Scott Brown

[Scott Brown]

Come to the webinar tomorrow night where we’ll be going over exactly these concepts. See the last issue of the Newsletter or look in the Webinars forum here for details.

[Scott Brown]

60v each

Very good, my fine young apprentice– you have snatched the pebble from my hand!

See? You didn’t even need to know the resistances of the loads yet you could determine the exact voltage drops across each load using a simple application of Kirchhoff’s Law.

Did this help answer your questions about voltage drops in the gas dryer valve circuits?

[Scott Brown]

You asked several questions in your OP about the parallel circuits question, one of which had to do with one of the element circuit. But your other questions had to with with the fan motor circuit. My reply was mostly directed to these latter questions.

To your question about the element and diode series circuit…

If current is flowing and then a diode fails, would that not break the circuit? If the circuit is broken then current will not flow therefore, would that not be a decrease in current?

In any series circuit, any open in that circuit will stop current flow in that circuit. That should answer your questions about that but let me know if you’re still confused.

On the gas dryer valve circuit…

The dryer circuit is showing the ignitor glowing, therefore its doing work and that means there is voltage drop on the ignitor.

This is correct. But the ignitor is in parallel with another load. Parallel circuits are really just independent series circuits connected to the same source. Given that, what do you know about the voltage supply in each of those circuits?

…if the coils are doing work, how can voltage drop be calculated without any resistance values shown?

In any circuit, the sum of the voltage drops will always add up to the source (or supply) voltage. No more, no less. So, in a series circuit with only one load, the voltage drop will always be equal to the supply voltage, regardless of the resistance of that load.

Now, if you had two loads in series with each other, you would need to know something about those load resistances to determine the voltage drop across each load. (This is called Kirchhoff’s Law.) Now, here’s a way to test your amazing new knowledge…

Pop Quiz: What is the voltage drop across each load? HINT: There is an exact, numerical answer based on the information given in the drawing.

PS – In the future, posting one question per post would help keep things much clearer.

[Scott Brown]

Why wouldn’t the convection element current (Decrease) if the diode was failed open? Why wouldn’t the fan motor voltage drop (increase) if the fan is the only load doing any work? Why wouldn’t the fan motor current (decrease) if it was the only thing doing work? and finally, why wouldn’t the total draw in the circuit have (No change)?

These questions all have one concept in common: What causes current to flow in the first place?

Let’s start with a refresher of what, exactly, current flow is. A common “tech myth” out there is that current has a mind of its own and moves in strange and mysterious ways. The reality is quite different and current flow is not mysterious at all.

When we talk about electric current, we are talking about the flow of electrons– tiny negatively charged particles attached to atoms. They are quite dumb and are content to remain attached to the atom they are with until they experience voltage pressure caused by a difference in voltage between two points. Then the law of electrostatic repulsion and attraction kick in. Electrons will always move away from more negative voltages to less negative (relatively positive) voltages. When the electrons “feel” a difference in voltage and there is a complete circuit, they will always move from the more negative voltage to the less negative voltage. So if the voltage does not change, then how can the flow of electrons change?

So in your analysis of the circuit in the question, you always begin with voltage and ask yourself, “Has the supply voltage changed in that circuit?” If not, then the current flow cannot change, either.

Think of parallel circuits as independent series circuits connected to the same source voltage. If something changes (say goes open) in one of those series circuits, does this affect the source voltage? If the source voltage is not affected, then what effect is there on the other circuits?

Review Units 1 and 2 in Basic Electricity

In the diagram of the gas dryer circuit, is the ignitor the only thing doing any work?

There are generally two types of circuit components that we deal with in appliance repair: loads and switches. There are other components shown in the circuit besides the ignitor. If something isn’t a switch, then it must be a load. Do you see components in the circuit that are not switches? What does this tell you?

[Scott Brown]

Hi Ryan,

Generally, everything (besides the wire) on a schematic is either a load or a switch. So, on that drawing, you have 4 loads and one switch.

[Scott Brown]

Hi Brett,

Happy Thanksgiving!

Both Maytag and Whirlpool used to make a kit of common replacement fasteners for their machines. I actually only found them to be marginally helpful. Over time, I eventually accumulated my own replacement fastener kit that has saved by hiney on numerous occasions.

You could start off with a kit like this: http://amzn.to/1R8hkxU

and then add to it and customize it over time.

[Scott Brown]

In an energized circuit with a single load, how much of the source voltage will be dropped across the load? (note that the resistance of the load doesn’t matter in this case because there is only one load)

Now suppose a second load of the exact same resistance is added in series with the original load. How much of the source voltage will be dropped across each load?

If you don’t understand how to think about this, then come to Office Hours tomorrow night where I’ll be review basic circuit concepts live with students.

[Scott Brown]

Thanks for the feedback, Josh. Based on yours and Tim’s comments, we are re-making that video to make some points, like the one you raised, more clear. The current video will be replaced by an all-original in-house Samurai Production. Stay tuned!

