Electrical Basics - Ohm's Law, Circuits & HVAC Components
Ohm's law (V=IR), series and parallel circuits, capacitors (MFD ratings), contactors, relays, transformers, and the 24V control circuit used in residential HVAC systems.
- Apply Ohm's law to calculate voltage, current, and resistance in HVAC circuits
- Distinguish between series and parallel circuits and their behavior
- Identify the function of capacitors, contactors, relays, and transformers in HVAC systems
- Trace the 24V control circuit from transformer to thermostat to equipment
Leçon 1
Ohm's Law & Basic Circuit Theory
The Three Electrical Fundamentals
Every electrical circuit in an HVAC system involves three fundamental quantities. Understanding these three values and how they relate to each other is the foundation of all electrical troubleshooting.
Voltage (V) - Measured in volts. Voltage is the electrical pressure that pushes current through a circuit. Think of it as the force behind the electricity. In a residential HVAC system, you will encounter 240V (compressor power), 120V (blower and accessories), and 24V (control circuit). Voltage is measured in parallel - you place your meter probes across (on both sides of) the component or power source.
Current (I) - Measured in amps (amperes). Current is the flow of electrons through a conductor. It is the quantity of electricity actually moving through the wire. Current is measured in series with the circuit, but in HVAC work we typically use a clamp-on ammeter around a single wire. Higher current means more electrical flow and more work being done.
Resistance (R) - Measured in ohms. Resistance is the opposition to current flow. Every component in a circuit has some resistance - motor windings, heating elements, contactors, and even wire itself. Higher resistance means less current flows for a given voltage. Resistance is measured with the circuit de-energized and the component isolated (disconnected from the rest of the circuit).
Ohm's Law: V = I x R
Ohm's law is the most important electrical formula you will use as an HVAC technician. It defines the relationship between voltage, current, and resistance:
V = I x R (Voltage equals Current times Resistance)
This formula can be rearranged to solve for any value:
- V = I x R - Find voltage when you know current and resistance
- I = V / R - Find current when you know voltage and resistance
- R = V / I - Find resistance when you know voltage and current
Practical example: A furnace heating element has a resistance of 12 ohms and operates on 240V. How much current does it draw?
I = V / R = 240V / 12 ohms = 20 amps
Power formula: Electrical power is measured in watts (W) and is calculated as:
P = V x I (Power equals Voltage times Current)
From the example above: P = 240V x 20A = 4,800 watts (4.8 kW)
This heating element converts 4,800 watts of electrical energy into heat energy every second it operates.
Watt's Law and BTU Conversion
In HVAC, we often need to convert between watts and BTU/h because heating capacity is rated in BTU. The conversion factor is:
1 watt = 3.412 BTU/h
So a 4,800-watt heating element produces: 4,800 x 3.412 = 16,378 BTU/h
This conversion is useful when sizing electric heat strips and comparing electric heating to gas furnace capacity.
Ohm's law (V = I x R) is the foundation of all HVAC electrical work. Voltage is measured in parallel, current with a clamp-on ammeter, and resistance with the circuit de-energized. The power formula P = V x I lets you calculate wattage, and 1 watt equals 3.412 BTU/h for converting between electrical and thermal measurements.