Susan Brown

[Susan Brown]

Hi Heath,

Unit 5 is describing various characteristics of series circuits and parallel circuits.

When two or more loads are in series, you can add the resistances of the loads together to get a “total” resistance. This can be useful when calculating the circuit current, since the “total resistance” is what determines that.

However, when loads are in parallel with each other, we talk about “equivalent resistance”. It’s a similar concept – it’s a way of describing the overall resistance that is present from all of the loads combined. It’s more complicated than simply adding them together, however.

We show the formula for calculating equivalent resistance in Unit 5. But we also give the rule of thumb in the 3rd video in that unit (you can even see it in the thumbnail for the video!). That’s perhaps more important to know than doing the actual calculation.

Take a look at that information again in Unit 5, and see if you understand it. Let me know!

[Susan Brown]

Hi Myles,

It could be, but we aren’t asking for that level of diagnosis in this question. You don’t have enough info to be that specific.

We tell you in the problem statement: *NOTE* These are simplified diagrams. There may be other components in the circuit that aren’t shown.

This means your answer will be a more general conclusion about the nature of the failure, not exactly which component in the entire circuit is bad.

What we want is for you to tell us what you can determine about the failure first from the measurements in Figure 1 (with the additional knowledge that the element has continuity), and then how the measurements in Part 2 give you even more insight into where the problem lies.

Back to your question – what is the basic thing that zero Vac across a good load tells you?

[Susan Brown]

If you got the answers correct, they will add up to 120 vac (or perhaps 119, depending on how you rounded the number for current). Because the sum of the voltage drops of loads in series will always add up to the source voltage.

You should have a copy of the Ohm’s Law chart handy whenever you are taking a quiz or exam! (Or, just open the site up in another window so you can have that unit available to look at when you need it.)

[Susan Brown]

Hi Ladarius,

First of all – just want to make sure you had gotten the feedback that I had emailed to you about the Midterm.

I’m going to copy a bit of our text from Unit 8. Can you follow how we describe to calculate voltage drop? If there’s something that is still unclear, please let me know. (This will help me see if I need to improve how we teach this.)

Voltage Drop

Here’s the key concept with voltage drop: A voltage drop across a load is produced when current flows through that load.

When we talk about voltage drop, we’re always talking about a specific load that has current flowing thorough it. That’s why it makes no sense to talk about the voltage drop at a wall outlet that we’re checking with our meter, for example, because there’s no current flow and no load (the meter doesn’t count as a load– a good meter should never load the circuit enough to make a difference).

In the course of doing its work, a load will have a voltage drop across it that is proportional to the resistance of that load. Remember E = I * R. You have to keep straight here what this is referring to. For example, if you’re looking at the voltage drop across a load, you would look at

E(voltage drop across the load) = I(current flow through the load) * R(resistance of the load).

If you’re analyzing an entire circuit, you would look at:

E(source or line voltage) = I (total current flow through the circuit) * R(total resistance of ALL the loads in the circuit)

[Susan Brown]

You got it!

[Susan Brown]

That’s correct – so you can already see why the answer you chose is not correct.

Now think about the terms “direct” and “alternating” current. Why are those terms used? (Describe it in terms of the flow of electrons)

[Susan Brown]

Hi Ladarius,

I’m glad you are asking a question!

First of all, what type of current comes from a battery?

And what type is provided at a wall outlet?

[Susan Brown]

Well, the explanation we give in Unit 5 is already pretty brief! As an appliance repair tech, you don’t have to have a deep understanding of how transformers do what they do, and you’ll be revisiting some of these concepts as you go along in the course. But I appreciate that you are trying to understand the material the best you can right now. Let’s see if I can help with that…

First: “what” they do. Here’s the main point (from the lesson): “Transformers get their name from the fact that they “transform” one voltage or current level into another. They are capable of either increasing or decreasing the voltage and current levels of their supply, without modifying its frequency, or the amount of electrical power being transferred from one winding to another via a magnetic circuit.”

Do you have any questions about that?

As for “how”, it’s good to understand this, at least in a general sense, because this is also basically how motors work (with windings and magnetic fields).

Here’s how we explain it in the lesson:

A single phase voltage transformer basically consists of two electrical coils of wire, one called the “Primary Winding” and another called the “Secondary Winding”. Normally, the “primary” side of the transformer is the side that takes power, and the “secondary” is the side that delivers power. In a single-phase voltage transformer, the primary is usually the side with the higher voltage.

These two coils are not in electrical contact with each other but are instead both wrapped around a common closed magnetic iron circuit called the “core”. This soft iron core is not solid but made up of individual laminations connected together to help reduce the core’s losses.

The two coil windings are magnetically linked through the common core allowing electrical power to be transferred from one coil to the other. When an electric current is passed through the primary winding, a magnetic field is developed which induces a voltage into the secondary winding as shown.

(see the figure in the unit)

Do you follow that explanation? You’ll be revisiting these concepts in various ways as you go along, so don’t feel you have to completely understand it yet.

[Susan Brown]

You’re welcome! Glad to help.

[Susan Brown]

Correct!

[Susan Brown]

Yep! So your meter reading would reflect that.

[Susan Brown]

Exactly!

[Susan Brown]

Right! So if you have a circuit with multiple loads, you can do that with one of them. If your hand has to travel through another load in order to reach either L1 or N, then that load is in series with the one that you have become. That’s one way to check if loads are in series. Loads that you do not have to go through, but that are still connected to the same power supply, would be in parallel to you.

Does that help?

[Susan Brown]

Correct – so would any current be flowing?

[Susan Brown]

Sure – but I was hoping you would give me the answer – the name of the situation I am talking about – so I can make sure you got it.

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