Dryer Schematic Quiz 2 – Click for Larger View (opens in a new window)

1. To calculate the current flow in this simple series circuit, you first need to calculate the total circuit resistance.

The heater resistance is given as a range so, for simplicity, let’s assume something in the middle, say 8 ohms.

The resistor ohms are also given as a range so let’s again use something in the middle, say 4500 ohms.

Similarly, a mid-range resistance on the timer motor would be 2200 ohms.

Adding these up, the total circuit resistance is about 6708 ohms. Now we can calculate total circuit current using Ohm’s Law:

I = E ÷ R = (total power supply voltage) ÷ (total circuit resistance) = 240 vac ÷ 6708 ohms = 0.036 amps.

2. The timer motor is a 120 vac synchro motor but the circuit is supplied with 240 vac. The purpose of the resistor is to drop some voltage so that the voltage across the timer motor is something less than 120 vac. How much less? You can calculate the voltage drop across the resistor because we know the circuit current, 0.036 amps. In a series circuit, current is the same at all points and through every load. Therefore, the voltage drop across the resistor is

E = I x R = (0.036 amps) x (4500 ohms) = 162 vac.

The remaining voltage is dropped across the heater and timer motor according to their resistances.

3. Ohm’s Law again:
P = I² x R = (circuit current)² x (heater resistance)
= (0.036)² x (8 ohms) = 0.01 watts

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Dryer Schematic Quiz – Click for Larger View (opens in a new window)

Looking at the timing chart, we see that the cycles on this dryer fall into two main groups: auto and timed. The big distinction between the two is timer contacts TM-OR (timed dry cycles) and TM-WB (auto dry cycles). Since the schematic shows that the TM-OR contacts are made, then we know we’re in a timed dry cycle. Checking the state of the other contacts listed in the timing, we see that the schematic shows them all as open. Therefore, we know that this dryer schematic is showing the dryer at the End-of-Cycle. Which also helps answer the next question…

2. Is the motor shown running or not?

We know from the forgoing analysis that the dryer is shown at end-of-cycle, so the motor is not running. But you can also tell by looking at the centrifugal switch contacts on the motor. They are shown in the retracted state meaning the motor is stopped.

3. Why won’t the timer advance in Auto Dry cycle when the operating thermostat is closed?

In Auto Dry, the timer chart shows timer contacts TM-WB are made. With this in mind, trace L1 to the timer motor. L1 comes through timer contacts BK-BU to one side of the timer. L1 also comes through timer contacts BK-R, through the thermal cutoff, operating thermostat, and high limit to one side of the heater. But it also branches off through the resistor and timer contacts TM-WB to the other side of the timer. Since both sides of the timer have L1, the voltage difference across the timer is zero and the timer motor does not run.

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Simple Circuit Survey

Ignore the thickness of the lines in the drawing; it doesn’t mean anything.

Since the ignitor is glowing, we know that 1) we have a valid power supply, and 2) current is flowing, so there will be a voltage drop across the ignitor (the load) and work is being done (producing heat).

Next, we note that the ignitor, booster, and safety are all in parallel with each other. From the Fundamentals of Appliance Repair training course, we know that the voltage in each parallel circuit is equal to the supply, in other words, 120 VAC in each leg. That’s just the way circuits work.

“But what about the voltage drop across the main coil— doesn’t that subtract from the voltage available to drop across the ignitor and booster?”

Ah, Grasshoppah, let’s look at that voltage drop across the main coil. Notice that the detector is closed. As such, it is shunting the current around the main coil because current will take the path of least resistance. Since there is no current flow through the main coil, the voltage drop across the main coil is effectively zero.

So the answers are:

1. Ignitor: 120 VAC
2. Booster: 120 VAC
3. Safety: 120 VAC
4. Main: 0 VAC

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Refrigerator Flow Chart Quiz

Also, the refrigerator is plugged in, the doors close properly, and the condenser coil is clean.

What’s the next thing to check?

Bad control board, right? Nope. What’s your next guess?

Check the defrost system, right? Nyet, tovarish!

This is an easy one IF you understand how refrigerators work. We reveal all the troubleshooting secrets for refrigerators in the Samurai Tech Academy’s Refrigerator Troubleshooting and Repair Training Course. From the simple to the fancy, from single evaporator units to fridges with two or more evaporators, from split-phase, single-speed compressors to inverter-driven, variable speed compressors, our Refrigerator course is the most comprehensive, up-to-date, and affordable training course out there.

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Oh, BTW, the answer to the quiz is to see if the compressor is running. 🙂

Bonus question: How do you check the compressor operation? To be continued…