This is the first article in Jordan Day’s series on Demystifying the Printed Circuit Board. Find Part 2 here.
Key Takeaways
- Component Identification: Learn to recognize common PCB designators (R=Resistor, C=Capacitor, U=IC) to demystify circuit boards
- Microcontroller Basics: The “brains” of modern HVAC boards operate on 5V DC and require careful diagnosis beyond simple input/output testing
- Practical Diagnostics: Visual inspection combined with component knowledge prevents costly misdiagnosis of control boards
- Safety Critical: Always ensure proper grounding and understand voltage requirements before handling PCBs
The Evolution of HVAC Electronics
Technicians who have been in the HVAC industry for 20-plus years have witnessed an explosion of printed circuit boards (PCBs) in their units. One popular manufacturer now boasts seven separate PCBs in their standard 10-ton rooftop units. In trade school, we were taught to think about PCBs in simple terms: “If you have the correct inputs but do not have the correct outputs, replace the board.” This troubleshooting technique works fine for PCBs with simple discrete inputs and outputs, but today’s PCBs demand a more sophisticated approach.
Modern PCBs utilize low voltage PWM (pulse width modulation) signals, hall effect sensors, analog inputs, and analog outputs. Others employ serial communication between separate PCBs. If you dabble in the controls business, you may encounter a protocol known as HART that uses serial communication and a 4-20mA analog signal over the same wires simultaneously! For those working with BMS control fundamentals, understanding these communication protocols becomes even more critical.
Due to ever-increasing levels of technology, misdiagnosed PCBs have become commonplace. Part of our due diligence includes a visual inspection for broken traces or components that look like they’ve exploded or caught fire. But what does the average technician do after spending several hours troubleshooting a unit with no success? He throws a board at it. We’ve all done it.
Starting Your Journey with Electronics
If you want to be a valuable service technician primed for the future, it’s time you begin your journey with electronics. Circuit board repair isn’t necessarily part of your job description, but familiarizing yourself with PCBs will greatly reduce the intimidation factor that these components present to most technicians.
I can give you a recent example: A boiler technician called me to assist with troubleshooting two boilers that had multiple circuit boards. Both independent boilers were giving the same error code. Knowing that a misdiagnosis would be a very costly mistake, he had called me for a second opinion. After thoroughly describing his troubleshooting process, he pointed to what he thought was the main board, explaining why it must be the culprit.
I agreed with his assessment but disagreed with which board was the “main board”. He had assumed that the larger board with all the “computer chips” on it must be the main board where the “brains” were located. Upon examination, I noticed that these ICs (integrated circuits) were just darlington arrays and comparators. On a much smaller board, however, I found a small IC that had ATMEL printed on it. Having programmed many ATMEL microcontrollers, I knew this board contained the “brains” we were looking for.
Two boards were ordered. Two boilers were repaired. Not a dime wasted.
Understanding PCB Basics
The first principle you need to embrace is that understanding boards, their layout, design, and component functions is well within your capabilities. A common misconception among HVAC technicians is that the engineers who design and program these boards are all summa cum laude graduates. While many are truly intelligent and hold degrees in electrical or electronics engineering, much of their trade study, like ours, has been self-taught at home.
Simply put, if you can grasp the complexities of HVAC systems and refrigeration, you can understand the basic operating principles of circuit boards found in HVAC equipment without too much trouble. This knowledge becomes particularly valuable when working with BMS network architecture where multiple boards communicate across complex control systems.
Examining a Common PCB: The Carrier Ignition Control Module
Let’s examine a common PCB found in many Carrier rooftop units: the Carrier Ignition Control Module (LH33WP002). I chose this board not only because it’s common but because it uses easy-to-identify TH (through-hole) components.
