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When you want to test a specific load, you have two options: take a resistance measurement across it, or measure its voltage drop. Measuring the voltage drop is almost always preferable because, while resistance measurements always need to be taken on a dead circuit, voltage drop measurements are taken on a live circuit.
This is important since there are times when loads or switches will fail under load — in other words, they appear fine when you take an ohms measurement, but once they are in a live circuit, they fail. This is something you can only catch by taking a voltage drop measurement, and this is why you should always take a voltage drop measurement instead of a resistance measurement whenever possible.
By “inputs”, we meant electrical inputs to the board, like those generated by sensors. That’s the standard meaning of that word in this context.
For the heater relay, you have to do a little deduction to figure out what it’s switching. First, you look at the picture of the heater PCB, and you can see that the relay has just two terminals, and that all the other connectors on the board have three or more. If you then look at the schematic, even though it doesn’t specifically call out the heater relay, you can tell which connector is the relay from the fact that it only has two terminals — and of course from the fact that the heater is connected to it. Then, to figure out what the relay is switching, you just have to follow the line that does not connect to the heater and see where it’s going.
As for the voltage going to the sub PCB, you don’t need the manual to tell you whether those are AC or DC to know which they are. You will never encounter AC voltages that are in the 5V or 12V range in appliance repair. In addition to that, you also know that control boards run on DC power, so it wouldn’t make sense for these supplies to be AC.
You were right the first time, actually. When the refrigerant arrives at the compressor, it’s a low-pressure, room temperature gas. As it gets compressed by the compressor, the increased pressure causes it to heat up, becoming a high-pressure, hot gas. As it passes through the condenser coils, it cools off, changing states and becomes a high-pressure, warm liquid.
January 12, 2017 at 1:18 pm in reply to: Module 3, Unit 4: DW drain pump circuit is melting my brain #11449I think the big thing that’s throwing you off here is a misunderstanding of how the float switch works. The float switch exists as a safety device in case of overfilling or leaks. It’s called a float switch because it literally floats on top of the water, and if the water level gets high enough to lift that switch, it makes contacts 11 and 14 which, as you correctly said, would run the drain pump for as long as those contacts continue to be made. This is so that it drains all of the excess water, preventing a flood. However, during normal operation, the float switch is in the position shown on the schematic — making contacts 11 and 12.
Another thing to point out is that A9 and A12 are the timer contacts that supply line to almost all of the loads in the machine, including the timer motor itself, which is why those contacts are closed for most of the cycle. All these loads have line present at them for as long as A9 to A12 is closed, but the timer only closes their paths to neutral when it’s time to run them.
And the reason that A1 to A3 close during the drain function is because they’re part of the drain pump’s path to neutral, not the timer’s. Knowing this, try to figure out exactly how the drain pump gets neutral. Let me know if you figure it out or get stuck!
Yes, you’re correct that a reed or centrifugal switch usually won’t have terminals marked as COM or NO. This question is specifically referring to a micro switch, and I’ve clarified its wording to reflect this. The main thrust of this question is just to see if you can determine what the voltage reading would be across the switch in its current state.
It varies depending on model, but most clogged defrost drains are a result of some kind of manufacturing defect that causes the drain to stick closed, such as the duck-bill grommet you’ll find in some machines. To identify this kind of problem, you need to familiarize yourself with the model you’re working on and see what kind of problems it’s prone to develop.
As for the path the water takes, in every case that I’ve seen, the water is simply flowing across the floor of the freezer and seeping out through the bottom of the door gasket. And some of it does freeze, forming the solid ice that you saw in the bottom of the freezer in the video (as opposed to the rime ice formed by water vapor freezing). But once it gets flowing well enough, it’t moving too quickly to freeze, and some escapes from the compartment.
If by positive and negative you mean DC voltage and DC ground, then yes, that would be correct for the four wire configuration you described.
I’ve never encountered a damper motor that was not a stepper, and this is because dampers are an application that stepper motors are uniquely suited for. Stepper motors move in set intervals, and they’re designed to only move so far in one direction or the other. This way, a control board can just tell a stepper motor to open or close the damper and not have to worry about sensing its movement or telling it to stop at a certain point.
