Vermont Energy Control Systems

Practical monitoring and control for the real world


Relay Basics

Relays are key components of most electrical control systems, and the Vesta controller is designed to operate up to 32 relays.

While detailed relay theory is beyond the scope of this manual, there is a tutorial document on this site, and there are many online explanations such as this one.

Relays are typically used for two purposes:

  1. Control - To allow the Vesta controller to control some external device such as a valve or a circulator
  2. Sense - To allow the Vesta controller to sense the presence of electrical power that's provided by some other means, such as another controller. For instance, a relay could be used to detect that an oil boiler was running.

Control relays have 12Vdc coils and are directly controlled by the Vesta controller. Each control relay is driven by a discrete output.

Sense relays have coils that match the voltage that needs to be sensed. For instance, a relay that's used to sense whether a 120VAC pump is running would use a 120VAC coil wired in parallel with the pump motor. The relay contacts would in turn be connected to a Vesta discrete input. An example is described in this application note.

Control Relays

The Vesta controller discrete outputs can drive any relay that has a DC coil that operates at 12 Volts and draws less than 200ma of coil current. This includes virtually all 12Vdc relays.

External Relay Enclosure

A picture of the RM-1207 External Relay Enclosure

It is desirable to keep high voltages outside of the Vesta controller enclosure for three reasons:

  1. To avoid accidentally vaporizing expensive circuit boards
  2. To keep the Vesta controller enclosure intrinsically safe and finger-friendly
  3. To reduce the distance that power has to travel to get to the high voltage loads

To achieve these goals, the preferred approach to controlling high voltage loads is to us a relay enclosure that is mounted near the loads and controlled by the Vesta controller. This relay enclosure can contain any combination of control and sense relays. The most common use of external relays is typically control, so the standard external relay enclosure is set up with control relays.

Since each discrete output connector on the Vesta controller carries four channels, the capacity of an external relay box is four relays.

The RC-4DD (formerly RM-1207) relay module has an RJ45 connector for connecting to Vesta discrete outputs. Internally, each relay has two electrically separate sets of contacts, each with common (C), Normally Open (NO), and Normally Closed (NC) contacts. These are brought out to screw terminals for easy connection to electrical devices.


A picture of the internal workings of a control relay

The RC-4DD is connected to any Discrete Output connector on the Vesta Controller using standard Cat5 Ethernet cable. The Vesta color code for discrete outputs is blue, so a blue cable is suggested.

The screw terminals in the relay module can accommodate up to 12 gauge stranded and 14 gauge solid wire if necessary, but smaller gauges are much easier to accommodate in the limited internal space.

As with all high voltage wiring, ensure that all relevant codes are followed.

WARNING: To avoid damage to the Vesta controller, disconnect the Vesta controller cable from the relay box before doing any wiring.

Sample External Control Relay Application

There is an application note that describes controlling a high voltage device with a Vesta discrete output using a relay.