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Do Ohms Lie?


Son of Samurai

1,167 views

"Ohms lie." It's become a popular phrase in the tech community, but what does it actually mean? Should you never do an ohms measurement? Are they completely worthless? Or is this phrase just another example of tech mythology?

The answer is more complicated than any of that. Let's run through a quick example to demonstrate. This is an old-skool Whirlpool gas dryer, just like they were cranking out left and right in the good old days.

Screen Shot 2021-11-16 at 11.51.33 PM.png

Now, our scenario is a simple one: when we run the dryer on a heat cycle, the ignitor doesn't glow, and we get no heat. We go through and take an ohms measurement of all the relevant components -- the ignitor, the coils -- we even measure the continuity of all the switches and thermal controls. Everything checks out.

How is this possible? If all the components are within spec on ohms, shouldn't the circuit run?

We've been doing all of our tests on a dead machine, since you can't do resistance measurements on a live circuit. Why don't we try a live test?

Here's what the ignitor's circuit looks like with the dryer running. We've got line and neutral all the way up to the ignitor, meaning that when we put our voltmeter leads across the ignitor, we read 120 volts. Our voltage supply is good. The only explanation for why the ignitor isn't running is that it has gone electrically open.

Screen_Shot_2021-11-16_at_11_51_33_PM.png

But wait -- we confirmed that it wasn't open with our ohms measurement. How could it be open now?

Let's step through what's going on in this circuit the instant that power is applied to that ignitor.

Screen_Shot_2021-11-16_at_11_51_33_PM-3.png

As soon as there's a voltage difference across that ignitor, electrons get blasted through it. This causes it to heat up -- and that's exactly what's supposed to happen.

But in our particular case...

Screen_Shot_2021-11-16_at_11_51_33_PM-4.png

And here we have the solution to our quandary: failing under load.

In its simplest form, this is exactly what failing under load looks like: thermal expansion due to the heat from current flow causes a tiny break to become an electrical open. Nothing more complicated than that.

This is not some whacky, once-in-a-blue-moon scenario. Failing under load happens all the time in various appliances. And it's precisely because of this that we say ohms lie. We went through and did an ohms test on every single component in the circuit and got no information out of it. And all we needed to do was take a single voltage measurement on a live circuit to tell us what was wrong.

Does this mean that you should never, ever do an ohms measurement? Of course not. There are some situations that will call for an ohms measurement, but it's up to you as the thinking technician to know when those situations are and how to interpret the results of the test.

"Ohms lie" is a pithy shortening of this whole concept, and I think a lot of techs who hear it don't know exactly what we mean when we say it. When we say, "Ohms lie," what we mean is, "An ohms measurement is one of the least informative tests you can do, and there's almost always a more useful test you can do instead."

But that takes a lot of breath, so instead we just say: ohms lie.

Want to learn more about how to properly make electrical tests and troubleshoot like a pro? Click here to check out the online Core Appliance Repair Training course over at the Master Samurai Tech Academy.

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Samurai Appliance Repair Man

Posted

Just to add a couple of thoughts...

All our troubleshooting comes down to loads and switches.

Loads are the things that do physical work (turn a motor shaft, light a light bulb, make an element give off heat, etc.). This physical work is called power in units of watts. Power is given by the ohms law equation: P=IxE, where P=power (watts), I=current (amps), and E=voltage supply (volts). No amps = no power = no work. No volts = no amps = no power = no work. In other words, loads are all about watts. 

Switches control the movement of electrons through the load; switch opens, no electron movement (ie., no current) hence no work being done. (switches can also be used for sensing but that's another conversation). 

So loads are all about watts. How many of you carry a watt meter? Yeah, me neither. But you should all carry an amp clamp. As we saw in everyone's favorite equation, P=IxE, amps are an excellent and convenient proxy for watts. How? Like this: 

P=IxE | I=P/E

In other words, you can measure the voltage supply to a circuit, measure the amps through that circuit, and then you can calculate the watts being converted by the load from electrical energy to mechanical or thermal energy. 

This is especially important for AC loads because of the relatively higher currents involved that can make things heat up and act differently (such as fail open under load). 

