Sam Brown

[Sam Brown]

You are correct that some inputs to the board require an output from the board in order to function. It sounds like you’re thinking of thermistors in particular.

A board outputs a small DC voltage to a thermistor, and then it measures the voltage drop across that thermistor. So in this case, the voltage supply is an output from the board, and the voltage drop is an input. In practical terms, we usually treat thermistors purely as inputs, since that’s how they operate functionally. We’re interested in that voltage drop, not the initial voltage supply from the board.

As for the second question, you identify the input/output that you want to measure by looking at the schematic and the board pinout (if present). For example, if we’re interested in checking the freezer thermistor in this Samsung refrigerator, we would first identify it on the schematic:

And if we need help to locate the connector where we would make that measurement, we can use the board pinout:

Pins 10 and 12 on connector 20 are our test points, simple as that.

Make sense?

[Sam Brown]

Without getting too much into the weeds, it all has to do with those computer-controlled switches that the inverter board uses and how the algorithm decides to operate them.

First, remember that the inverter board rectifies the 120 VAC input it receives into DC. Then it takes that DC voltage and, using those software-controlled switches, it inverts the DC into three-phase AC. The voltage of this newly-created three-phase AC voltage depends on exactly how long the board keeps each of those switches closed at a time. The longer each of those switches stays closed over a particular stretch of run time, the higher the voltage will be.

That’s a very broad-brush explanation, but it’s about as particular as we need to get as appliance techs.

[Sam Brown]

As I understand it, if the condenser is at room temperature the compressor is bad because there’s no compression.

When you measure a very low condenser split like that, it always indicates a sealed system problem of some kind. Yes, it could be a failed compressor, but it could also be a number of other sealed system failures, such as a refrigerant leak.

As for why you got that small of a split on a machine that was still marginally cool, that would seem to indicate that you caught a problem that was just beginning to manifest.

[Sam Brown]

It depends on the specs of the pressure switch in question. We always make measurements with comparison to specifications. Could you provide the model number you’re talking about so we can take a look at the particulars?

[Sam Brown]

When it comes to loading down, you don’t confirm it by taking a voltage measurement. Instead, you disconnect DC loads from the control one at a time until the control begins operating normally again. Whichever one had to be disconnected to restore proper operation is the bad actor.

Make sense?

[Sam Brown]

Ok wait I think I just answered my own question.

Pretty much! Technically, the defrost heater is “running”, but as you said, the watts it produces would be so minuscule as to be negligible.

When you do an amps measurement on that circuit, you’re going to read the total circuit current. In other words, the amps you read at the defrost heater will be the same that you find at the evaporator fan. The big difference is in their respective voltage drops: the defrost heater will drop almost no voltage, while the evaporator fan will drop almost the full 120 VAC supply voltage.

Make sense?

[Sam Brown]

All 4 possible positions of the 3-way valve are the same, regardless of the evaporator configuration. How exactly these positions are used and when depends on how the control board is programmed. A TDM system still has the same 4 positions that a parallel one does.

Let me know if anything still doesn’t make sense.

[Sam Brown]

Yep, you’ve got it exactly right! Choose three adjacent keys and repeat the 1-2-3 sequence until you enter service mode.

[Sam Brown]

Saturation occurs in the Evaporator @ 41 F. 210 – 41 = Superheated 169 F.

This is where you’re going astray. The compressor divides the high side and the low side of the sealed system. Once you’re measuring at the compressor discharge, you’re dealing with radically different pressures compared to what you were dealing with in the evaporator: 312 psig vs. 84 psig.

The saturation temperature changes with pressure. If you enter 312 psig into the Danfoss app, you’ll see that it corresponds to a saturation temperature of 120 F. That’s the temperature you need to calculate your superheat from.

If that makes sense to you, let me know what your estimate of the superheat would be now.

[Sam Brown]

Hi Richard,

The purpose of the Saturated, Superheated, Subcooled spaces are for you to identify which of those three states the refrigerant is in and give the degrees of superheat/subcooling, if applicable. Is the refrigerant at saturation at that point in the system? Just put a check mark into the saturation spot and leave the others blank. Is it superheated? Put in how many degrees of superheat you have (e.g., 10 F if it’s 10 degrees above saturation). Same idea for subcooling — just put in how many degrees below saturation the refrigerant is at that point.

Make sense?

[Sam Brown]

Which direction a start winding causes the motor to spin in is determined by the physical configuration of the start winding relative to the run winding. So for example #3, each of those start windings is configured such that, when current is moving through it, it causes the rotor to spin in a specific direction.

In other words, you can’t tell just by looking at the lines on the schematic which start winding is going to make the motor spin in which direction. You would need additional information to figure that out. But fortunately, that’s not really necessary. We just need to know that those two windings make the motor run in opposite directions.

[Sam Brown]

How much information about the control board’s algorithm you’re given and in what format it’s presented depends a lot on the manufacturer and model. One way you might find it in a service manual is when you’re reading information about how the control responds to a failing load. Something like this:

When the control board senses no motor speed feedback signal within 10 seconds of supplying voltage to the motor, it will stop supplying voltage to the motor for 5 seconds, then retry for another 10 seconds. This will repeat two more times, and if no speed feedback signal is sensed at the end of the last attempt, the control board will stop supplying voltage to the motor and display an F5 error code.

I made that one up, but it’s a variation on the kind of theme you’ll see.

The reason it’s important to be aware of things like this is because, in the case above, you might measure that the control board isn’t supplying voltage to the motor and immediately assume an issue with the control. But that’s in fact just a symptom of the control board doing what it’s programmed to do.

While it’s very important to know information like this, it’s often not presented in the clearest way. It can be scattered about the manual/tech sheet in descriptions of load functions, error code descriptions, board documentation, etc. So it’s always important to do a thorough analysis of everything relating to your LOI, including algorithmic information.

[Sam Brown]

Dunking a thermistor in ice water is a way to test it at a specific temperature — 32 degrees. Usually, this is only necessary if you do not have a thermistor chart and the manufacturer is instead only telling you the resistance at 32 F. In this case, it’s usually easiest to completely disconnect the thermistor and check its resistance, since dunking it in ice water while it’s still connected to the control is often impractical.

[Sam Brown]

You’re correct that we should never use chassis ground as a reference for any AC measurement. Remember too what was talked about in the video for this unit: how the DC power supply has its own ground that it produces, separate from the AC world and separate from the chassis.

Sounds to me like a DC measurement with respect to chassis ground just isn’t a valid measurement. Is there an answer in the quiz that reflects this?

[Sam Brown]

Are you referring to the fact that we’re not always drawing the red and blue lines up to the drive motor?

If so, that’s only because it’s not the load we’re currently looking at. For example, in case study #3, we’re looking at the heating circuit and timer, and so we only draw out the power supply for those circuits. But if you look at the drive motor’s circuit and compare it with the timer chart, you’ll see that it is receiving power whenever a cycle is running.

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