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 an example.


A relay is just a remotely controlled switch that opens and closes using an electromagnet. The electromagnetic portion that provides the opening 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. This open or closed circuit will often be described as “making” a circuit, meaning the switch is closed, or “breaking” a circuit, meaning the switch is open.


When connecting a relay, it is important 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 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 or 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 previously open points (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 those points, 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 controls 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

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8 responses to “Understanding Relays With the 90-340”

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

  2. For the longest time I have not yet to understand relays! I know 24volts will open or closed the contacts, right?

    My questions is why terminal g and r get conected if so why?

    • ” I know 24volts will open or closed the contacts, right?”
      – Yes, close the contacts if the contact is a N.O. contact, and vice versa.

      “My questions is why terminal g and r get conected if so why?”
      – The R supplies power to all of the terminals in the thermostat, when the thermostat closes the terminals for either cooling or heating. When the thermostat calls for the HVAC system to run, the G wire will now be carrying the 24 volts (control voltage) to the indoor blower motor relay/contactor coil. G is connected to one side of the coil (think L1 on a wiring diagram), and the other side is connected to neutral (think L2).
      As to why? I dont know about you, but when I need to run my HVAC system, I do not want to be running around the house, manually turning on the switches needed to make the house cool. Then when it gets too cold, run to all of the switches in different parts of the house, just to shut everything off. The relays/contactors do this for us, so everything is automatic. No need for running around like a chicken without a head.
      Then from there, the above article comes in.

  3. It might help the student to know that the coil leads are tiny, and must be soldered to a hollow rivet.
    (The higher the coil voltage, the smaller the wire.)
    All of the contacts are riveted, with no solder.
    This is also true with “peanut” relays.
    Great article.
    Keep up the good work!

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