For this reason, good techs never waste their time using ohms to make diagnostic conclusions about AC loads in particular. The key spec for loads is watts for which we use amps as a surrogate parameter

So why do manufacturers bother including ohms for AC loads? Because manufacturers know that most techs do not understand watts or how to use them in troubleshooting. Most techs have never had formal training in electricity and circuits and learn bad habits from older techs or by watching Youtube videos which are all about ohms testing and other amateur hour goofiness. So manufacturers have had to dumb down their training to meet techs where they are. I have personally see this in manufacturer tech literature and training over the past three decades I've been in the trade. 

None of this is rocket science and anyone can learn it. We teach all this and more in the Core Appliance Repair Training course. What I tend to see is that many techs don't think they need to know it. They're comfortable doing ohms checking all day long because they understand ohms (or at least think they do.) They literally don't know what they don't know and so are condemned to working harder, not smarter. 

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This is a great post, very well presented!

Ohms don't lie any more than tea leaves lie. ;) If you make assumptions in your interpretation, your interpratation could be wrong, or at least unsupported by the facts. 

If you measure the resistance of a light bulb filament (or a motor winding) and read 3 ohms, you could assume that must be a 4800W light bulb, but you'd be wrong. You're assuming the resistance is the same when current is flowing, but when the filament heats up its resistance increases substantially

 

On 11/17/2021 at 7:07 AM, Samurai Appliance Repair Man said:

How many of you carry a watt meter?

Those of us that run Whirlpool warranty calls are expected to carry a Kill-a-Watt meter, since the factory only kind of knows how the new ant-style linear compressor works, and want watt readings to diagnose some sealed system issues

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mastertech011

Posted

Very good explanation, thanks very much. Just another opinion but I think we get in trouble when we call a "continuity test" an ohms test. If you get a true measurement in ohms it should have told you there was a problem in that ignitor. Just passing a continuity test is where you fail in most cases. If in doubt do a wiggle test and watch the meter go crazy.

 

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2 hours ago, mastertech011 said:

If you get a true measurement in ohms it should have told you there was a problem in that ignitor.

I'm pretty sure this lesson was based on a call I had a couple of weeks ago. If not, it's still the same as a call I had a couple of weeks ago. 8-)

The ignitor measured correctly when cold and off but opened immediately when powered up, leading to a dryer burner that wouldn't light even though each of the components clearly had the correct resistance.

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I had a fuse years ago on a GE Electric dryer waste my time years and Years ago. No heat. 220 at the plug. Long story short, after checking everything three times with a meter for continuity, I tested the dryer at the plug while running. No 220v only one side of the line 110v. Yea that was the Last Time I tested a electric dryer at the receptacle with out running it. 30 A cartridge style. Aha that's how long ago it was.

Also had a Flame Sensor on a gas dryer sticking closed. out of the blue it would stick keep the ignitor on and not heat. The customer was furious. I already replaced the burner coils (you know that story right? Almost always them messing with you) I told him what to look for and to check the heat a various times in the cycle and call me when it acted up. When she called I dropped everything and went straight there. Sure enough, there it was plain as the nose on my face. Ignitor on, a little Tappy- tap and it switched open... We all have hundreds of story's but some stand out more than others. What's that you say? Did the customer ever call you again? Yes! she was a good customer for many years. Didn't charge her for the sensor or my time. whatchagonnado.

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Rhubarb Tau

Posted (edited)

15 hours ago, mastertech011 said:

If you get a true measurement in ohms it should have told you there was a problem in that ignitor.

I don't think so, at least not always. What he describes is the cold resistance on the ignitor being within the spec (and it's a very wide spec), but physically changing when under load and heat expansion opened up the hairline crack. Fundamentally components behave differently when being powered by the appliance circuit, versus being powered by the 9V (or 3 volts) battery supply in a meter's resistance test. That difference may not be significant nine times out of ten, but sometimes it's enough to throw you off the trail.

Although you may be right, measuring while doing the tappy-tappy dance around the circuit might have shown the crack

Edited by Rhubarb Tau
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