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HVAC School - For Techs, By Techs

Dec 5, 2017

In today's podcast, Bryan talks about voltage (volts) and resistance (ohms), specifically using a voltmeter and an ohmmeter for diagnosis. We also discuss voltage drop.

In many cases, Ohm's law is impractical for field usage because of the additional resistance from inductive reactance. We also don't typically measure impedance and only care about resistance on the windings. However, Ohm's law is still a valuable concept because it teaches technicians the relationship between voltage, amperage, and resistance (ohms).

Ohm's law states that volts equal amps multiplied by ohms (E = I x R). Therefore, if the volts stay constant, ohms will increase as amps decrease and vice versa.

We distinguish lines from loads in circuits; we say that loads are the parts that "do" something due to resistance in a circuit. There are two kinds of loads: inductive and resistive. Inductive loads generate expanding/collapsing magnetic fields, which can also cause rotational force or activate a solenoid. Resistive loads generate light and heat, so heat and resistance are related.

Of course, the diagnostic tools we use (multimeters, voltmeters, ammeters, ohmmeters, etc.) also have their limitations. A voltmeter merely determines a difference in charges between two points. When using a voltmeter on a low-voltage circuit, try to plant one of your leads on the common side and take readings throughout the circuit with your hot lead. Ground is also NOT a reliable reference point for diagnosis.

The point of measurements is to prove what we suspect to be true; we must understand what our data mean for system operation and what our tools' diagnostic limitations are. For example, when we ohm out contactors, we check to see if they're open.

Bryan also discusses:

  • Fixed wattage or resistance
  • Reading between wires
  • Meter lead placement
  • Amperage (dynamic current/electrons)
  • Undiagnosed shorted circuits
  • Contact points
  • Voltage drop and resistance
  • Infinite ohms
  • Wire length

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