April 1, 2011

sub-par electronic design in GE appliances

by dwm — Categories: Uncategorized1 Comment

Two days ago my 1994-vintage GE countertop microwave oven started misbehaving. It would not start unless I jiggled the door. I assumed the door switches needed cleaning.

Last night it went further awry. The blower would run when I _opened_ the door and stop when I closed the door. I’m assuming the interlock switches prevented it from running the magnetron, but nonetheless it’s disturbing for the microwave to seem to be running while the door is open.

The root cause: a small, low quality Korean-made microswitch with 120V across the contacts and 10A of current (more during start-up). The switch is rated for 15A and 250V, but… every time the microwave door is opened while the microwave is running, arcing will occur. What happened in my case: the contact that is supposed to be depressed by the plunger when the door is closed had deteriorated to the point where a piece of it fell off. That piece had positioned itself such than when I opened the door, the piece would straddle the remains of the contacts and the microwave would start. Closing the door moved it out of the way, opening the circuit.

I can’t really complain since this was an inexpensive microwave that lasted nearly 17 years. However… it goes to show what can happen when a designer thinks he’s doing the right thing for the target price range but uses a sub-par part. Why a mechanical switch instead of a relay or contactor? Because in theory it tends to have safer modes of failure (high mechanical leverage against the contact spring to prevent contact sticking, etc.). But if you use an el-cheapo part and don’t test it for the many thousands of cycles it’s expected to see over the long haul, you’re gambling.

Arcing wasn’t the only issue. The microswitch housing was deformed from heat. If you ask me, a switch of this size and design shouldn’t be carrying 10A of current on a regular basis. Once the contacts are dirty from arcing, there’s more heat being generated than the housing is designed to handle, which causes the contacts to melt the housing. This accelerates the demise of the switch. When I tried to remove the 2-pole connector from the switch, one of the contacts came right out of the housing instead of the connector coming off of the contact. And it was black from arcing and melted plastic (the switch housing is gray).

Since I’m an electrical engineer by training (but not by profession), I diagnosed the problem and ordered a replacement part. Hopefully I can make it a habit to turn the microwave off from the front panel before opening the door to prevent the switch arcing. There’s nothing I can do about the fact that it carries 10 amps of current when the microwave is on, unless I get motivated to replace it with a normally-open relay and energize the relay coil (low voltage, low current) from the switch instead of letting the switch carry the load current.

I wonder if there are any microwaves with the smarts to open and close the load relay at the zero-crossing of the AC input voltage…

1 Comment »

  1. dwm says:

    I received the new switch today from easyapplianceparts.com. It took me all of 2 minutes to install it, and my microwave is back in service. $14 for the part, an outrageous $7 for shipping. Still much better than buying a new microwave oven and recycling my old one prematurely.

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