which allows its set point to be manipulated by an external source. A common application would use one ramping controller with a set-point retransmit output to ramp multiple controllers using the remote set point. Or you could use an analog output from a PLC to send set point values to an EZ-ZONE®PM.

The controller must have two process inputs to use the remote set point feature.

You may select between local and remote set points at the front panel, with an event input, from a remote computer using the communications feature or from an external switch using an event input.

Make sure all input and output impedances are compatible.

Switch to the remote set point with Remote Enable [`r;En] (Operations Page, Loop Menu). Select whether the remote set point controls an open- or closed-loop set point with Remote Set Point Type [`r;ty].

Assign the function of switching to a remote set point to a digital input with Digital Input Function [``Fn] (Setup Page, Digital Input Menu).

Assign the function of switching to a remote set point to the EZ Key with Digital Input Function [``Fn] (Setup Page, Function Key Menu).

Outputs

Duplex

Certain systems require that a single process output control both heating and cooling outputs. An EZ- ZONE® PM controller with a process output can function as two separate outputs.

With a 4 to 20mA output the heating output will operate from 12 to 20mA (0 to +100 percent) and the cooling output will operate from 12 to 4mA (0 to -100 percent).

In some cases this type of output is required by the device that the EZ-ZONE®PM controls, such as a three-way valve that opens one way with a 12 to 20mA signal and opens the other way with a 4 to 12mA signal. This feature reduces the overall system cost by using a single output to act as two outputs.

Outputs 1 and 3 can be ordered as process out- puts. Select duplex [dUPL] as the Output Function [``Fn] (Setup Page, Output Menu). Set the output to volts [uoLt] or milliamps [`MA] with Output Type [`o;ty]. Set the range of the process output with Scale Low [`S;Lo] and Scale High [`S;hi].

No-arc Relay

Ano-arc relay provides a significant improvement in the life of the output relay over conventional relays.

Conventional mechanical relays have an expected life of 100,000 cycles at the rated full-load current. The shorter life for conventional relays is due to the

fact that when contacts open while current is flowing metal degradation occurs. This action produces unavoidable electrical arcing causing metal to transfer from one contact to the other. The arcing conditions continue on each subsequent contact opening until over time the resistance through the contacts increases causing the contacts to increase in tempera- ture. Eventually, the contacts will weld together and the relay remains in the on state.

The Watlow no-arc relay is a hybrid relay. It uses a mechanical relay for the current load and a triac (solid-state switch) to carry the turn-on and turn-off currents. No-arc relays extend the life of the relay more than two million cycles at the rated full-load current.

Although a no-arc relay has significant life advan- tages, a few precautions must be followed for acceptable usage:

Do not use:

•hybrid relays for limit contactors. A limit or safety device must provide a positive mechanical break on all hot legs simultaneously;

•dc loads with hybrid relays. The triacs used for arc suppression will turn off only with ac line voltage;

•hybrid switches to drive any inductive loads, such as relay coils, transformers or solenoids;

•cycle times less than five seconds on hybrid switches;

•on loads that exceed 264V ac through relay;

•on loads that exceed 15 amperes load;

•on loads less than 100 mA;

•no-arc relays in series with other no-arc relays.

Retransmitting a Process Value or Set Point

The retransmit feature allows a process output to provide an analog signal that represents the set point or process value. The signal may serve as a remote set point for another controller or as an input for a chart recorder documenting system performance over time.

In choosing the type of retransmit signal the operator must take into account the input impedance of the device to be retransmitted to and the required signal type, either voltage or milliamps.

Typically applications might use the retransmit option to record one of the variables with a chart recorder or to generate a set point for other controls in a multi-zone application.

Outputs 1 and 3 can be ordered as process out- puts. Select retransmit [rMt] as the Output Function [``Fn] (Setup Page, Output Menu). Set the output to volts [uoLt] or milliamps [`MA] with Output Type [`o;ty]. Select the signal to retransmit with Retransmit Source [`r;Sr].

Watlow EZ-ZONE®PM Integrated Controller

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Chapter 9 Features