Before we can start to use meters, we need to understand the basic electrical circuits in a piece of equipment. How I was taught about electricity is pretty simplistic but it made me really understand circuit diagnostics and that was — in every circuit you have a power source , you have a switch(es), and you have light bulbs. It does not matter what you are working on, each circuit has at least these 3 items.
In today’s heating and air conditioning equipment, there can be multiple power sources, switches, and light bulbs. Some power sources can be light bulbs — a transformer is a power source for one part of the circuit and a light bulb for another part of the circuit. Power terminates at one side of the transformer making it the light bulb. But, now this light bulb becomes a power source for another part of the circuit.
Every electric circuit has a power source–24 volts, 110 volts, 220 volts, 46o volts. Most control system have multiple power sources — 110 volts to 24 volts, 220 to 110 or 24 volts, and so on.
Light bulbs are basically anything that “consume” power — Motors, Gas Valves, Contactor coils, Transformers, etc. They are an end point.
When we look at switches, switches are anything in a circuit that can open or close, either making or breaking the power source or varying the power to the light bulb. They can be a simple temperature Klixon switch. They can be a contact closure in a relay. They can be part of a circuit board. They are there to act as safeties, on/off controls, sequencers, etc. Even a temperature sensor can be considered a “switch” (think of it as a “dimmer switch” since it can vary voltage to the light bulb). To put it simply: Switches control the power source. Control Boards are nothing more than multiple light bulbs and switches — the on-board relays = light bulbs and the relays = on/off switches. All they are there for is to control what happens between the power source and the light bulb. We do not need to worry about the “electronics” in the board since we are not rebuilding boards. We need to see if the switches are working when we have the power source present. If the power source is present and the switches are working but the “light bulb” is not activating, then we need to diagnose the light bulb.
There are different types of circuits, but the 3 most common on today’s HVAC equipment are;
Series circuits in which the switches are connected one to the next. This is common in a safety string
Parallel circuits in which the power can have more than one path:
And then there is the combination of both the above, the Series-parallel circuit
Understanding how circuits are constructed will help in making proper diagnosis of the system. This can then be applied to reading the wiring diagram, which is the road map for the system you may be working on.
Where some technicians get lost when doing electrical diagnostics is the start in the middle of a circuit at a “switch”. WRONG! When performing electrical checks, you need to start at either the power source and work forward or at the light bulb and work back. When you start in the middle, I can guarantee you are going to miss something.
No matter how complicated a circuit may be, no matter how many controls you may see in that circuit, if you just remember that everything you are looking at is either a power source, a switch, or a light bulb, and track it through with that in mind, starting at one END or the OTHER, electrical diagnostics will become easy.
Now that you have an understanding of how a control circuit is constructed, we need to understand how electricity “flows” through these circuits and what you meter is “looking for”. In my next posting, I will try to explain electricity since this is necessary to understand so we use meters correctly.