Microwaves Running with the Door Open and Meter Voltage Ratings Explained

Every other week, all the finest techs of Appliantology gather in cyberspace for a Live Dojo workshop, where we discuss hard-hitting technical topics, industry secrets, and all else appliantological.

You may have missed the live workshop on January 6th, but never fear! We’ve documented the discussion for posterity here. Topics covered include:

  • Microwave door interlock switches
  • Control board algorithms and relays
  • Turntable motor circuit analysis
  • Theorizing a reasonable failure scenario for multiple failures
  • What meter categories mean and how they should influence what meter you buy

Keep scrolling for a summary of the conversations. Here’s a recording of the first half of the Live Dojo, covering a microwave that runs with the door open – viewable only by premium members of Appliantology.

Not a premium member? You can become one today for free by enrolling in any technical course right here at Master Samurai Tech.

Microwave Turntable Interlock Switch Investigation

Sam discussed a problem with a malfunctioning microwave where the turntable motor ran while the door was open, though there was no heat. He suggested a possible link between these two failure conditions and proceeded to examine the circuit. Sam explained the role of interlock switches, monitor switches, and analog switches, emphasizing that the primary interlock switch must be closed for the turntable motor to function, and it should not be able to operate while the door is open.

Machinery Interlock Switch Malfunction

Sam, Scott, and Tim discussed how the primary interlock switch could be malfunctioning. Sam pointed out how the primary interlock switch should be closed when the door is closed and vice versa to prevent a dead short. However, the team observed that the turntable motor in this case operated with the door open, indicating a potential problem. Sam, Tim and Scott further discussed the issue, hypothesizing that the primary interlock switch might be faulty, sticking closed when the door was open. They agreed to test for coil voltage and check the state of the switch with the door open and closed.

Microwave Failure: Cause and Solution

The team further discussed the problem with the malfunctioning microwave, focusing on the sequence of failures that led to the appliance’s non-functionality. Sam and Wally speculated that the primary interlock switch might have malfunctioned and gone unnoticed for a period before a relay started sticking open, leading to a lack of heat. The team agreed that this sequence of failures was plausible, but noted that they did not have the appliance’s history to confirm this theory. The team also discussed the benefits of walking through schematics logically to troubleshoot similar issues in the future.

Want to watch the second half of the Live Dojo, when we discussed meter categories? Click below to view the full video – viewable only by premium members of Appliantology.

Not a premium member? You can become one today for free by enrolling in any technical course right here at Master Samurai Tech.

Voltage Ratings and Meter Setup Importance

Scott discussed the importance of understanding voltage ratings when dealing with probes for a Fluke meter. He explained that the amp ratings are only used when setting up the meter to work in series with a circuit. Scott stressed that the Category and working voltage ratings of a meter were more important than its amp rating. They also discussed the concept of input impedance, explaining that it controls the current through a circuit. Scott concluded by discussing voltage spikes and why they might be relevant, even if the typical highest voltage we deal with is 240 volts.

Electrical Measurement: Impact of Voltage Surges

Scott discussed the issues related to electrical measurement, focusing on the impact of voltage spikes on measurement readings. They explained how transient noise, lightning strikes, and other sources could create these spikes that could potentially damage measurement components if not adequately protected. Scott emphasized the importance of understanding the electrical measurement categories, ranging from Category one (Cat I), the lowest risk category suitable for low voltage DC electronics, to Category four (Cat IV), which provides protection against all categories below and is often necessary for industrial electricians working with 3-phase power.

Transients, Spikes, and Meter Specification

Scott discussed the issues related to transients and spikes on electrical lines and how they can damage equipment. He explained that even meters rated for higher voltage were failing due to these transients. He then went on to discuss the development of a category system to deal with these issues, which rates meters based on their ability to handle transients. Scott also clarified the confusion around the specification of leads, emphasizing that the working voltage and the category are the most important aspects. He further explained the concept of high input impedance in meters, which allows them to measure voltage without significantly affecting the circuit. The team agreed that high input impedance is crucial when measuring delicate low voltage DC logic circuits.

Voltage Ratings and Meter Amp Ratings Explained

Scott explained the differences in voltage ratings and their compatibility. He clarified that even though the meter is only measuring 240 volts, it can handle transients of 6,000 volts. Scott also addressed the issue of the amp rating of the meter and the probes, assuring that even a full amp rarely flows through the meter. He further explained that the actual current flowing through the meter is much lower due to its high input impedance. Scott then asked Sam to do a quick calculation to confirm this point.

Electrical Safety and Product Information Concerns

Wally expressed concerns about the suitability of their gold-plated items for handling a thousand votes and 20 amps, without knowing their cat rating. Scott suggested that product literature should contain this information and that it might be legally required. Wally had difficulty finding further information and decided to return the items. Scott assisted Wally in searching for the product online and discussing its features, but they couldn’t confirm if Wally’s items were the same as ones viewed online. Scott provided guidance on electrical safety and explained the potential impact of a 1,000-volt cat. He clarified that devices rated at 3 amps, such as Pomona leads, would still be safe to use.

Door Switches and Dryer Heater Testing

Scott clarified that all interlock switches are door switches, actuated by the door opening and closing. Sam provided a picture of the microwave with three switches, each actuated by the door. The team also discussed testing a heater on a dryer when it’s open, with the understanding that the meter would be acting like a load. Tim Wanat shared his experience with a malfunctioning microwave motherboard, and Scott advised him to unsolder one leg of the suspected diode to test it properly.