Using Volts and Ohms in Diagnosis

HVAC School - For Techs, By Techs

Bryan Orr

Training, Careers, Airconditioning, Self-improvement, Hvac, Business, Education, Refrigeration, Heating, Ac, Apprenticeship

4.8 • 985 Ratings

🗓️ 5 December 2017

⏱️ 74 minutes

🔗️ Recording | iTunes | RSS

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Summary

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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Transcript

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0:00.0	This episode of the HVEC School Podcast is made possible because of the generous support in so many ways from
0:15.0	Testo, Rector Seal, and Carrier. And I want to mention I'm going to be at
0:20.7	AHR, the AHR conference in Chicago this January, starting on January 22nd, which is
0:27.9	a Monday.
0:28.9	January 22nd, 2018.
0:29.9	I'm going to be at the AHR conference and for those of you who listen to the
0:33.6	podcast and would like to hang out. I'm going to try to grab lunch and dinner with
0:38.0	some of you, maybe breakfast, who knows? My brother Nathan's going to be there. Some of
0:41.0	you know him from the Facebook group and we're going to have a great time.
0:44.7	I hope to see many of you there. But one way to see me and also see a pretty cool tool is by going to,
0:50.8	actually seeing me and seeing a cool tool might be the same thing anyway I
0:53.7	digress is by going to the rector seal booth and so that is booth 2545 at
0:59.6	a hr and from 2 p.m. to 4 p.m. on January 23rd which is Tuesday I'm going to be
1:05.6	demonstrating the pro fit tool so we'll probably demonstrate it a couple times if
1:09.2	you have questions for me or anything you want to talk about you can come by and
1:11.6	talk to me there at the director seal booth but you can also enter to win at the Rector Seal Booth for a pro-f-f-f-fel, if I'm not mistaken, don't quote me on this, but I think they're giving away one of the pro-fit flare kits and then also a swedge kit the new
1:25.1	swedge kit that they make and it's a really pro it for those you don't know a
1:28.0	pro fit is pro fit is pro fit is a really great flaring and swaging tool that you can use. It just goes onto a drill or an impact. It has a chuck on it.
1:36.4	It's made primarily for high RPM drill so you've got to have a good quality drill, but you can make a really, really nice flare and sledge with that tool.
1:43.9	The flare kit's already out.
1:45.7	Swedge kit will be out very soon.
1:48.4	I would also encourage you to go check out the Testo Booth, 7365.
	...

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