Did you know the RES-1E resistance output transducer can be used to.....
- control a 135 ohm 3-wire potentiometer valve or damper actuator?
- reset the leaving water temperature of a chiller from a Building Automation System?
The RES-1E can be used for these as well as other unique applications. Specific information on the two applications mentioned can be found in the following technical briefs.
Don't replace the actuator!
Use the RES-1E Transducer to Control a 135 ohm 3-wire Potentiometer Valve or Damper Actuator.
An early form of electric control used a 3-wire 135 ohm slide-wire potentiometer as the controlling element. This type of control was commonly used in the "old days" because it provided modulating control of valves and dampers. It is not uncommon to find this type of control when retrofitting an older building. Today, however, Building Automation System manufacturers have not adopted the 135 ohm 3-wire potentiometer as an industry standard analog output. With current, voltage, and pulse width modulation being the most commonly available types of analog outputs on BAS controllers, retrofitting a building could require replacing all of the valve and damper actuators, a potentially expensive and labor intensive option. If the actuators are in good working condition, another option is to use the RES-1E transducer. This device is ideally suited to convert BAS controller outputs to a 135 ohm signal that can drive the existing actuators.
An Example: An existing Honeywell M945 valve actuator is to be controlled by a 0-10 VDC analog output signal from a new BAS controller using a standard RES-1E transducer with 135 ohm output. The analog output from the BAS controller has been programmed to vary based on room temperature. As the controller output increases, the resistance between the RES-1E terminals "R" and "B" will increase proportionally from approximately 2 ohms toward 135 ohms; the resistance between terminals "R" and "W" will decrease from 135 ohms towards approximately 2 ohms. As the controller output decreases, the resistance between "R" and "B" will decrease and the resistance between "R" and "W" will increase. The actuator will respond accordingly to the changes in resistance. As shown in the wiring diagram, an existing 135 ohm thermostat has been connected to the RES-1E. When 24V power is interrupted at the RES-1E power terminals, the fail-safe mode internally connects the RES-1E "OUTPUT" terminals to the "FALLBACK POT" terminals allowing the existing thermostat to be used for backup control.
Note: When powering the RES-1E from 24 VAC, a separate dedicated transformer is suggested. Otherwise, exercise caution when powering multiple electronic devices from a common transformer. (See 20/20 Insights, April '94)
Take Control of Your Chiller
Use the RES-1E Transducer to Reset the Leaving Water Temperature of a Chiller from a Building Automation System
Many chillers are provided with a factory installed control package that allows the chiller leaving water temperature (LWT) setpoint to be reset according to the input from a resistive outside air temperature (OAT) sensor. In certain applications it may be desirable to eliminate the OAT sensor and control the reset of the LWT setpoint from a Building Automation System according to varying building load requirements, the outside air temperature as monitored by the BAS, or other factors. This is easily accomplished by using a device that can be controlled from a BAS controller and will mimic the resistive signal of the OAT sensor. The RES-1E transducer is the perfect device for this application.
An Example: A 4-20 mA analog output signal from a BAS controller is to reset the LWT setpoint of a chiller that is normally reset from a resistive OAT sensor wired directly to the chiller. The first step is to determine the desired setpoint reset range and the equivalent resistance range of the OAT sensor. This information should be obtained from the chiller manufacturer. If the sensor is a thermistor, the setpoint reset range should be as narrow as possible to minimize non-linearity. The RES-1E can be ordered with a standard range of 135, 500 or 1000 ohms or it can be ordered with a custom range to match the application.
As shown in the wiring diagram, the RES-1E terminals "R" and "B" have been connected to the terminals on the chiller where the OAT sensor would normally have been connected. The analog output from the BAS controller has been programmed to vary from 4-20mA according to the application. As the controller output increases, the resistance between the RES-1E terminals "R" and "B" increases proportionally from minimum to maximum. As the controller output decreases, the resistance from the RES-1E will decrease. This change in resistance mimics the change in resistance of the OAT sensor and will reset the chiller setpoint accordingly.
In the event of a loss of the input signal, the RES-1E can remember the last signal and continue to provide an output based on the last commanded signal. This insures that the same setpoint will be maintained even if the BAS control signal should be lost. The signal loss feature is enabled by turning DIP switch 2 "ON".
There is an alternative to using the RES-1E to replace the OAT sensor as described. If the chiller is furnished with the capability of remote setpoint control from a potentiometer, the RES-1E can be used in place of the remote setpoint potentiometer. Be sure to use an RES-1E with an output resistance range as required by the chiller remote setpoint input.
Note: When powering the RES-1E from 24VAC, a separate dedicated transformer is suggested. Otherwise, exercise caution when powering multiple electronic devices from a common transformer. (See 20/20 Insights, April '94)