Sam Brown

[Sam Brown]

In order to run the motor, an inverter board needs two inputs:

• 120 VAC
• A good PWM signal from the main control board

If the inverter is receiving both of these things and still not running the motor, then that implicates the inverter.

You’re correct that when you measure a PWM signal, your multimeter will read an average of that signal, since it’s an oscillating square wave. If you test for that PWM signal coming into the inverter board and do not detect it, that does not implicate the inverter board, but rather the main control board, since that’s what is supposed to generate the PWM signal.

[Sam Brown]

When you say that you tested the thermistors, do you mean that you just ran the board’s built-in thermistor test, or did you measure the resistance/voltage drop accross each thermistor and check it against a temperature chart to make sure it’s in spec?

If you only ran the board’s test, that’s not conclusive. The board can only check if a thermistor is completely open or completely closed. However, this is rarely how thermistors fail. Usually, a failed thermistor’s resistance goes out of specifications, but without opening or shorting, and you can only test this by doing it yourself with your meter.

But, if you did test the thermistors in the correct way, and they are indeed still good, then we’re left with a situation in which the evaporator fan is good, the thermistors are good, and yet the board is not running the fan. In this case, the board’s logic is for some reason saying that it should not run the fan.

To figure out why this is the case, you have to find the service manual for the specific unit you’re working on and look up how this board’s algorithm functions. Without knowing those details, your diagnosis can’t get any more specific.

[Sam Brown]

Nice find! Hope it works out well for you.

[Sam Brown]

Those specs look like they should be more than enough for measuring data communications. However, I would recommend that you take a look at the oscilloscope that the Samurai uses in this video:

https://youtu.be/gSWn6D-PP8o

Rather than having to lug around a whole separate scope, you can just keep this one tucked away and plug it into your iPad whenever you need it. Much more convenient. If you’re interested in checking it out, you can find it by searching for the Oscium IMSO-204L.

[Sam Brown]

No problem! Glad you figured it out. 🙂

[Sam Brown]

Pay careful attention to the wording of the question. Yes, you will often take measurements of DC voltages, mostly when it comes to troubleshooting control boards. However, that’s not what the question is asking. Read over the question again carefully and then tell me if you get it or if you’re still confused.

[Sam Brown]

If you ignore everything else going on in the schematic and just focus on the ignitors, the answer is actually pretty simple. Just take a look at which side of the load the terminals are on, and that’ll give you your answer.

[Sam Brown]

I’m glad that you were able to fix the problem with your own dryer! The reason the Samurai didn’t check the other devices in the control circuit is because the point of that video was just to demonstrate a concept, not to demonstrate an exhaustive troubleshooting procedure.

[Sam Brown]

I don’t think that I can clarify this any further than I already have, so I recommend that you review Unit 4 of the Troubleshooting module in the Fundamentals course. You should pay special attention to the second video in that unit, since it gives a very concrete example of how EEPs are used with schematics for troubleshooting.

After you’ve reviewed that unit, if you have a more specific question, please feel free to leave another reply here.

[Sam Brown]

Electrically equivalent points are found in any kind of electrical circuit that exists. That term simply refers to any two points in a circuit that have the same electrical potential. To quote something I said in my first response to this topic:

“It doesn’t matter if your two measurement points are both at 0V, 12V, 120V, 240V, 3.5 bazillion V, or any other voltage. As long as the voltage at those two measurement points is the same, you will always read 0 volts.”

As for identifying EEPs, just look at a schematic and follow one of the lines. Any two points along that line are electrically equivalent, as long as the points don’t have any obstructions between them, such as an open switch or a load. If you were to take a voltage measurement of any of those two points, you would read 0, since, being electrically equivalent, there is 0 difference in potential between them.

[Sam Brown]

Before I answer your question, I need to clarify the difference between a short and a shunt.

A short is a malfunction where line voltage somehow finds a 0 (or very low) resistance path to ground or neutral. For example, this could happen if a bare wire lead makes contact with the chassis of the appliance. In this case, the appliance would probably not function at all, and in all likelihood, it would trip the circuit breaker whenever it was turned on.

A shunt, on the other hand, refers to an intentionally engineered piece of a circuit that allows current to bypass a load by providing an alternate, 0 resistance path. Note, however, that a shunt would never be designed to bypass every load in a circuit, since this would in fact create a short.

A shunt is not the same as a short. A short is a malfunction, and a shunt is an intentional piece of design.

So, to answer your question: a short can have various effects on an appliance depending on where the short occurred. Sometimes, it will just trip the circuit breaker, and once the short is corrected, the appliance will function as it should. However, other times a short can damage sensitive electronics, such as the control board, so there are some cases where after you have corrected the short, there is still a follow-on problem to address.

In other words, there’s no general rule as to whether or not a short will ruin an appliance, though I would say that more often than not, a short will not cause a terminal event.

[Sam Brown]

A very important thing for you to remember is this: Whenever you take a voltage reading, you are actually measuring the difference in voltage between those points.

This is why, for example, if you want to check whether a load is getting 120 volts, you take your measurement with one probe on the hot side of the load and the other probe on the neutral side. The neutral side of a circuit has an electrical potential of 0 volts, so the difference between those two points will equal whatever voltage is present at the hot side.

Now, knowing that voltage readings tell us the difference in voltage between two points, we can also see why you always read 0 volts when you are measuring two Electrically Equivalent Points. It doesn’t matter if your two measurement points are both at 0V, 12V, 120V, 240V, 3.5 bazillion V, or any other voltage. As long as the voltage at those two measurement points is the same, you will always read 0 volts, because that’s the difference between them.

[Sam Brown]

Answering this question requires doing a load analysis, a very important electrical troubleshooting technique. Put very simply, load analysis involves identifying the suspect load, and then using the schematic to see how it gets line and neutral.

We teach everything about load analysis in Fundamentals, so you may be interested in that course. We’ll contact you privately with more information.

[Sam Brown]

When a compressor motor fails due to heat stress, it will sometimes manifest as the compressor starting for a few seconds and then shutting off, but you can’t tell for sure just from that.

The much more informative thing to do is measure if compressor is drawing too much current. This will give you a better idea of whether or not there is something wrong with the compressor.

If you want to learn more about the different failures compressors can have and how these manifest, I recommend that you take our Refrigerators course–it’s packed with all kinds of information about that, plus a whole lot more.

[Sam Brown]

Remember what PWM stands for: Pulse Width Modulated. The signal sent to the inverter board is a series of DC square wave pulses. During a pulse, the board is sending out 5 VDC, and between pulses, the board is sending out 0 VDC. If you measure this signal with your meter, it will take the average of these two voltages, meaning that you will read 2-3 VDC.

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