Imagine you want to use a USB relay controller to control a heater, or maybe you want to control a water pump for irrigation applications.  How about using a relay board to control a gas valve.  Don’t think of using standard on/off commands for these applications, this is how you get into some serious trouble.  Computers are not fool-proof, communications is not fool-proof, you should be able to rely on the relay board to handle dangerous applications automatically.  A quality relay controller must have a timer mechanism for controlling relays for a period of time.  This will force a relay to turn off automatically if communication is lost.  Relay commands must be powerful enough to handle dangerous loads in the event of any communication malfunction.

The proper way to control a room heater from a USB relay controller:

  1. Send a command to activate a Relay for 10 Seconds

  2. Monitor the Temperature of your Room

  3. If the Room is too Cold, Send Another Command to Restart the Timer for 10 More Seconds

  4. When the Room comes to temperature, Stop Sending Turn-On Commands

In the above application, the relay is turned on via a timer by sending a USB command to the controller.  The relay board will automatically turn the relay off if you stop sending timer commands to the controller.  The relay cannot stay energized using this strategy, keeping the heater safe from overheating.  This is often referred to as a watchdog timer, and should be considered a basic requirement for any USB relay board handling a dangerous electrical load.  NCD ProXR Enhanced and Advanced relay controllers include 16 timers that run concurrently, allowing 16 fail-safe relays to be activated for different time duration.

Standard Relay Configurations

SPST Relays: Single Pole Single Throw:

A relay SPST relay has two connection terminals, acting like a basic switch.  Inside an SPDT relay, two pieces of metal make contact when the relay is activated.  This type of relay is normally open (though some manufacturers make a normally closed version) and completes a circuit when the relay is energized.  We do not use this type of relay very often as SPDT relays do the same thing with nearly identical cost.  The only exception being 30-Amp high-current relays and solid-state relays, which we offer only in SPST configuration.

SPDT Relays: Single Pole Double Throw:

A SPDT relay has three contact points whereby two contacts (common and normally closed) are connected when the relay is off.  When the relay is energized, the common contact disconnects from normally closed and reconnects to the normally open connection.  This is the most common type of relay we offer, as this type of relay can be wired to turn a light on when the relay is on or it can be wired to turn a light off when the relay turns on.  We offer SPDT relays in 1-Amp (for low-power signals), 5-Amp, 10-Amp, and 20-Amp (for high-power signals).

DPDT Relays:

A DPDT relay is exactly like having two separate SPDT relays inside a single relay with both SPDT relays switching simultaneously.  There is no connection between each of the two internal SPDT switches.  Both SPDT switches inside a DPDT relay share a single electromagnet, forcing both switches to activate or deactivate simultaneously.  These relays are frequently used in signal switching applications, and are not ideal for high current loads above 5 Amps.

Exotic Relay Configurations:

Other more exotic (and rare) relay configurations can be simulated using our relay grouping functions.  Have you ever heard of a 8PDT relay?  A 8PDT relay contains 8 poles that switch between two positions simultaneously.  This could be used to switch 8 high-power signals simultaneously.  The only problem is, if you could buy such a relay, it would be very expensive, as it is rarely needed, and likely produced in extremely low volumes.

ProXR series relay controllers include relay grouping commands.  Use these commands to activate multiple relays simultaneously.  Relay grouping commands make the following relay configurations possible using low-cost relays:

3PDT Relays: 3 Poles that Move Simultaneously Between Normally Closed and Normally Open
4PDT Relays: 4 Poles that Move Simultaneously Between Normally Closed and Normally Open
5PDT Relays: 5 Poles that Move Simultaneously Between Normally Closed and Normally Open
6PDT Relays: 6 Poles that Move Simultaneously Between Normally Closed and Normally Open
7PDT Relays: 7 Poles that Move Simultaneously Between Normally Closed and Normally Open
8PDT Relays: 8 Poles that Move Simultaneously Between Normally Closed and Normally Open

Relay grouping commands make it easy to control relays in groups, perfect for simultaneous switching applications.

While there are many more types of switches (and terminology), this gives you a basic run-down of different types of commonly available switches.