[Scott Brown]

There are two choices: electrostatic repulsion and electrostatic attraction. Electrons are negatively charged. Given this, which do you think it is?

[Scott Brown]

It depends on the circuit and what you’re using as the reference point for the measurement. If you’re using the regular voltage measurement function on your DMM, you may well measure 120 VAC even without neutral present. This is due to the prevalence of ghost voltages in circuits with open neutrals. Read more about that here: https://mastersamuraitech.com/the-low-down-on-loading-meters-and-dmms/

[Scott Brown]

Are variable speed compressors considered BLDC? Or are they AC motors?

Most variable speed compressors use BLDC motors. Some of the LG refrigerators use linear motors, which are also variable speed drives. You can always tell them apart by looking at the schematic.

Recall from the the motors screencast on electronic commutation that all motors actually run on AC. I spent some time emphasizing that key point in that video. You need to re-watch that video and make sure you understand this very important concept about motors.

[Scott Brown]

Hi Josh,

Great question!

AC induction motors, also known as asynchronous motors, use a rotating magnetic field to produce torque. This happens naturally in motors running on a three-phase power supply because it alreeady have a rotating magnetic field. Single-phase motors, however, do not have a rotating magnetic field– it just pulsates at 60 times a second but doesn’t rotate around the stator so it push the rotor and make it turn.

So, to make motors run on single-phase AC, they need extra circuitry to create a rotating magnetic field. This extra circuity consists of things like a second motor winding, called a start winding, and sometimes a start capacitor.

Single-phase AC induction motors usually only have one stator winding to generate a rotating magnetic field, which produces torque on the rotor. It can generate a pulsating magnetic field, which is enough to sustain rotation, but not sufficient to start the motor from a standstill. That’s why like on e a refrigerator compressor when the start relay goes out, you hear the motor humm and trying to start but failing because the start winding is not in the circuit. What you’re hearing is the rotor twitching back and forth 60 times a second but not rotating and drawing lots of current. If you were to just nudge the rotor either CW or CCW, it would happily continue rotating in that direction.

But since that’s not practical, the engineers came with other ways of nudging the rotor fro a standstill. They do this by using a start winding and sometimes a start capacitor to help the rotor start.

A start capacitor is sometimes used when high starting torque is required for the motor application. It works by increasing the electrical phase angle between the main winding voltage and the starting winding voltage. With motor, it’s not about zapping or kick-starting, as many people mistakenly believe. It’s all about phase angle. The greater the phase angle, the greater the starting torque.

Run capacitors are different. They have nothing to do with starting. They’re whole purpose is to smooth out the back EMF ripples on the main winding and help the motor run smoother, quieter, and with a higher power factor. It remains in the circuit in series with the start winding and in parallel with the main winding while the motor is running. You’ll commonly see run capacitors on refrigerators using a single-phase motor.

[Scott Brown]

Abe, did you read the explanation that appears in the quiz for that question? I emphasis this concept throughout the Basic Electricity training in the Fundamentals course: voltage is not the complete story! Having voltage isn’t enough to make current flow. Current also needs a complete circuit. That’s where the neutral comes in.

The only states that Line voltage is present. It says nothing about there being a valid Neutral. That’s why you need to always test power supply voltages with a loading meter before making diagnostic conclusions that a part is bad. Since the correct answer was given as one of the choices, it should have prompted you to think along those lines.

You mentioned wording. Yes, wording is vitally important when dealing with technical matters! Words mean specific things. Voltage means one thing; power means something else. Part of what you are learning in this course is how to think and talk like a real technician.

[Scott Brown]

i remember having to test for 3.5v dc at that harness- coming from the board. so is pwm a signal of pulsing dc voltage???

YES! A Pulse Width Modulated signal is a square wave with varying pulse width depending on the information being encoded into that pulse width by the microprocessor.

To properly see a PWM signal, you would use an oscilloscope. Are you willing ton spend $3,000 for a decent portable O-scope? No? Neither am I nor most other techs I know. Especially when we don’t have to because, as clever Master Samurai Techs, we know how to use our regular ol’ DMMs as a proxy because we know how to interpret the reading and because we know what the DMM is actually doing when it reports a DC voltage.

When you measure a PWM with your DMM, your meter is actually reporting and AVERAGE DC voltage to you. The manufacturers, in their mercy, have told a reasonable range of DC voltage that we should expect. Some manufacturers, including GE, will actually tell you a FREQUENCY that you could measure, too.

Does this make more sense?

[Scott Brown]

On the PCB HEATER P-2 (of 32) it has a red wire connecting to heater terminal 1 with a notation of [WH] next to it. Not sure what I’m looking at there?

The “WH” is a box indicating the name of the component; in this case, “Water Heater.”

Look over at the motor and you’ll see “MT” in a box. What does that mean?

A red wire connects to the heater terminal marked 1 [WH]. Is that a specification that the color of the wire is now considered white?

See above.

A bracket connects P-1 (WHT) to P-2 (of 33) and is empty. Also, not sure what that is?

It’s showing what any line on a drawing shows: a path for electrons.

[Author]

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