Sam Brown

[Sam Brown]

Let’s take a step back and talk about how voltage measurements work for a moment. Whenever you do a voltage measurement, what you’re doing is measuring the voltage difference between two points. That’s why you need two probes to make a measurement. You’re always comparing one point in the circuit to another.

A standard (non-loading) VAC measurement compares those two points without putting a load on the circuit. In other words, the meter’s resistance is so high that no current flows through it while it’s doing a measurement. What this means is that you can get fooled by ghost voltage, which is what the video that Scott posted was showing. The meter showed about 108 volts, but it was just measuring a floating wire with no valid power supply.

The point of a loading meter is that while it’s measuring the voltage difference, it’s also acting as a load in the circuit itself. That’s always desirable when you’re measuring AC power supplies, since you want to make sure that the voltage you’re measuring is actually capable of driving current.

So yes, for your example of measuring the AC voltage voltage supply to a van, you would certainly want to use a loading meter, since you want to make sure that you have both a good line and a good neutral going to the load. A loading meter tells you that with 100% certainty.

Does that clear things up?

[Sam Brown]

Did you read the section just a couple paragraphs above the section you mentioned? It’s the one with the title “Loading vs Non-Loading Meters”? That gives a pretty thorough explanation of what loading meters do and why they’re useful.

As for videos only showing a “regular multimeter”, many multimeters have the LoZ function — it’s pretty standard. And we always use it for AC voltage measurements. So odds are that the LoZ setting was being used on those meters to make those measurements.

Let me know if you still have questions after re-reading that section I directed you to.

[Sam Brown]

Remember that there can be no voltage drop without current. Since the switch is currently open, there’s no current flowing through the circuit, and therefore no voltage is being dropped anywhere. That’s why you see a 120 VAC difference across the switch — it has neutral on one side, and 120 volts standing on the other side.

Make sense?

[Sam Brown]

Pay special attention to the phrasing of the question. It specifically states that this is in the timed dry cycle — that’s important! Which of those answers do you think would be affected by whether the dryer is running in auto dry vs. timed dry?

Might be helpful for you to rewatch the third video, the one that talks about auto dry, and imagine how it would work differently in timed dry. The timer chart will be a huge help in this.

[Sam Brown]

Yes, you should be able to test it just like any other diode.

[Sam Brown]

Yep, you’re basically right — the LoZ meter is acting as a high resistance jumper. Or, more accurately, it’s acting as a load in series with the fan motor. But yes, it is completing the circuit and allowing current to flow through that fan.

[Sam Brown]

For question #12: A failed defrost system, especially once it’s gone on for long enough, would cause both compartments to warm. Remember the main topic of this unit! In a single evaporator unit, if you have normal cooling in the freezer, what is the only thing that could cause that cold air not to reach the fresh food compartment?

For question #18: Think about how the air distribution works in single evaporator refrigerator. If the customer has the freezer set to maximum cool, how will that affect the fresh food compartment?

[Sam Brown]

Those rule-of-thumb amp draw specs we give are referring to the total amp draw of the microwave, so no need to measure after the primary winding.

[Sam Brown]

When measuring capacitance, you put one lead on each side of the capacitor — just like you would if you were using any other test on any other component.

[Sam Brown]

Remember that we always make voltage measurements with a point of reference. That’s because our measurement is just telling us what the voltage difference is between those two points. Neutral has a potential of 0 volts with respect to line, and line has a potential of 120 volts with respect to neutral. Either can be used as a reference, but you have to know how to interpret the results.

So in our case, J2-8 is at Neutral electrical potential, since all of its voltage is getting dropped across the fan. That means when you measure that point with respect to Neutral, you’re going to read a difference of 0 volts — because those two points are at the same electrical potential.

And if you switch your reference from Neutral to Line, now you have one probe still on J2-8, which is at 0 volts, and the other at Line, which is 120 volts. The difference between those two points is, of course, 120 volts, so that’s what we read on our meter.

Make sense?

[Sam Brown]

Sorry for the delay in getting back to you — to answer your question:

You’re trying to plot a point based on enthalpy, but temperature does not equal enthalpy. They are two separate factors. You can tell this because, while the enthalpy increases within the saturation dome, the temperature does not. They are also measured in different units: degrees Fahrenheit for temperature, and BTU/lb for enthalpy. So 41 F does not equal 40 BTU/lb.

Point #7 was plotted based on the measured temperature and pressure at that point in system. And that just so happens to place us at 0.3 refrigerant quality.

Make sense?

[Sam Brown]

I was wondering how you arrived to the answer on “block 7” of 0.3 refrigerant quality inside the saturation dome in the video at the 24:00 minute mark?

Could you clarify which part of the video you’re referring to here? I went to the 24 minute mark in the Refrigeration Sealed System Thermodynamics part 2 video, but I didn’t hear any talk about refrigerant quality.

[Sam Brown]

I was under the impression that EEP’s were supposed to be in continuity with each other.

P25-2 and p30-1 do have continuity — they are directly connected by a conductor. But keep in mind that EEPs aren’t just about continuity. EEPs are primarily concerned with voltage. So the main reason that we call those two points EEPs is because they have the same electrical potential — that is, 0 volts. For two points to be EEPs, they need to both have continuity and have the same voltage.

[Sam Brown]

I don’t know the exact example you’re referring to — could you tell me which unit you’re referring to? And where in that unit?

But in general, you can calculate the resistance of a circuit component using Ohm’s law. For example, if you have a 120 VAC circuit with a single load with 10 ohms of resistance, you would expect to measure 12 amps of current through that circuit. That’s just I = E/R — 120/10 = 12.

But let’s say that instead of measuring 12 amps, you measured only 8. From this, you can figure that the total circuit resistance has increased from 10 to 15 ohms. That’s just R = E/I — 120/8 = 15. That’s the effect of a loose connection with 5 ohms of resistance.

Make sense?

[Sam Brown]

The centrifugal switch on this model does have two poles — one that opens the start winding circuit, and one that switches between the main and slow windings. But note that that the state of that second pole doesn’t matter if the timer doesn’t close contacts 16 – 31 (you can find these just below the start winding, labeled with an 8). If those contacts are still open, then the start winding will never receive power, regardless of the state of the centrifugal switch.

This means that it’s the timer, not the centrifugal switch, that controls which winding receives power, since it all has to do with which timer contacts are closed. And that makes intuitive sense — the customer needs to be able to select whether they want to run the machine on regular or slow, and that selection is made at the timer.

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