Measurement should not influence the circuit you are examining: that is why the inserted resistor must have a very low resistence in the range of only a few milliohm. This ensures that the real load “won’t notice” the presence of the additional shunt resistor, and no excessive energy is consumed by the resistor and converted to heat. Such special resistors are called shunt.
When you add a shunt to a circuit, all of the current passes the low-resistance shunt. The current flow is equal to the voltage drop across the shunt divided by the resistance of the shunt.
The result is typicall a very low voltage that can be measured. Since this voltage drop is in the range of a few milliamperes, special amplifier circuits are used. A regular multimeter would not be able to measure the minimal voltage drop across a shunt with the necessary precision.
Selecting a Shunt
The most important spec for a shunt is the maximum current it is supposed to measure. Shunts are typically designed to drop 50mV, 75mV, or 100mV when operating at their maximum rated current. When the maximum current is known, the necessary resistance can be calculated based on the anticipated voltage drop.
Depending on maximum current, a shunt can be a tiny SMD resistor or a huge external stripe of metal. For large currents, you can even build your own shunt from a piece of wire.
Here is an example calculation for a maximum current of 5A and a desired voltage drop of 100mA:
- 0.02 Ohm: The required shunt resistance is according to Ohms Law: U = R * I, R = U / I, thus 0.1V / 5A = 0.02 Ohm. Keep in mind that shunt resistors must be very precise and should not change their resistance when they heat up.
- 0.5W: The shunt would also need to be rated for P = U * I, thus 0.1V * 5A = 0.5W. This would be the minimum rating, and you should double it as a safety margin and to prevent heating and its influence on resistance.
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(content created Feb 27, 2024)