Charge-pool-power-supply

Description: It is generally advantageous for the remote transmitter of a 4 to 20 mA current loop system to be powered directly from the transmission line. However, in certain scenarios, high power demands of the remote sensor/transmitter system may preclude this option. An alternative to using a separate power supply can be implemented if the remote transmitter operates in a pulsed mode, activating only briefly to perform its functions. In such cases, a charge pool power supply can enable the transmitter to be powered directly by the current loop. In this circuit, a constant current (I1) is supplied to the charge pool capacitor (CP) from the HA-5141, with I1 approximately equal to 3 mA. The voltage (V1) across the capacitor continues to increase until the output of the HA-5141 approaches the positive supply voltage (V+), or until it reaches the optional voltage limiting provided by Z2. The LM2931 voltage regulator then provides the transmitter with a stable +5 V supply derived from the charge accumulated in CP. The available power supply current is determined by the duration of the pulse, the allowable voltage droop on CP, and the required repetition rate.

In the described circuit, a 4 to 20 mA current loop serves as the primary means of communication and power supply for the remote transmitter. The design leverages a charge pool capacitor (CP) to store energy, allowing the transmitter to operate intermittently. The HA-5141 operational amplifier functions as a current source, providing a constant current (I1) to the capacitor. The choice of I1 at approximately 3 mA is critical, as it ensures that CP charges sufficiently during its active state without exceeding the power limitations of the system.

As the capacitor charges, the voltage (V1) across it rises until it reaches a predetermined threshold, defined by either the maximum supply voltage (V+) or the clamping voltage of Z2, which serves to protect the circuit from overvoltage conditions. The LM2931 voltage regulator is employed to convert the stored voltage in CP to a stable +5 V output, which is essential for the operation of the remote transmitter. This regulated voltage ensures that the transmitter receives a consistent power supply, vital for reliable performance.

The performance of this circuit is influenced by several factors, including the duration of the charging pulses, the maximum allowable voltage droop on CP during the transmitter's operation, and the repetition rate of these pulses. The design must carefully consider these parameters to ensure that the transmitter operates efficiently and effectively within the constraints of the current loop system. By utilizing a charge pool power supply, this method provides a practical solution for powering remote transmitters that have high power requirements while still maintaining the integrity of the 4 to 20 mA current loop.It is usually desirable to have the remote transmitter of a 4 to 20 mA current loop system powered directly from the transmission line. In some cases, this is not possible because of the high-power requirements set by the remote sensor/transmitter system.

In these cases, an alternative to the separate power supply is still possible. If the remote transmitter can be operated in a pulsed mode where it is active only long enough to perform its function, then a charge pool power supply can still allow the transmitter to be powered directly by the current loop. In this circuit, constant current II is supplied to the charge pool capacitor, CP, ~! the HA-5141 (where II ~ 3 mA). The voltage VI continues to rise until the output of the HA-5141 approaches + V, or the optional voltage limiting provided by Z2. The LM2931 voltage regulator supplies the transmitter with a stable + 5 V supply from the charge collected by CP.

Available power supply current is determined by the duration, allowable voltage droop on CP, and required repetition rate.