Core Factors of Transmitter Output Signal Transmission Distance
The transmission distance of a transmitter output signal is not a fixed value, but is jointly determined by the supply voltage, load resistance, wire resistance, and transmitter type. Typically, transmitters use 24V DC power supply, with a maximum output current of 22.5mA, a minimum operating voltage of 10.5V DC, and a load resistance of 600Ω. Understanding these parameters is the basis for analyzing transmission distance.
Transmission Distance of Explosion-Proof Transmitters
For explosion-proof transmitters, as long as the sum of the wire resistance and the resistance of the equipment connected to the transmitter does not exceed 600Ω, the transmission distance of the wire is not limited. This is because explosion-proof transmitters do not rely on safety barriers, and the resistance in the loop is the only limiting factor.
- Key condition: Wire resistance + equipment resistance ≤ 600Ω
- Conclusion: When the above condition is met, the transmission distance is theoretically unlimited
Transmission Distance Limitations of Intrinsically Safe Transmitters
Intrinsically safe transmitters are different. Intrinsic safety is a system, including not only the transmitter itself but also associated equipment (such as safety barriers) and external wiring. Safety barriers usually indicate the maximum allowable inductance and maximum allowable capacitance, and these parameters directly limit the transmission distance of the wire.
Composition and Requirements of Intrinsically Safe Systems
- Transmitter: Unprotected inductance and capacitance
- Associated equipment: Safety barrier, marked with maximum allowable inductance and capacitance
- External wiring: Distributed capacitance and inductance of the wire
If the distributed capacitance and inductance of the wire plus the unprotected inductance and capacitance of the transmitter exceed the range specified by the safety barrier, then the entire instrument system is no longer intrinsically safe, posing a safety hazard.
Comparison of Transmission Distance Between Explosion-Proof and Intrinsically Safe Types
| Type | Limiting Condition | Transmission Distance |
|---|---|---|
| Explosion-proof | Wire resistance + equipment resistance ≤ 600Ω | Unlimited (meeting resistance condition) |
| Intrinsically safe | Wire distributed capacitance/inductance + transmitter capacitance/inductance ≤ safety barrier allowable value | Limited, requires calculation |
Engineering Selection and Wiring Recommendations
In actual engineering, transmission distance requirements must be considered when selecting transmitter types. If the transmission distance is long, explosion-proof transmitters are more flexible; if intrinsically safe types must be used, the distributed parameters of the cable must be carefully calculated, and a safety barrier with matching parameters must be selected.
- Confirm the transmitter type (explosion-proof or intrinsically safe)
- Explosion-proof: Calculate the total loop resistance to ensure it does not exceed 600Ω
- Intrinsically safe: Check the maximum allowable inductance and capacitance of the safety barrier, and calculate cable distributed parameters
- Select appropriate cables, and if necessary, shorten the distance or use low distributed capacitance cables
Following the above steps ensures stable transmission of the transmitter output signal while meeting safety regulations.