What is the importance of a 4-terminal shunt resistor? What is real purpose behind using two seperate terminal for the voltage measurement across the shunt resistor?
Take a look at this. It shows two terminals as the main current flow (thick tracks) and uses 2 more terminals for measurement of the volt drop generated by that current: -
The "better" way is on the left because it takes measurement connections from a defined place and at no point on those measurement connections is there load current flowing.
The picture on the right shows the measurement connections at some small distance from the shunt resistor terminals and therefore there is a small volt drop that forms an error - in effect you can't rely on the stated value of the shunt resistor in order to convert the measured voltage to an assumed current flow.
I will just go out on a limb and say that it's probably because of this:
- 2 terminals for the current to flow through.
- 2 terminals for measuring.
By having 4 connections, the supplier of the shunt resistor can make sure that what you are measuring across will be the shunt resistor where the current will flow which you are trying to measure.
If there was only two terminals, then you would have to decide for yourself where you want to branch off for measuring. There are many viable positions to measure across, but which one will most correctly measure the voltage across the shunt resistor? When you are making high quality stuff, this is no simple question.
Shunt resistors are used where there are high currents. As such the point at which you tap of the voltage to be measured is important. Moving it by even 1mm may make a big difference especially if there are large currents.
The four tap shunt is built to have the voltage points there where they represent the correct conversion from amps to voltage. See it is part of the calibration.
On your question of putting them in parallel.
That would be possible if the two resistors where really equal in value. In that case you can measure one of them and multiply the result by two.
But we all know components are never exactly accurate. Thus a small difference in resistance will make the current split unequal. I would say you have to measure both voltages and add them up.
Shunt resistors for measuring large currents have very low resistance so they don't consume excessive power and produce excessive heat. Shunt resistors are usually far less than 1 ohm, usually being 0.1 or 0.01 ohms or even 0.001 ohms. The problem with using resistors of such a low value is that the resistance of the solder joints and circuit board traces can be a similar order of magnitude and hence will create a voltage drop similar in magnitude to the voltage drop across the sense resistor itself. This makes accurately measuring the voltage drop through the precision sense resistor a bit tricky. The most robust solution to this is to separate the high current path from the voltage sense path with a 4 terminal shunt. The high current path may have a few extra milivolts of drop due to the extra miliohms of resistance from the traces and solder joints, but the voltage sense connections will not see this due to the different current path and can therefore much more accurately read the shunt voltage.