Understanding PCB Designators
| Designator | Component Type |
|---|---|
| R | Resistor |
| C | Capacitor |
| D | Diode |
| Z | Zener Diode |
| T | Transformer |
| J | Terminal Block (sometimes “P” for Pin) |
| JW | Jumper Wire |
| U | Integrated Circuit (IC) |
| K | Relay (Key Switch) |
| Q | Transistor |
| F | Fuse |
| LED | Light Emitting Diode |
| Designator | Component Type |
| ———— | ————— |
| L | Inductor |
| X | Crystal |
| SW | Switch |
PCB Components and Critical Markings
The silkscreen contains important information like “CUT IF CS USED” and the model number for this specific board. Look carefully and you’ll find “GROUND SCREW REQUIRED” in the bottom right corner. If you turn the board over, you’ll see that this tubular stand-off is electrically connected to the ground plane of the PCB.
Understanding Microcontrollers
Let’s examine the microcontroller more closely. In many cases, the public identifier will be printed on the IC. For example, a PIC16C57C can be found on the CXM board once used by Carrier and ClimateMaster in many of their water source heat pumps (not to be confused with the CXM2 which uses the STM32 microcontroller). We can work with this part number to dive deeper into the brains of the Carrier Ignition Module.
One thing you’ll quickly learn is that finding data sheets on electronic components with public identifiers is much easier than finding service manuals for your HVAC equipment. Using a search engine, you can find what’s called a “pinout” for this microcontroller.
Understanding Voltage and Component Markings
Applying high voltage directly to this controller will destroy it. This controller operates on 5 volts DC. Look at the pin assignments for pin #2 and pin #4. Pin #2 is marked VSS and Pin #4 is marked VDD. Here’s where things might get confusing and counter-intuitive. VSS stands for Voltage Source Supply and VDD stands for Voltage Drain-to-Drain.
One might assume that Voltage Source Supply would be the positive voltage supplied to the IC and that the “drain” would be the negative. It’s actually reversed in most cases. The reason for this is rooted in the structure of a component called a MOSFET, where the “drain” terminal is connected to the positive supply voltage in an N-channel device. Don’t let this technical detail derail you—just remember that VDD is typically positive and VSS is typically ground.
Decoding Capacitor Markings
This ceramic capacitor is marked “103Z”. The number 103 tells us the capacitance value, similar to how our common run capacitors are marked 30μF or 45MFD. However, this isn’t a 103μF capacitor—it’s actually only 0.01 microfarads, or 10 nanofarads.
Here’s how to decode it: The first two numbers in 103 are the significant figures (10), and the last number (3) is the multiplier. Ten to the power of 3 equals 10,000, but our units are in picofarads. So 10,000 picofarads = 10 nanofarads = 0.01 microfarads. The letter “Z” at the end is manufacturer-specific but likely indicates the tolerance (±20%).
The purpose of this specific capacitor, and the reason it’s located so close to the microcontroller, is that it’s a “decoupling” capacitor. These are almost universal components for microcontrollers, connected between the positive and negative pins. It’s imperative that they be located as close as possible to the microcontroller VSS and VDD pins. The main purpose of a decoupling capacitor is to filter out high frequency noise and fluctuations from the power supply. The microcontroller is a very sensitive device and its processes can be interrupted by the slightest instability in the power supply.
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Practical Application and Next Steps
The next time you have to swap one of these boards out because the induced draft motor won’t come on, take the old one home and spend some time studying it. Just being able to identify each component will help alleviate any apprehension and hopefully spark curiosity to dive deeper.
Understanding PCB components becomes particularly valuable during September’s strategic maintenance window when you’re inspecting control boards before the heating season. Early detection of failing components can prevent those emergency calls in January.
For technicians working with heat pumps, this knowledge becomes even more critical when diagnosing issues with reversing valve controls where the PCB manages the O/B terminal operations.
Conclusion
Modern HVAC systems rely heavily on sophisticated electronics, and understanding PCB basics is no longer optional for serious technicians. By learning to identify components, understand designators, and recognize the “brains” of a board, you’ll diagnose problems more accurately and avoid costly misdiagnoses. Remember, if you can master the complexities of refrigeration cycles and psychrometrics, you can certainly understand the circuit boards controlling them.
