Tag: relay



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

Look closely at a contactor on your truck and you may find some interesting ratings you never noticed. Things like terminal ambient temperature ratings and torque specs. One reading you may overlook is the RES AMPACITY of the contactor or relay. The RES rating is the RESISTIVE LOAD AMPACITY (amperage capacity) or rating.

Remember, a contactor and relay is both a switch (contacts) and a load (coil) that controls the switch (contacts). The FLA, RES and LRA are the ratings of the switch / contacts / points portion of the relay / contactor, not the coil portion. The coil is just rated for voltage and cycle rate (hz).

You will notice that the RES (Resistive load) rating is higher than the  FLA (Full Load Amperage) rating and much lower than the LRA (Locked Rotor Amperage) rating. So the contactor above the contactor above would be sufficient to control/switch a 40 amp full load INDUCTIVE load and a 50 amp RESISTIVE load.

So what is the difference between an inductive and a resistive load?

An inductive load is a magnetic load that converts electrical energy into kinetic energy (motion) through electromagnetic force (magnetism). This would usually be motors and solenoids in HVAC/R, really anything where magnetism and motion are involved (Transformers and inductive rages being examples of induction WITHOUT motion for the purist).

An inductive load will experience a spike in current when voltage (potential) is first applied, this is called locked rotor amps (LRA) because it is the current a motor will draw when it starts up from a locked or stalled position.

A Resistive load is a load that converts electrical energy directly to light or heat as the electrons flow through a resistive conductive path. These would be things like heat strips, crankcase heaters and incandescent light bulbs. Resistive loads do not have any internal variation in load and the voltage and amperage are completely in phase. This means that when a contactor, switch or relay are controlling a resistive load there will be less load variation.

The conclusion is that often contactors and relays can handle a higher amperage across the contacts when the load is resistive (Light / Heat)  than when it is inductive (Motor / Motion).

— Bryan

If you do any larger commercial work you’ve probably seen a DIN rail without knowing what it is called. It is simply a mounting standard that originated in Germany in the 80’s and slowly worked its way over here.

DIN rails can be used to mount terminal blocks, relays, starters, breakers… just about anything electrically. They aren’t designed to conduct electricity like a busbar, though they are used in some cases as a grounding assembly.

The most common DIN rail type is the “top hat” or TS35 shown in these photos.

Components that attach to a DIN rail have little release clips so that they can be easily installed and removed. In addition to the “top hat” style there are also some less common, heavier DIN rails with a C and G style configuration.

So if you ever see one of these don’t be alarmed, you can just sound cool when you call your boss and tell him “yeah, it’s one of those DIN rail mounted relays” and just wait for him to say “What??”

— Bryan

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