Vermont Energy Control Systems

Using Relays


Relays are an essential component of almost any control system. They are most often used to allow a small low voltage signal to control a large high voltage load, but there are many other applications. This page will show some examples of different ways that relays are commonly used with the Vesta system.

Relay Operation and Terminology

A relay is an electrically operated switch. A relay has a coil and one or more sets of contacts. When power is applied to the coil, the contact(s) change position. A relay with power applied to the coil is said to be energized. In the first example on the right, a circuit is wired with a battery, a light bulb, and a relay. If power is applied to the relay coil, the relay will move to connect the 'C' contact to the 'NO' contact. Current can then flow through the circuit and the light bulb will be illuminated.

Coil Voltage: The coil can be almost any voltage, and may operate on alternating current (AC) or direct current (DC). The Vesta system uses an RM-1026 relay module which has four relays with 12vdc coils. These coils can be operated by discrete outputs on the Vesta controller. The Vesta controller can handle multiple RM-1206 modules.

Poles: In relays, a set of contacts is called a pole. A relay with one set of contacts is called a 'single pole relay', and a relay with two sets is called a 'double pole relay'. If there are two or more sets of contacts, they are electrically isolated from each other. The top two relays on the right both contain a single set of contacts and are therefore single pole relays.

Single vs. Double Throw: There are two basic configurations of contacts. The simplest is two contacts which are either connected or not connected. In relay terminology this configuration is referred to as single throw. A more complex configuration is a common (C) contact which is connected to a seocnd contact when the relay is not energized, or to a third contact when it is energized. This is referred to as double throw. A wall switch with this configuration is known as a three-way switch. The top relay on the right has contacts in a single-throw configuration, while the second one has double-throw contacts.

Contact Terminology: A contact that is connected when the relay is not energized is called normally closed (NC), while a contact that's not connected when the relay isn't energized is called normally open (NO). In schematics, relay contacts are drawn in the 'normal' (un-energized) position.

The drawings at the right show a variety of relay configurations. The relays in the RM-1206 have double pole, double throw (DPDT) contacts. All three contacts (common, normally open, and normally closed) for each pole are available on a screw terminal header inside the RM-1206 enclosure.

The drawing below shows a pair of DPDT relays. The one on the left shows the contacts in the non-energized position, while the one on the right shows the contacts in the energized position.

Controlling A High Power Load

One of the most basic applications of relays is to allow a low voltage signal to control a high voltage load. In this example, a discrete output of the Vesta controller is used to control a 24vac zone valve. We use the RM-1206 Relay Module, which has four relays with 12vdc coils. These relays can be controlled by the Vesta controller, and can switch loads up to 220vac.

In this example, power for the zone valve comes from a 24vac transformer. One motor connection from the zone valve is connected directly to the 'black' lead from the transformer. The other lead is connected to a normally open contact. Finally, the common contact from the same contact set is connected to the 'red' lead from the 24vac transformer.

When the Vesta controller energizes the relay, the circuit is completed and power is applied to the zone valve motor.

Sensing High Voltage

Sometimes the Vesta controller needs to be able to determine whether a high voltage is present - perhaps to see if AC power is on, or to see if another controller has turned on a high power device.

In this example, a Vesta controller will provide heat for a hot tub by turning on a zone with a heat exchanger that will heat the hot tub in place of the hot tub's original electric heating element. In order for this to work, the Vesta controller needs to know when the hot tub controller is calling for heat. Unfortunately, the only available signal is the 220vac power that the hot tub controller applies to the heating element.

In this case, we use a relay with a 220vac coil. We disconnect the electric heating element and connect the relay coil in its place so that the relay is energized when the hot tub controller calls for heat.

NOTE: This example uses a relay with a 220vac coil. This is NOT one of the relays in an RM-1206 relay module. Relays with different coil voltages and contact configurations may be purchased from most electrical suppliers. Contact us if you have any questions.

NOTE 2: This example is for illustration purposes only. Actually doing this would almost certainly void your hot tub warranty. This type of wiring involves high voltages and should only be performed by an experienced electrician.

The relay 'common' and 'normally open' contacts are connected to one channel of a DB-1195 discrete breakout box, which is connected to a set of discrete inputs on the Vesta controller. When the hot tub contoller calls for heat, the relay is energized and the Vesta controller can detect the relay contact closure and respond appropriately.

Failsafe blower control

This is a more sophisticated application of relays. In this case, we have a variable speed blower on a boiler that was originally managed by a controller on the boiler. Since the boiler control logic somewhat simplistic and can't be modified, we want to see if we can develop rules that provide more effective control of blower speed using the Vesta system. However, we want to be able to return to the original configuration in case our experimental rules are not providing the desired results. In fact, we'd like the boiler to work the same way as it originally did if we simply turn off or disconnect the Vesta controller. This is known as fail-safe operation. If the Vesta controller is taken out of the picture, the system works exactly as it did before.

There are two wires going to the blower. We'll use a single relay in the RM-1206 relay module to switch these wires between the original boiler controller and the Vesta's VS-1108 variable speed control module.

Each relay coil in the RM-1206 is connected to a discrete output channel in the Vesta controller. This allows the Vesta to activate any relay based on rules that you define, or through the Graphical User Interface.

Here's a schematic of what we need to do. This example shows a single relay in the RM-1206.

In this example, the wires from the blower are connected to the relay's common contacts. The blower control outputs from the boiler are connected to the normally closed contacts. This way, the blower is connected to the boiler controller any time the relay is not activated.

The blower control outputs from the VS-1108 are connected to the relay's normally open contacts. When the relay is energized, the blower will be connected to the VS-1108 and the Vesta will be able to control its speed.

During development, the relay might be controlled by a clickable button on the Graphical User Interface so that the user can switch instantly between Vesta control and Boiler control of the blower. Once the Vesta rules are satisfactory, an 'always true' rule can keep the relay permanently in the energized position.

Note: This is only a partial solution to illustrate a possible use of relays. In order to protect the variable speed circuitry, there would need to be some logic or safeguards to prevent the blower being switched beween controllers when either controller is 'on'. Suddenly switching the load while under power can cause damage.

Note: In some cases the only intent might be to allow the Vesta to turn the blower off. In this case the wiring between the boiler, the relay, and the blower is the same. The VS-1108 is not required, and nothing would be connected to the relay normally open contacts. Wired this way, energizing the relay simply removes power to the blower, turning it off.