P+F safety barrier circuit voltage issue
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I have a question recently and hope to get some advice from everyone. In a temperature circuit powered by a P+F safety barrier, when I disconnect the wiring from the output side of the safety barrier to the field instruments, the voltage at the output of the safety barrier is 24 volts. However, once the wiring is reconnected to the temperature-sensing load, the voltage at the output of the safety barrier drops to 19 volts. Why is there a decrease of 5 volts? Welcome everyone's advice
It is caused by the loop resistance, which equals the safety barrier resistance + wire resistance + receiver resistance. The greater the loop resistance, the greater the voltage drop.
Thank you for your advice. You are right; I conducted experiments this afternoon. The issue lies in the fact that the voltage division across the safety gate resistor is relatively large – the greater the circuit current, the larger this voltage division, and consequently the lower the voltage across the transmitter.
When the components are connected in a loop, it forms a series circuit; the total voltage is equal to the sum of the voltages across each component in the circuit, including the voltage drop along the wires； An open circuit is equivalent to a load resistance at its maximum value, resulting in the highest voltage drop across it. The total voltage is equal to this voltage drop across the load, and thus reaches its maximum value.
Voltage drop – the resistance of the entire circuit must be taken into account.
Consider the safety barrier as a power supply with a high internal resistance (that is, 24V supplied after connecting a resistor in series)； The 24V voltage is distributed based on the voltage in the circuit, with the transmitter acting as a variable resistor; the voltage across various components in the circuit changes as this variable resistor changes ； The safety barrier provides the 5V voltage that was missing after the temperature sensor was connected; this is actually the operating voltage required by the temperature sensor ； Since the actual power consumption of the transmitter is roughly: circuit current × voltage across the terminals. Therefore, the voltage requested by the transmitter as the loop current changes also changes ； After the transmitter has been in use for a long time, its power consumption increases. If the voltage supplied to the circuit is not sufficient, the transmitter will stop working (the signal level becomes too low or the current cannot increase). In such cases, everything seems normal when connected to a voltage regulator, but the transmitter stops working when connected to a safety barrier.