Stepper motors have two separate windings, one for rotation in one direction, and one for rotation in the other. This would explain your four wire configuration–the board activates one winding when it wants the damper to open, and the other when it wants it to close.
As for their power supply, all stepper motors run off DC power. If it looks like a stepper motor has an AC power supply, that just means that there is a rectifier built onto the motor, which turns that AC power into DC power for the motor to use.
November 4, 2016 at 12:43 pm in reply to: Question about schematic used in quiz question 4 and 5, unit 5 part 2 #11319Since you asked, I can explain a little about how noise filters work. That’s not a transformer in there. That resistor and those capacitors and coils that you see all serve to filter out high frequencies generated by the appliance (“noise”) and prevent it from leaking back into the main power supply. There’s no stepping up or down of the power.
However, how the noise filter works is actually not important to answer that question. In fact, that question was created specifically to see if you can apply basic troubleshooting strategy to something that looks very complicated. All that you need to do is identify where your inputs and outputs are and how you would test for them. In fact, even though this diagram shows all the individual components of a noise filter, in reality, noise filters nowadays are just self-contained little chips. If your tests were to show that the noise filter is bad, you would just change it like any other part.
You answered your own question! Yes, that yellow wire carries the PWM signal from the board, and by hot-wiring that to the positive terminal of the battery, it’s simulating the board telling the fan motor to run as fast as it can.
We pulled the unit out because we needed to access the components in the heater circuit. There’s no way to test the voltage across each component individually without physically getting access to them.
To answer your first question, yes, we had the unit in diagnostic mode to test the voltage supply to the heater. That way we could reliably know that the board was in a state where it was supposed to be supplying voltage, and we could test whether it was doing its job.
As for your second question, I recommend you re-watch the video, because the Samurai spent the whole first half of it showing how he proved that the board was providing a good voltage supply. Once he had done that, he moved onto testing the voltage across each load in the heating circuit to see which component was open.
Remember that when you do a voltage test, you’re measuring the voltage difference between your two probes. Measuring 0V across the heater proves that it’s good because it shows that it is closed. In fact, there was actually 120V on either side of that heater, since no current was flowing through the circuit and thus no voltage was being dropped across the heater. The difference between 120 and 120 is 0, so that’s what the meter showed.
Towards the end of the video, he shows that he’s measuring 120V across the float switch, indicating that it is open (when it should be closed) and implicating it as the bad component in the heating circuit.
Technically, yes, the compressor and condenser fan are dropping some voltage. But to tell exactly how much voltage, you have to calculate the equivalent resistance of those parallel circuits. Scott goes through this in that unit’s video. I recommend that you review it from the 9 minute mark to the 12 minute mark.
As shown there, the resistance of those parallel circuits is so low in comparison to the resistance of the evaporator fan that they would be dropping a minute amount of voltage, so little that the compressor and condenser fan would not actually do any work.
October 17, 2016 at 10:56 am in reply to: What characteristic of a heating element does NOT make it an electrical load? #11009The wording of this question is a little tricky. As you know, when a heating element is in a circuit, it is a load. So clearly the question isn’t saying that a heating element isn’t a load.
A heating element has various characteristics, not all of which are necessary characteristics of a load. What it’s asking is this: which quality does a heating element have that is not an essential quality of a load?
What Scott said about looking up the technical literature is key. I found what I’m pretty sure is the correct model number for your machine, and a quick look through the documentation answers your question.
In the parts diagram, you can see exactly why you can’t put this model into any kind of diagnostic mode: it has no interface for doing so! The only controls that this model has are some mechanical knobs for controlling the temperature. While that ADC board may technically be a “control board”, it is not a microprocessor board, so there is no way to communicate with it.
This highlights exactly why you need to find whatever technical documentation you can for the appliance you’re working on and look through all of it carefully. Most of the time the answer to your questions is right there waiting for you.
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