Do you know how the PLC’s 4 to 20 mA analog inputs work and why they are so sensitive?
This article addresses this and proposes a simple solution to protect the 4 to 20 mA analog inputs of the PLC.
How the PLC’s 4 to 20 mA analog inputs work
Most 4 to 20 mA inputs in CLPs have a resistor of about 150 to 200 ohms at their input. See below for a typical circuit.
In the example in the picture above, we show a hydrostatic level transmitter.
This type of sensor is widely used to measure the water level in reservoirs belonging to the municipal water supply system.
The hydrostatic level transmitter works submerged, and because it is in direct contact with water, it is a path for electrical surges that normally enter the network and seek ground.
When a hydrostatic level sensor burns out due to a surge, it often ceases to function as a 4 to 20 mA regulator and delivers the unrestricted 24 V at the output.
Whether it is a hydrostatic level transmitter, a pressure sensor, or any other field instrument that, instead of delivering a 4 to 20 mA current, delivers the 24 V of the supply directly to the analog input, this will damage the analog input by the excess voltage and current.
What happens when the sensor shorts out and supplies the 24 V, with no current limit, to the 4 to 20 mA analog input?
Let’s say that the analog input is provided with a 200 ohms resistor. The current over the resistor will be:
I = 24V / 200 ohms = 120 mA and the Power over the resistor P = 24 V x 120 mA = 2.88 W
The resistors used in the analog inputs of the CLP are not sized to withstand this power and fatally burn out.
Solution to protect the analog input against excess current
The solution is simple; we need a voltage limiter and a current limiter working together.
As a voltage limiter we use the TVS diode and as a current limiter we use the PTC thermistor.
Using the solution presented, when the field sensor is shorted, and the 24 V from the source goes straight through, the TVS diode will conduct, limiting the voltage at the analog input to 6 V.
The current on the PTC thermistor trying to exceed 50 mA will cause the PTC to heat up and change its original resistance from about 2 ohms to a resistance that limits the current to 50 mA.
In the case of the circuit shown, the resistance of the PTC will change to about: R = (24 V – 6 V) / 50 mA = 360 ohms.
Over the 200 ohms resistor of the analog input the resulting voltage will be 6 V, and the current 30 mA, resulting in a maximum power of 180 mW, which is not enough to damage the component.
The thermistor works like a resettable fuse, because after the replacement of the damaged (shorted) sensor, and having ceased the excessive current, the PTC will cool down and return to having only 2 ohms of resistance.
The PTC selected is of a type specially designed for overcurrent protection. The Resettable Fuses – Multifuse® PPTC line from Bourns is an example of such components.
The TVS diode is a fast diode specially designed to absorb surge voltages and is widely used in SPD (Surge Protection Device) circuits.
Complete protective circuit for analog 4 to 20 mA PLC inputs
We now present a complete circuit of an SPD for the protection of 4 to 20 mA inputs.
The circuit presented protects not only the analog channel, but also the 24 V supply that is provided to the field sensor.
The protection is in three stages, by means of the three types of surge suppressors:
- Gas spark gap;
- Metal oxide varistor;
- TVS diode.
The inductors that separate each stage of the protection help to delay and dampen the surge.
SPD PCB for analog inputs 4 to 20 mA
QTY DESCRIPTION
- 4 CN1, CN2 – AKZ-700 – 2
- 1 D1 – P6KE30A (TVS)
- 1 D2 – P6KE6A (TVS)
- 1 F1 – Resetable fuse (PTC) 50 mA
- 1 F2 – 50 mA resettable fuse (PTC)
- 4 L1, L2, L3, L4 – Inductor 100uH
- 2 RV1, RV2 – S10K30 (Varistor)
- 2 SA1, SA2 – 75V (spark gap)
- 1 Spacer 15 mm
- 1 RS75 Female Foot
- 1 RS75 Male Foot
- 1 SS2701 PCB
Request additional information or a quote for the Analog Input Protector
The module SS2702 module is an analog channel protector against electrical surges caused by over-voltages in the field wiring. Assembled in printed circuit board and housed in a plastic DIN-rail mount, the module incorporates five surge protection circuits, one to prevent surges from damaging the 24V power supply circuit and the other four for protecting analog channels. Each circuit is fitted with a fuse, gas spark gap, metal oxide varistor, suppressor diode, and inductors. The module replaces, with advantages in cost, space, and assembly time, an arrangement of four protectors, five fuses, and sixteen terminal blocks. One of the product’s differentials is the fact that it is the only one in the market equipped with resettable fuses (PTC).
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