EMS - Industrial Sensors and Controls 

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Current Loop Sensors 
The thermostat - The basic control system

The classic thermostat with a bimetallic coil that operates a mercury switch is a good illustration of a sensing element that effects a control function. When the mercury switch tilts enough for the switch contacts to be closed by the mercury, a 24VAC loop is established that allows a control relay at a remote location to operate and current to be applied to a furnace, heating element, or ventilation fan.

Two wire current loop

Modern sensors often use 24VDC along a two wire connection. Two wires supply a voltage a portion of which is used to operate the circuitry for both the sensing element and the transmission circuit. By combining both the sensing and transmitter functions, the sensing function can be located up to miles from the control function.

Three wire current loop

Some sensors require enough power to operate that there wouldn't be enough voltage left to provide for a 4-20mA signal. For these situations a power wire is added so that sensor operation is separate from the current signal output. 

Current Loop - Basic Circuit
The transmitter circuit at the sensor end of the 2 wire 4-20mA connection constantly adjusts the voltage drop across itself so that a current proportional to the sensor output is maintained regardless of variations in cable length and resistance. At the control circuit end of the connection the current is converted by running it through a 250 ohm resistor to a 1-5V DC signal.

If the transmitter just sent a voltage signal, ambient electrical noise might interfere with the accuracy of the measurement circuit.

                  The Sentry Monitor in conjunction with an EMS current loop sensor can provide a complete measurement and control system for many different applications.





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