【Daily Question 20090208】Why does the technical specification for instruments with 4–20mA output require a load resistance of 250Ω?

I think it’s a problem with impedance matching inside the instrument!

I think it’s an impedance matching issue. Impedance matching refers to a operating condition in which the load impedance and the internal impedance of the driving source are matched with each other, thereby achieving maximum power output. For circuits with different characteristics, the matching conditions vary. In a purely resistive circuit, the output power is maximum when the load resistance equals the internal resistance of the driving source; this operating condition is known as matching, whereas otherwise it is referred to as mismatching. When the internal impedance of the excitation source and the load impedance contain reactive components, in order to deliver maximum power to the load, the load impedance and the internal impedance must be in a conjugate relationship, that is, their resistive components must be equal, while their reactive components must have equal magnitudes but opposite signs. This matching condition is called conjugate matching. From the perspective of energy transmission, in order to avoid any reflections in the transmission system, it is required that the load impedance be equal to the characteristic impedance of the transmission line or four-terminal network; that is, the resistive and reactive components must be equal to each other, rather than being conjugate pairs. This matching condition is called reflection-free matching or object matching.

If the load resistance is much larger than this value, it will cause the signal to decrease； The operating voltage for two-wire instruments is 24 volts; the load resistance must match the internal resistance of the transmitter in order to achieve optimal operation ； I encountered a malfunction once, and it’s possible that this was the cause: the signal measured by the two-wire transmitter was displayed differently in the DCS and on the local digital display ； I started connecting the signals in series; neither the digital display nor the DCS showed any output. Later, I measured the supply voltage and found it to be 21VDC. Only by connecting a separate 24V power supply in series did things work properly, and the DCS was connected using a four-wire configuration. It’s possible that the series resistance is too high, and the load exceeding the voltage provided by the DCS prevents proper operation.

It is usually instruments that provide a output of 4–20 mA. The technical specifications specify load resistances of 100Ω, 250Ω, and 350Ω—what is the meaning of this parameter? What will happen if the load resistance is much larger than this value? Why? Answer: This parameter requires that the total of the internal resistance of the instrument connected after the output instrument and the line resistance must not exceed this value. If the load resistance is much larger than this value, the output current will decrease significantly, preventing the device from functioning. The main reason for this is that it disrupts the constant-current characteristic of the constant-current source in the output circuit; in severe cases, it’s as if the load is essentially an open circuit, which can lead to damage to the output device.

The friend upstairs is absolutely right! The value of the load resistance is determined by two factors ； Firstly, the output of a constant current source is not unlimited; there is definitely a limit value. If this value is too high, it will surely damage the constant current source ； The second factor is determined by the 24V voltage; since a DC voltage of 24V has been chosen, and the standard current limit for instruments is 20 milliamps, the internal impedance of the amplification circuit also plays a role in determining the load resistance. Depending on the choice of amplification circuit and current source, different load resistances can be matched, which is why there are loads with various resistance values.

The current-carrying characteristic of the constant current source in the signal output instrument is fixed; if the load impedance increases indefinitely, this will cause the load voltage to drop sharply, and in severe cases normal operation will not be possible. An open circuit of the constant current source (when the load is nearly open circuit) can ultimately lead to damage of the output instrument.

The value of the load resistance is determined based on the measurement circuit; different integrated circuits have different parameters that require corresponding matching impedances. When the wiring and load resistance are within the specified range, their impact on the current can be ignored; otherwise, the measurements will be inaccurate. If the load resistance is much greater than the specified value, there will be a significant drop in current. This post was last edited by zhaohh3211 on 2009-2-10 10:19.]

The requirement is that the sum of the internal resistance of the instrument connected after it and the line resistance must not exceed this value. If the load resistance is much larger than this value, the output current will decrease significantly, preventing the device from functioning. The main reason for this is that it disrupts the constant-current characteristic of the constant-current source in the output circuit; in severe cases, it’s as if the load is essentially an open circuit, which can lead to damage to the output device. A DC voltage of 24V was chosen, and the current limit for standard instruments is 20 milliamps; together with the internal impedance of the amplification circuit, this also determines the load resistance. Depending on the choice of amplification circuit and current source, different load resistances can be matched, which is why there are loads with various resistance values.