Working principle of sampling resistor

火辣西米秀

Working principle of sampling resistor [Copy link]

1. The basic principle of current detection resistor:
According to Ohm's law, when the measured current flows through the resistor, the voltage across the resistor is proportional to the current. When the current passing through the 1W resistor is several hundred milliamperes, this design is no problem. However, if the current reaches 10-20A, the situation is completely different, because the power loss in the resistor (P=I2xR) cannot be ignored. We can reduce the power loss by reducing the resistance value, but the voltage across the resistor will also decrease accordingly, so based on the sampling resolution, the resistance value of the resistor is not allowed to be too low.

Second, long-term stability
is very important for any sensor. Even after a few years of use, people hope to maintain the early accuracy. This means that the resistor material must be corrosion-resistant during its life cycle and the alloy composition cannot change. To make the measuring element meet these requirements, an alloy composed of homogeneous composite crystals can be used, and the basic thermodynamic state can be achieved through annealing and stabilization production processes. The stability of such an alloy can reach the order of ppm/year, making it suitable for standard resistors.
After aging for 1000 hours at 140°C, the surface mount resistor has only a slight drift of about -0.2%, which is caused by lattice defects caused by slight deformation during the production process. The drift of resistance is largely determined by high temperature, so at lower temperatures such as +100°C, this drift is actually undetectable.

Third, the terminal is connected
to a low-resistance resistor. The influence of the terminal's resistance and temperature coefficient cannot be ignored. These factors should be fully considered in the actual design. The additional sampling terminal can be used to directly measure the voltage across the metal material.

The electron beam welded copper-manganese-nickel-copper resistor actually has such a low terminal resistance that it can be used as a two-terminal resistor through reasonable wiring and approach the performance of a four-terminal connection. However, when designing, it must be noted that the signal connection line of the sampling voltage cannot be directly connected to the current channel of the sampling resistor. If possible, it is best to connect to the current terminal from the bottom of the sampling resistor and design it as a microstrip line.

Fourth, the low resistance
four-lead design is recommended for high current and low resistance applications. The usual practice is to use manganese-nickel-copper alloy strips to directly stamp into resistors, but this is not the best way. Although four-lead resistors are beneficial for improving temperature characteristics and thermal voltage, the total resistance is sometimes 2 to 3 times higher than the actual resistance, which will lead to unacceptable power loss and temperature rise. In addition, the resistance material is difficult to connect to copper by screws or welding, which will also increase contact resistance and cause greater losses.

Constantan wire resistor
Speaking of current/voltage sampling circuits, like the one used in the multimeter in the picture above, what is a constantan wire resistor?

Simply put, constantan wire resistors are made of high-precision alloy wire and processed through special processes. They have low resistance, high precision, low temperature coefficient, no inductance, and high overload capacity.

Because of the excellent electrical properties of constantan wire, it is widely used in communication systems, electronic equipment, power supply circuits for automation control, etc. for current limiting, current sharing or sampling detection circuit connections.

Constantan wire has a low temperature coefficient of resistance, a wide operating temperature range (below 500°C), good processing performance, and good welding performance (this is very important!).

In addition, there is a new constantan resistance alloy, which is a copper-iron based alloy. It has the same resistivity as constantan, a basically similar temperature coefficient of resistance, and the same operating temperature.

Manganese
copper wire resistor is the same as constantan wire resistor. It is made of precision alloy wire and processed by special process to make it have low resistance, high precision, low temperature coefficient and good stability. It has no inductance and high overload capacity.

Manganese copper wire resistors are also widely used in communication systems, electronic equipment, power supply circuits for automation control, etc. for current limiting, current sharing or sampling detection circuit connections.

After reading the description, we found that it seems that manganese copper wire and constantan wire are actually similar, and their resistivity is also similar.

Which sampling resistor is better?
There is no essential difference in the performance and use of the two resistors, but if it is used as a sampling resistor, it is more inclined to the manganese copper wire resistor, which has better stability. The
resistance of the constantan wire resistor ranges from 0.1 milliohms to 100 milliohms, the power ranges from 1 watt to 30 watts, and the product accuracy can reach up to 0.5%.

The resistance of manganese copper wire resistors ranges from 2 milliohms to 1 ohm, the power is available from 1 watt to 10 watts, and the accuracy is 1% and 5%.

From this table, we can conclude that the temperature coefficient of resistance of constantan is more than 4 times that of manganese copper; the thermoelectric potential of constantan to copper is more than 20-40 times greater than that of manganese copper; in addition, due to the high nickel content of constantan, constantan is not as easy to weld as manganese copper when soldering with ordinary flux.
In general, both can be used as materials for manufacturing precision resistors, but each has its own advantages: manganese copper has a higher precision level; constantan can also be used to manufacture high-power resistors with a certain accuracy.

The implementation of a simple sampling circuit is
simple but not simple. Three formulas: R=U/I; Since it is a sampling circuit, it can be divided into two practical applications, one is current sampling, and the other is voltage sampling. Sometimes these are just two different names, and the implementation methods are similar, but the quantities required in specific applications are different. Even so, according to different circuit parameters and requirements, the corresponding sampling circuits may also be very different, so here we only talk about the application ideas of sampling resistors, and no longer talk about those "boring" circuit principles.

For ordinary enthusiasts, the most commonly used function should be the sampling of small currents or small voltages. For this type of circuit, in layman's terms, if you want to use sampling resistors to achieve current or voltage sampling, another important component commonly used is a chip with A/D conversion function. If necessary, the sampled current or voltage needs to be amplified first, and functional chips such as operational amplifiers are used here.