Understanding Relays With the 90-340



Relays can be used for many different control applications including controlling fans, blowers, other relays or contactors, valves, dampers, pumps and much more. A 90-340 is a very common, versatile relay that many techs have on their truck so we will use it as the example.


A relay is just a remotely controlled switch that opens and closes using an electromagnet. The electromagnetic portion that provides the opening and/or closing force of the switch is called the coil. Relay coils can come in many different voltages depending on the application, but in residential and light commercial HVAC 24-volt coils are the most common.

The portion of the relay that opens and closes can be called the switch, contacts or points. These contacts can either be closed meaning there is an electrical path or open meaning there is no electrical path. Often this open or closed circuit will be described as “making” a circuit, meaning the switch is closed or “breaking” a circuit meaning the switch is open.


It is important when connecting a relay to distinguish which two relay points connect the coil. In the case of the 90-340, it is the bottom two terminals of the relay. Even though the coil is unmarked on most 90-340 relays, you can find it easily by locating the terminals with the small strands of wire connected. These two points connect together through the electromagnetic coil. When 24 volts of potential is applied across the coil the switch portion of the relay will switch from open to closed and closed to open depending on the terminal. Keep in mind that in a normal 24v circuit one side of the coil is connected to a 24v switch leg such as the thermostat “G” circuit for blower control, and the OTHER side of the coil is connected back to common.

The other six terminals are switch/contact terminals and the relay has a diagram embossed right on the top for easy reference. The way the circuit is drawn shows the de-energized state of the relay, meaning the state of the switches when no power is applied to the coil. When power is applied to the coil the points that were previously open (broken) now become closed (made) and the ones that were closed become open. When two points are closed when no power is applied to a relay coil we call them “normally closed” when they are open when no power is applied they are called “normally open”.


So based on this embossed diagram on the relay 1 to 3 and 4 to 6 are open (normally open) with no power to the coil and closed when power is applied. 1 to 2 and 4 to 5 are closed (normally closed) with no power and they open when the coil is energized. There is never a path between 2 & 3 or 5 & 6 because between them, at least one of them is always open. There is also no path or circuit between the top three terminals and the bottom three terminals or between the switch and coil portions of the 90-340 relay.

The data tag on a 90-340 shows both the coil voltage as well as the LRA (locked rotor amps) and FLA (full load amps) that the contacts can handle at various voltages for inductive (magnetic) loads like motors. It also lists the amp rating if the relay is controlling a RES (resistive) load like a heater or an incandescent light.


This relay can control a 39.6 LRA and 6.9 FLA Motor or a 15 amp heater at 240 volts based on the data tag.

— Bryan

2 comments

  1. Pete singh says:

    I really love to read your column , keep it coming

  2. Travis Curth says:

    I wish some of my teachers in service school would have been this clear on stuff like this…keep doing what your doing!!

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