Shunt resistance is the simplest, cheapest and most accurate way to measure current. Think of Ohm's law, V = IR. Here, if you put R = 1 ohm, V = I. That is, when 1A of current flows, the voltage across the resistor becomes 1V, so the current can be converted into a voltage immediately.
The second method is to measure AC current. When measuring high voltage current, there is a risk of electric shock. So, when measuring high voltage current, it is measured using a current transformer called CT.
However, CT can only measure AC current. This is improved by using the third Hall sensor and magnetic force.
Both DC and AC current can be measured. However, the price is high and the precision is somewhat inferior.
The shunt resistor is mainly used to measure low voltage and DC current.
CT is used to measure current in a non-contact method in high voltage AC.
Transformer + Hall sensor can measure both DC and AC current and is mainly used for measuring instruments and special purposes.
By understanding the current measurement circuit below, you can understand the role of the current measurement resistance.
1. Connect the shunt resistor in series to the circuit where you want to measure the current.
In the circuit below, the current values flowing through the shunt resistance and load resistance are the same.
2. The current value can be found by measuring the voltage value across the shunt resistor.
When current flows in the circuit below,
A voltage drop occurs in the shunt resistor. The voltage drop is RxI according to Ohm's law.
For example, when measuring the voltage across the shunt resistor
If the voltage difference is 1V and the resistance value of the shunt resistor is 1 ohm, then the current is 1A.
When the voltage difference changes to 3V, the resistance value is fixed, so the current value becomes 3A.
In other words, if you measure the voltage difference across the shunt resistor in the circuit below, you can easily measure the current value.
3. The shunt resistor has a low resistance value.
Because, as the shunt resistance value increases, the voltage applied to the load decreases significantly.
If possible, use a shunt resistor with a low resistance value to avoid affecting the load voltage.
4. If the shunt resistance is unconditionally low, current measurement may be difficult.
If you use an excessively low shunt resistance value to reduce the voltage drop,
The voltage drop decreases proportionally, but in some cases it is difficult to measure or the error becomes large.
Therefore, the shunt resistance value should be as low as possible,
Consider the expected range of current values in the circuit and select an appropriate value considering the resolution of the voltage measurement circuit.
5. If the shunt resistance value increases, the resistance value changes due to heat generation and the error of the measured value may increase.
If the shunt resistance value is too large, the voltage drop also becomes a problem,
The resistance value may change due to the heating phenomenon of the shunt resistance due to voltage drop.
In this case, of course, the error of the measured value of the current increases.
Therefore, the shunt resistance value should be selected as small as possible within the measurable range.
You must use a resistance with a power resistance that can withstand the range of the current flowing in the circuit.
Answer C
There are roughly three ways to measure current.
Shunt resistance is the simplest, cheapest and most accurate way to measure current. Think of Ohm's law, V = IR. Here, if you put R = 1 ohm, V = I. That is, when 1A of current flows, the voltage across the resistor becomes 1V, so the current can be converted into a voltage immediately.
The second method is to measure AC current. When measuring high voltage current, there is a risk of electric shock. So, when measuring high voltage current, it is measured using a current transformer called CT.
However, CT can only measure AC current. This is improved by using the third Hall sensor and magnetic force.
Both DC and AC current can be measured. However, the price is high and the precision is somewhat inferior.
The shunt resistor is mainly used to measure low voltage and DC current.
CT is used to measure current in a non-contact method in high voltage AC.
Transformer + Hall sensor can measure both DC and AC current and is mainly used for measuring instruments and special purposes.
Answer B
By understanding the current measurement circuit below, you can understand the role of the current measurement resistance.
1. Connect the shunt resistor in series to the circuit where you want to measure the current.
In the circuit below, the current values flowing through the shunt resistance and load resistance are the same.
2. The current value can be found by measuring the voltage value across the shunt resistor.
When current flows in the circuit below,
A voltage drop occurs in the shunt resistor. The voltage drop is RxI according to Ohm's law.
For example, when measuring the voltage across the shunt resistor
If the voltage difference is 1V and the resistance value of the shunt resistor is 1 ohm, then the current is 1A.
When the voltage difference changes to 3V, the resistance value is fixed, so the current value becomes 3A.
In other words, if you measure the voltage difference across the shunt resistor in the circuit below, you can easily measure the current value.
3. The shunt resistor has a low resistance value.
Because, as the shunt resistance value increases, the voltage applied to the load decreases significantly.
If possible, use a shunt resistor with a low resistance value to avoid affecting the load voltage.
4. If the shunt resistance is unconditionally low, current measurement may be difficult.
If you use an excessively low shunt resistance value to reduce the voltage drop,
The voltage drop decreases proportionally, but in some cases it is difficult to measure or the error becomes large.
Therefore, the shunt resistance value should be as low as possible,
Consider the expected range of current values in the circuit and select an appropriate value considering the resolution of the voltage measurement circuit.
5. If the shunt resistance value increases, the resistance value changes due to heat generation and the error of the measured value may increase.
If the shunt resistance value is too large, the voltage drop also becomes a problem,
The resistance value may change due to the heating phenomenon of the shunt resistance due to voltage drop.
In this case, of course, the error of the measured value of the current increases.
Therefore, the shunt resistance value should be selected as small as possible within the measurable range.
You must use a resistance with a power resistance that can withstand the range of the current flowing in the circuit.
Answer A
Put in a very small resistor
This method is used because the current flowing in the reverse direction can be obtained by obtaining the voltage between them.
The reason for putting in a resistance is that we can get the current value by putting in the resistance we know.