No, but they don’t tell the whole story either
By Sam Brown, MST/Appliantology.org
“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 an example in an old-school gas dryer to demonstrate. Here’s the schematic:
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’ve already gone through and taken an ohms measurement of all the relevant components like the ignitor and the coils. We’ve even measured 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?
Since we’ve done all our tests on a dead machine, as you can’t do resistance measurements on a live circuit, why don’t we try a live test next?
Below we’ve got what the ignitor’s circuit looks like with the dryer running. Line and neutral make it 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.
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.
As soon as there’s a voltage difference across that ignitor, electrons get blasted through it. This causes it to heat up, which is exactly what’s supposed to happen.
But in our particular case…
The heat causes the ignitor material to shift just enough for a tiny, invisible crack expand, causing the ignitor to go electrically open. 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 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.” Our ohms testing told us nothing. All we needed to do was take a single voltage measurement on a live circuit to tell us what was wrong.
Why? Because of the nature of loads. Loads are the things that do physical work (turn a motor shaft, light a light bulb, make an element give off heat, and so forth). This physical work, called power, is expressed in units of watts. Power is given by the Ohm’s 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.
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 the failure under load that happened in our example.
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.
None of this is rocket science, and anyone can learn it. We teach all this and more in the Core Appliance Repair Training course at Master Samurai Tech (MST). What we 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 are therefore condemned to working harder, not smarter.
Don’t be that guy! Use your 15 percent BrandSource discount coupon and up your repair skills today.
Sam Brown is “Professor of Appliance Repair Mastery” at BrandSource partner MasterSamuraiTech.com (MST), an online training academy for appliance repair personnel, and administrator of its sister tech support site, Appliantology.org. For more information, email Sam at firstname.lastname@example.org or call (603) 290-5516.