistor feeding that leak is 1 MOhm, that will translate to an error of 1V, while a 10k resistor will translate to an error of 10mV.
Of course, if the leakage is in the order of several nA or less, you might not care much about the error a 1 MOhm resistor creates. But you might, depending on what exactly you are designing.
Smaller resistors in feedback systems, e.g. with inverting amplifiers using op-amps, may cause errors on the incoming signal if the incoming signal is relatively weak.
It's all checks and balances, and if that's not enough information at this point, you might want to ask a more direct question about specifically what you are doing. With schematics and that.