Control Circuits As A Foreign Language
Control circuits are a foreign language for many of us. Relays compound the problem considerably. Although relays add more than their share to the complexity of a circuit, they're worth it. They serve as a voltage translator.
The complexity of relays comes from several facts:
- A relay often has two different voltages going through it at the same time.
- A relay is both a load (the coil) and several switches.
- When a relay has power, it tells lies.
Two Different VoltagesAs you know, each circuit can have only one voltage. But in control systems, we often have two voltages. The equipment - the circulator for example - is line voltage (somewhere around 120V). The controls, though, are usually low voltage (around 24V).
Why not simply match the voltage of the controls to the equipment? Of course there are line-voltage controls. Why do we use low-voltage controls instead?
Some of the reasons for using low-voltage controls are:
- Low voltage is safer.
- In some localities, nonelectricians aren't supposed to work on line voltage.
- Low voltage allows tighter temperature control, which means more comfort for the user.
Line-voltage controls can't keep the temperature in the living space within the “comfort zone.” The industry standard for “comfort” is a temperature swing of no more than a two degrees. That's sometimes stated as “plus or minus one degree.”
We have some good reasons for putting low-voltage controls with line-voltage equipment. However, it's just a fact that we can't put a low-voltage control in the same circuit with line-voltage equipment. So how are you going to get the two to “talk” to one another? If we think of voltage as the language of the circuit, the thermostat and the equipment speak different languages.
We need a translator. A relay is a voltage translator.
To understand the translator idea, we need to look at the parts of a relay. A relay consists of one coil and a number of switches. You know that every control must be one of three things: a power supply, a switch or a load. A relay is tricky - it's two of these. The relay coil is a load. The switches, of course, are switches.
The coil and switches can be different voltages because there's no physical connection between them. The only relationship between the coil and the switches is magnetism. When we apply electricity to the coil, the coil creates magnetism inside the relay.
The magnetism causes each switch to change position. If a switch was open before electricity was applied to the coil, the switch closes. If the switch was closed, it opens.
Not all of the switches are necessarily the same. Some can be “normally open,” and some can be “normally closed.” More on that in a minute.
Yes, indeed, translating voltage is what a relay lives for. It translates a message - “turn on the heat” - from a low-voltage thermostat to a line-voltage circulator.
Here's how it works. Let's start with the thermostat since that's where the call for heat comes from. In the thermostat circuit, the thermostat is the switch. The power supply is a 24V transformer. The load is the coil of the relay. Period. That's the circuit.
You know that a load always changes electricity into some other form of energy. In this case, the coil load changes 24V into magnetism.
Our second simple circuit is a line voltage. The power supply is house current, the load is the circulator motor and the switch is a relay switch.
Here's how the communication goes between the circuits: The thermostat closes on a call for heat. The thermostat is a switch in a 24V circuit. That allows electricity to flow to the relay coil. The relay coil changes the 24V into magnetism inside the relay. The magnetism closes the normally open relay switch. That now-closed relay switch is in the line voltage circulator circuit. The closed switch allows line voltage house current to flow to the motor and bring on the circulator.
With the relay as voltage translator, here's the simple thing we accomplished: The low-voltage comfort-providing thermostat brought on the heat in the heat-providing line-voltage equipment.
The same thing happens with air-conditioning. But then we're using a contactor to translate between the low-voltage thermostat circuit and the line-voltage compressor circuit.
The Politics Of PowerA relay often isn't what it says it is. Kind of like politics. When a relay gets power, it tells lies.
On the top of a relay are a number of switch terminals. Between the terminals you'll find some printing. There might be the letters “N.O.” or “N.C.” N.O. stands for “normally open.” N.C. means “normally closed.” Normally is when there is no electricity going to the relay coil, as in when the relay is still in the box. N.O. means that those two terminals form a switch that is open (turned off) if there is no electricity applied to the coil of the relay. Did you get that?
As soon as there is electricity to the relay coil, the switch labeled N.O. (open) is closed. The switch says it's one thing, but it's actually the opposite. It lies.
The same principle goes for terminals labeled N.C. An N.C. switch is closed as long as there's no power to the coil. As soon as the coil is powered, the N.C. switch is open.
There can be both normally open and normally closed switches on the same relay. Sometimes the normally open/normally closed designation is shown by a symbol rather than letters. The open switch symbol means normally open, and the closed switch symbol means normally closed.
Voltage & Switching SpecificationsA relay has specifications for voltage and switching. The voltage rating applies to the coil. If it's a 24V relay, you can't apply more than 24V to the coil. Well, of course you can, but the coil won't live through it. However you can put the switches of a 24V relay in line voltage circuits. Sometimes the switch terminals are made so that you can even use them for millivoltage.
Switching is shown as SPST, SPDT, etc. This is the same as with any other switch. SPST (single pole, single throw) is usually all we need. That means the switch closes to turn the circuit on, and it opens to turn the circuit off. SPDT (single pole, double throw) means that each switch can turn one thing on while turning something else off, and vice versa. (See March 2003 PM issue for more about these designations.) You can always use an SPDT as an SPST. DPST (double pole, single throw) simply means that there are two switches available rather than one. You can choose to use just one of them.