What Is a Voltage Converter?

Voltage / current conversion is V / I conversion. It is to convert the input voltage signal into a current signal satisfying a certain relationship. The converted current is equivalent to a constant current source with adjustable output. The load changes. In general, the voltage-current conversion circuit is implemented by the form of negative feedback, which can be negative current feedback in series or negative feedback current in parallel. It is mainly used in industrial control and many sensor applications.

The role of the voltage-to-current converter is to convert the output voltage of the multiplying and dividing unit into a direct current. The voltage-current converter is composed of a self-excited oscillation modulation amplifier and a power detection amplifier. Power detection amplifier composed of components such as transistors, and

If the input signal is a voltage source. And to be transmitted to a farther load. The load current depends on the series resistance between the input signal and the load. Even a small reduction in the voltage drop across the series resistor can greatly change the percentage error of the load voltage. Any change in load resistance due to losses or temperature will cause errors. The simplest type of voltage-current converter is shown in Figure 3.40 (a). The current flowing through the resistor R ₁ is

In this way, the output current i ₀ flowing through the load resistor R depends only on VS and R ₁ , not on R itself. For a certain value of R ₁ , i _{0 is} directly proportional to v _s . Note that no port is connected to ground in Figure 3.40 (a), that is, the load is floating. The advantage of this structure is that no common-mode signals (such as noise) appear on the load. ^[1]

The measurement of the signal is generally performed by the sensor after converting the non-electrical physical quantity into a voltage signal, and then sent to the amplifier via the transmission line for amplification. However, there are three problems in the transmission process:

(1) Weak voltage signals are easily interfered by various external factors during the transmission line transmission process;

(2) The transmission wire itself has resistance, and the voltage signal must generate a voltage drop on the wire, that is, there is an error between the amplifier input signal and the signal source output signal. Different detection points have different lead resistances, causing inconsistencies.

(3) When the ambient temperature changes, the lead resistance also changes.

The above problems can be improved by using compensation wires, shielded wires, twisted wires, and increasing the input resistance of the amplifier.

If the voltage signal of the sensor is first converted into a current signal with a very high output resistance, it is transmitted to the instrument terminal through a transmission line, and then converted into a voltage signal and sent to the amplifier, display instrument or analog-to-digital converter, that is, the current signal transmission is used to solve the problem. . Its advantages are:

(1) Because it is a current signal transmission, the influence of the wire resistance on the signal is negligible. The input voltage of the meter is determined only by the current signal and a fixed load resistance. The transmission line can use ordinary wires, and the transmission distance can be from several meters to several hundred meters. ^[2]

In the industrial control and application circuits of many sensors, when analog signals are output, voltage output is generally used. When transmitting analog signals over a long distance by voltage, the signal source resistance or the DC resistance of the transmission line will cause voltage attenuation. The lower the input resistance at the signal receiving end, the greater the voltage attenuation. In order to avoid signal attenuation during transmission, only the input resistance of the signal receiving end is increased, but the increase of the input resistance of the signal receiving end reduces the anti-interference performance of the transmission line, is susceptible to external interference, and the signal transmission is unstable. When analog signals are used, the voltage output mode cannot be used to convert the voltage output into a current output. In addition, in many conventional industrial instruments, the current connection also requires the output terminal to convert the voltage output into a current output. V / I converter is to convert the voltage output signal into the current output signal, which is beneficial to the long-distance transmission of the signal. V / I converters can be composed of a variety of devices such as transistors.