Sensors and other devices with 4-20mA current loop output are very popular in industrial fields. We can find them from Process Control to data logging systems. A basic current loop has parts as stated below:
- Sensor: a device which draws current from its power source in direct proportion to the value it measures (level, pressure, etc.).
- Power source: supplies DC power to sensors or any device in the loop.
- Reading devices: a PLC, a controller or data logger which reads the current value provided from the sensor and uses it for the next step of controlling or data logging.
In order to isolate, convert, filter and amplify or duplicate control signals we use signal conditioners. Signal conditioner makes a simple 4-20mA current loop into a field loop and control loop like below:
The devices in both the field loop and the control loop need to be powered. They can be powered by a DC Auxiliary power supply or by another active device in the loop. That being said, we have many types of Signal Conditioners (According to how it is powered or can it supply power for other device) that can be used in this scenario.
Types of sensors
The most common 4-20mA sensors in industrial fields are 4-wires sensors and 2-wire sensors. The difference lies in how they powered.
A 4-wire sensor is an active device which has a separate power supply for itself (from DC or AC power supply).
On the other hand, a 2-wire sensor is a passive device which draws power from a DC auxiliary power supply in the loop or from other active devices in the loop (like a repeater power supply signal conditioner)
Types of 4-20mA Signal Conditioners:
- Passive Signal Conditioner: A passive signal conditioner doesn’t need a separate power supply for itself. It draws power from the field loop or control loop. This means there are 2 types: one powered by a field loop and one powered by a control loop. As passive signal conditioners can cause a drop in voltage in the loop, if there are too many passive devices in loop it could lead to inaccuracies in the measured values. In the picture below is a field loop powered signal conditioner with 2-wire sensor, where the devices in the field loop are powered by an auxiliary power supply.
- Active Signal Conditioner: This type of signal conditioner is powered by a separate power supply so it doesn’t cause voltage drops in the loop. It can also provide power for the control loop. For example, an active mini MCR Pro signal conditioner can drive up to a 600 Ohm load in a control loop. The field loop still needs an auxiliary power supply when using a 2-wire sensor. Pictured below is an active signal conditioner and 4-wire sensor. In the control loop is a passive analog input controller.
- Repeater Power Supply Isolator: This type of signal conditioner is powered by a separate power supply and it can supply power for the sensor’s loop and also for the control loop (with a maximum load of 600 Ohms). It can be used for 2, 3 or 4-wire sensor without the need of a power supply for the field loop.
So depending on the type of application, we can choose the right 4-20mA signal conditioner. Based on information about the type of sensor, whether the field loop has an auxiliary power supply as well as if the control loop has a passive or active controller, we can choose the suitable 4-20mA signal conditioner.
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