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Son of Samurai's Blog

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Why Amps are the Definitive Measurement in AC Circuits

Son of Samurai

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Volts, ohms, and amps -- these are the three types of electrical measurements from which we draw our diagnostic conclusions as appliance techs. They all have their uses, but watch out -- they're not all equal in usefulness or reliability! Let's go through them one at a time.

Ohms: Despite being a lot of techs' go-to measurement, ohms is actually the least reliable of the three. This is due in large part to the fact that it can only be performed on a dead circuit. This means that it completely misses failures that occur when power is applied to the circuit, such as failure under load. Ohms tests are by no means useless, but you shouldn't use them as definitive tests in AC circuits unless given no other choice.  The rule of thumb is this: if it tests bad on ohms, it's bad. If it tests good on ohms, it may still be bad -- you don't know until you do more tests.

Volts: Volts are a much more reliable test than ohms, but only if you're using a loading meter or the LoZ function on a multimeter. Otherwise, you're going to get faked out by ghost voltage. Still, even a loading voltage test doesn't tell you everything. You can see if voltage is being supplied to a circuit, or how much voltage a load is dropping, but you can't catch any opens or out of spec loads in the circuit with a single voltage measurement. You have to follow up with more voltage or resistance measurements. If only there were a way to test the entirety of the circuit...

Amps: Oh wait, there is! With a single amps measurement, you can perform a thorough checkup on an entire circuit. Does the circuit measure within spec? You have just definitively confirmed that the circuit is in perfect working order electrically. Are the amps lower than spec? Now you know that the circuit has higher resistance than the spec, either due to a high-resistance load or a loose connection. Are you getting no amps? Now you know that you need to confirm a good voltage supply and check for opens in the circuit. Don't get me wrong -- I'm not saying amps is the only test you're ever going to use. I'm just saying that it's the test that gives you the most information.

So if amps is such a good test, why don't more techs use it? Probably because current specs are almost never given by manufacturers, meaning that you have to calculate the current spec from the other specs that you are given. This isn't hard to do; all it requires is basic knowledge of Ohm's law and the ability to do 7th grade math. But sadly, it's an extra step that a lot of techs aren't willing to take, and their troubleshooting suffers because of it.

Don't let yourself be allergic to math -- it's the best tool you have to really understand electricity. Amps measurements are one of the most powerful troubleshooting tools at your disposal. You can learn how to use them, Ohm's law, and much, much more in our Core Appliance Repair Training course over at MasterSamuraiTech.com.

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MVrepairs

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Great blog entry. Thank you for the great information and encouragement. 

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Rhubarb Tau

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So if amps is such a good test, why don't more techs use it? Probably because current specs are almost never given by manufacturers, meaning that you have to calculate the current spec from the other specs that you are given

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This can lead to some confusion too, when the measured current doesn't match the calculated spec exactly, or when a tech infers the expected current based on an ohm reading. Heating elements and motors come to mind. 

 

 

 

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Son of Samurai

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On 1/9/2020 at 9:30 AM, Rhubarb Tau said:

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So if amps is such a good test, why don't more techs use it? Probably because current specs are almost never given by manufacturers, meaning that you have to calculate the current spec from the other specs that you are given

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This can lead to some confusion too, when the measured current doesn't match the calculated spec exactly, or when a tech infers the expected current based on an ohm reading. Heating elements and motors come to mind.

Like all troubleshooting techniques, you can't just blindly use amps without being aware of other factors and expect good results. For example, if a tech doesn't know that motors are non-ohmic devices due to inductive reactance (which is inherent in the way electric motors work) and tries to calculate the circuit current based on the resistance of the windings, then the tech doesn't even understand the technology he's working with. And of course he's going to get erroneous results. In the case of motors, you would need to use motor's wattage spec to calculate the expected amp draw. 

Heating elements, on the other hand, are ohmic, so you can safely use the Ohm's law to derive expected amps from a given resistance. We actually showed doing exactly this in one of our most recent webinars -- check out the fourth case study in this recording.

When I say that amps are a reliable test, it's implicit that they're only reliable in the hands of a competent tech. For a tech who knows his stuff, an amp measurement is a gold mine of information. For a tech who doesn't, it's just a meaningless number.

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Quick test- 0 amps probably bad. 100 amps, really bad!

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