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Connecting External Relays


Realy theory is beyond the scope of this manual. There are many online explanations such as this one: http://www.controlanything.com/Relay/Device/A0001. Relays are key components of most electrical control systems, and the NFCS is designed to operate up to 32 relays.

Relays are typically used for two purposes:

  1. Control - To allow the NFCS to control some external device such as a valve or a circulator
  2. Sense - To allow the NFCS 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 NFCS. 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 wiried in parallel with the pump motor. The relay contacts would in turn be connected to an NFCS discrete input.

Control Relays

The NFCS 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. The 'standard' NFCS control relay is the Schrack PT270012, which has a coil current of about 65ma and can control loads up to 10 amps.

Relays come in many contact configurations and mounting styles. The Schrack PT270012 is designed to plug into a socket which in turn can be mounted on a standard DIN rail. The Schrack PT270012 has double pole / double throw (DPDT) contacts rated for 12 amps of current. It also has a window on the top which shows an orange flag if the relay is activated as well as a lever to manually actuate the relay. These features can help with troubleshooting.

External Relay Enclosure

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

  1. To avoid accidentally vaporizing expensive circuit boards
  2. To keep the NFCS 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 NFCS. 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 NFCS carries four channels, the logical capacity of an external relay box is four relays.

Accordingly, he NFCS Remote Relay Enclosure has an internal DIN rail and carries up to four relays. It has an RJ45 connector for connecting to NFCS discrete outputs and has four standard electrical cable strain reliefs.

Other configurations are possible. If the enclosure held only sense relays, then an RJ45 would be provided to connect to NFCS discrete inputs instead. If the enclosure held a mix of both control and sense relays, then two RJ45 connectors would be needed. Any configuration other than four or less control relays is 'custom' and would need to be site-built or made to order.


Because of the tight space inside the enclosure, it is recommended to use smaller gauge stranded wire for the high voltage connections to the relays.

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

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

The RJ45 connector is prewired to the four relay coils, with relay 1 being the leftmost.

The relay socket is not labeled with helpful legends. To translate the socket markings, refer to this schematic. A copy is pasted on the inside of the relay enclosure cover.

Sample External Control Relay Application

In this example, the NFCS controls a 120Vac circulator. The 120Vac neutral goes directly to the circulator motor. The 120Vac 'hot' goes to one of the 'Common' terminals on a relay, The corresponding 'Normally Open' (NO) terminal is connected to the circulator motor. The relay coil is connected to an NFCS discrete output.

When the NFCS discrete output is asserted (set to 'TRUE'), the relay coil will be energized. That will change the relay contacts so that 'common' is connected to 'NO'. That will allow current to flow through the circulator motor.

Many variations are possible. If the circulator motor were connected to the NC terminal instead, then the circulator would run whenever the relay was NOT activated. Each relay has a second pair of contacts which could be used to switch a completely different load or perhaps a status LED.

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