More on Electricity

Feature Story

by George Lanthier

In a previous article we looked at an excerpt from one of our newer books WIRING & Fuel Burning Equipment, Volume One, Fundamentals. In this article we will finish our discussion of switches and look at relays from that text, which is available at our Web site at �

Switch Differential
Differential is probably the most difficult thing about switches to understand. In actuality it is not that difficult, but many of us try to make it too complicated.

Figure 1

If I told you to turn on the lights in a room, you would go to a light switch and turn it on. If I told you to turn the lights off, you would reverse the process.

The action that you would take in both cases is mechanical. You would turn the switch on and you would turn the switch off. Your finger on that switch handle is a mechanical action and would require your manual motor skills to perform it. In the switches around oil, coal and gas heating equipment, we use magnetic, mechanical, sensible and timed actions to make a switch react.�
Magnetic switch actions are created through the use of electromagnetic coils in relays and by using solid-state devices.

Figure 2

In sensing type switches, the actuator recognizes the presence of light, pressure or temperature. A sensible light application is similar to a cad-cell on an oil burner. A sensible heat application is similar to a thermocouple or flame rod on a gas burner.

In steam systems, and any pressure application, pressure is used to push on a diaphragm or a bellows to open or close a switch.

In older hot water systems, temperature is used to expand the medium fluid in a sensing well and open or close a switch through hydraulic action. In newer systems, it’s done through an electronic sensor called a thermistor. A thermistor, Figure 1, is a heat sensitive resistive device that increases or decreases resistance based on temperature.

And finally, in warm air systems and any air related application, such as a room thermostat, heat once again is used to expand a bi-metal and cause a mechanical reaction.

Figure 3

In timed applications, timing mechanisms and resistors are used to control switches.

Now, what about that differential?  Well, in all of these cases there will be different conditions.  We will either have a certain amount of pressure or temperature, or we will not.

Let’s say that you have a normally closed switch.  That switch is set to remain closed and allow ‘something’ to run until the temperature reaches 180°F, and then it opens. Now, when does it turn that ‘something’ back on? Aha, you would have to know the differential, right?

Let’s say that the differential was 10°F, then it would turn that ‘something’ back on again at 170°F.  If the differential was 80°F then that ‘something’ couldn’t come back on until the temperature dropped to 100°F.  Now that’s a difference in temperature, and one heck of a differential!

Figure 4

Now another thing about differentials — they come two ways in this business: fixed and adjustable. With a fixed differential you cannot change the switch’s action. If you have a 10 degree differential, then that’s what you have, and it will take 10 degrees of change from the set point to make something happen.

With adjustable differentials, you can set the differential and get the switch and the controlled ‘something’ to do what you want it to do. Now, it doesn’t matter whether the switch is single pole, single throw (SPST) or single pole, double throw (SPDT), it has to have a differential to work, got it?

Figure 5

Relays are switches that are opened or closed and require electricity to work. They not only direct the path of electricity in a circuit, but also they need electricity to work themselves.  They consist of one or more electrical switches that can be SPST, SPDT, or double-pole, double-throw (DPDT) and a coil to activate the switches.

Many people think of basic relays incorrectly. When most of us think of a relay, the ones that we are really thinking of are switching relays. A relay switches power only, switching relays do two things: switch the controlled loads and convert line voltage (120 Vac) to low voltage (24 Vac).

Let’s take a look at relays first. Since we have already discussed switches, let’s look at the coil first, then a transformer and then put a switching relay together.

Figure 6

Relays are made up of contacts and a coil.  The coil is an electrical load that can be put to work. When powered, a coil will activate and will either move a piston, in the case of a valve, Figure 2, or move the contacts of a switch, see Figure 3. As shown in Figure 3, the switch contact assembly is where the power is actually passed from one side of the switch to the other.

The relay coil consists of five basic components: the coil or solenoid; the iron core; the relay armature; the relay hinge; and the fixed and movable contacts.

Figure 7

The relay, Figure 4, operates on the principle of electromagnetism, and so when the coil is powered, the armature is attracted to the iron core and moves on its hinge. When the armature is attracted to the core, it allows the moving contacts to join with the fixed contacts, and so current passes through the switch, Figure 5.  These switch contacts can be either low voltage or line voltage, but must be constructed to accept the higher voltage used. 

Did you also notice that in Figure 3 there are two sets of contacts? One set of the contacts is shown as N.C. (Normally Closed), and one set is shown as N.O. (Normally Open). The N.C. sets of contacts are normally closed, which means that until the coil is powered, current will pass across the contacts. When the coil is powered, the N.C. set opens, current is stopped, and now current will flow across the N.O. contacts.

Also, keep in mind that relay coils come in different voltages and so may be powered by 24 Vac, 120 Vac and 220 volt AC circuits. Most of the relays you will use have coils that are powered by 24 Vac and 120 volt AC, but they may control the switching of higher voltages through their contacts. Most commercial gas, oil or combination gas-oil burners use 120 volt coils to control the power through a ‘contactor’ to the burner motor operating on 220 volt power.

In Figure 6 a typical contactor is shown on the left, and it is also shown as a schematic on the right. What is the difference between a contactor and a relay, you may ask? Electrically nothing, but physically contactors are much larger and can carry much higher voltage and current potentials than relays.

Step-Down Transformers
Transformers come in two distinct types, step-down and step-up, and for fuel burning equipment and HVAC service, both types are used. The primary use of step-up transformers is for ignition systems, 120 Vac to 10,000 Vac or more. The primary use of step-down transformers, 120 Vac to 24 Vac, is for control systems such as thermostats, zone valves, dampers, gas valves and primary controls. In this section we will discuss low-voltage step-down transformers.

All of the transformers that we use consist of three major components: an iron core, a primary coil and a secondary coil.

A transformer changes power from one voltage and current potential to another through electromagnetic induction without changing the phase or frequency. Since we use 60 cycle Alternating Current (AC) in the U.S. and Canada, this is the only factor that will not change in a transformer.

In the step-down or low-voltage transformer, Figure 7, the primary or entering voltage is 120 volts, and the secondary or leaving voltage is 24 volts.  The transformer is shown graphically on the left, and its schematic form on the right.

The current also changes as shown in Figure 8. This table is for a particular manufacturer and you should always ‘check the specs’ for the transformer you are using.

Another thing that is important to note is the output of the transformer in volt-amperes (VA). VA is the output or amount of work that the transformer will perform, and you must know this output rating when sizing zone control devices such as valves and dampers.

Did you notice that the input voltage may have no effect on the output voltage? It’s very important to remember that it’s how the transformer is constructed internally ‘ that’s important, not just the primary and secondary voltages.

George Lanthier is the owner of Firedragon Enterprises, a teaching, publishing and consulting firm. He can be reached at 132 Lowell Street, Arlington, MA, 02474-2756. His phone is 781-646-2584, fax at 781-641-7099, his e-mail is and his Web site is

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