What is the usage of Zero Ohm & MiliOhm Resistor?
There's a huge difference between a 0 Ω resistor and a 1 Ω resistor: the latter has an infinitely larger resistance :-).
The 0 Ω has different usages:
- selective connections. You can create variants of your circuit by placing or leaving out the jumper. Just like you would delete a connection in your schematic capture program (= remove jumper) and make a connection to a different point (= place jumper)
- facilitate routing. A couple of jumpers over traces may allow you to use a single layer board instead of a double layer, which would cost you more. You'll typically use 0603 or 0805 size jumpers for this; 0402 are too small to bridge an average trace.
- provide a current measurement point. During development and testing you can place a low resistance shunt resistor to measure the current, and for production replace it with a zero ohm jumper. Then you don't have to cut traces to insert the shunt resistor in the circuit. Probably less applicable, since you should have measured the current before creating the final PCB, but for very low current circuits the layout and PCB material may matter, and then you do want to measure on the final board.
Zero Ohm "resistors" are frequently used as links on single side boards because they can be placed by component insertion machines that can insert resistors.
High volume single sided board manufacturers often use a separate link inserting machine - whose frighteningly fast speeds need to be seen to be believed.
A 1 Ohm resistor is "just another component".
It may be used as a current sense resistor or for some other circuit function.
If using resistors for current sensing for measurement purposes.
Worst case voltage drop across them should be small compared to total circuit voltage so that they do not affect operation. eg if a circuit draws 1 amp and has a 5V supply then a 1ohm resistor would drop 1 Volt. This is 20% of total circuit voltage and would be excessive in essentially all real world cases.
A 0.1 Ohm resistor would drop 0.1 V at 1A = 2% of supply and MAY be acceptable depending on circuit.
A 0.01 Ohm resistor will drop 0.01V at 1A = 0.2% and would almost always be acceptable.
The 0.1 Ohm resistor will drop 100 mV per Amp so 1 mA will produce 100 uV.
Many low cost DMMs have a 200 mV range with a resolution (but not accuracy) of 0.1 mV = 100 uV, so they can read current in a 0.1 Ohm resistor to 1 mA resolution. Similarly they can read current in a 0.01 Ohm resistor to 10 mA resolution.
Placing the sense resistors with one side grounded allows ground referenced measurement which may be convenient. The Voltage drop must not affect circuit operation.
Sometimes bypassing the sense resistor with a capacitor - maybe 10 uF or 100 uF depending on circuit, will further reduce impact on the circuit.
Where high frequency noise is present use of a DMM or other meter to measure voltage so as to calculate current will give bad results die to noise entering the meter. In such a case use an eg 0.1 Ohm sense resistor, feed the voltage via a series 1k resistor to the meter and add a say 10 uF across the meter terminals.
I have seen 0 ohm resistors used in calibration/testing. For example, if you put an RC lowpass on a board but realise that it isn't required, you just put a 0 ohm instead of any resistor and leave the capacitor off.
This selective building of noise reducing circuits is quite common; if you open up some commodity hardware that is relatively complex (DTV receiver for example), you might see that a lot of decoupling capacitors are left off. This is because they test the boards after manufacture, and if they are too noisy after QA, they just put more capacitors on in different places until it passes. Some extremely sensitive instrumentation devices can have a completely unique denoising circuits (as tuned by a grey-haired, bearded man of course)
Also: You can use them as a kind of soldered down DIP switch to select features for a device.