Friday, June 4, 2010

Teaching Ladder Diagrams

Reading electrical diagrams is a crucial skill that HVAC/R students must master. That is why all HVAC/R curriculums spend a lot of time discussing how to read electrical diagrams. Many students struggle with this even though every HVAC/R instructor I have ever met emphasizes diagrams. I believe this is because students must learn several pieces of information and then weave them all together in order to truly understand electrical diagrams. You really can’t understand diagrams if you don’t understand the basic function and operation of standard electrical components. Learning symbols for components that you don’t understand is like memorizing words in a foreign language without knowing what they mean. You still can’t read. Understanding basic circuits is also a must. Students should be able to construct basic circuits consisting of loads and switches. Finally, they must have a good concept of how the system they are working on heats or cools. If all this is in place they usually will have no problem learning how to read electrical diagrams. I find that students who are the weakest with diagrams are usually also weak in one or more of these foundation areas. After all, a diagram is simply a representation of a collection of circuits that are made up of electrical components. You really can’t understand an abstract representation if you don’t understand the actual component that you can see and touch. So we start by having students build basic circuits with switches and lights in order to learn how loads and switches work in a basic circuit. Loads should have a voltage drop when current passes through them, switches should not. Then we introduce components and relate them to either loads or switches. So thermostats are switches while motors are loads. Introduce symbols and have students draw symbols for selected components. Then have them identify components on a unit using the unit diagram. The most difficult components for students to grasp are usually contactors and relays. Understanding relays is a big stumbling point for many students. I have a method for teaching relays, but I will save that for another article. Just know that there is no use trying to explain a ladder diagram that contains 24 volt control relays to someone that does not understand how relays work. After explaining basic ladder diagrams and showing several examples ask the students to draw a simple diagram of a packages air conditioning unit without looking at an example. You don’t want a copy of the book; you want to see how much they understand. There will be lots of weeping and gnashing of teeth. Students will complain that they are not artists and that drawing diagrams is not necessary to service systems. However, you will find out quickly how much they understand about circuits, components, and diagrams. People that understand diagrams can draw them. Be patient, suggest corrections, and have them revise their diagrams until they have something that will work. This typically takes several revisions for many students. Finally, have them wire the circuit that they drew using the actual components as much as possible. For safety we have a simple rule: the instructor has to check the circuit before it is energized. The pride the students get from seeing their circuit operate will offset the anguish they experienced getting there. Gradually add more complex circuits – two stage cooling, cooling and heating, heat pumps, or whatever makes sense for your area. After the students have a good command of ladder diagrams you can start discussing the many variations they will see in different manufacturer’s diagrams. There are as many variations as there are manufacturers, adding to the difficulty for aspiring technicians.

Diagrams are discussed extensively in Fundamentals of HVAC/R in units 31 Electrical Components and Wiring Diagrams, 32 Controls, and 33 Control Circuits and Diagrams. There are some great exercises in MyHVACLab to help students learn electrical diagrams including an interactive electrical symbol recognition activity and a virtual wiring exercise that aligns a ladder diagram to the actual components as they are wired.

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