Vernon Larson

April 13, 2009

The following letter was written in reply to Lawrence Solomon’s column entitled "Vampire Hour," which appeared in the 28 March 2009 edition of the National Post.

Dear Mr. Solomon:

I live in Calgary, and I am a retired professional engineer

The information Dave Walton, Director of Home Ideas at Direct Energy, gave you about LEDs in microwave ovens contains some significant errors.

Microwave ovens contain a special type of electron vacuum-tube called a magnetron. Magnetrons require an electrically powered heater or, to use an old-fashioned term, filament. The heater heats a tiny oxide coated metal bit called a cathode. When heated the latter emits electrons that whiz around the interior of the magnetron and, by a kind of magic involving a couple of thousand volts and a magnetic field, produce the microwaves.

In the olden days, before Shockley invented the transistor, we all had radios containing from four to eight vacuum tubes. At least once per year one or more of these tubes used to "burn out"; that is, the heater in the tube gave up the ghost. We looked inside to see which tube wasn’t lit and we paid two to four dollars for a new one.

Nowadays, if the heater of a magnetron burns out, we will pay two to four hundred dollars for a new one, plus another hundred dollars for another exalted technocrat to replace it.

Have you ever heard of a microwave oven tube burning out? I haven’t. I own two microwave ovens, one 30 years old and another 13 years old. Both are still working. Why would that be? I do not actually know; but, I’ll bet you a new microwave tube that it is because we keep them plugged in all the time and that the manufacturer has arranged that the microwave tube heater is always connected to a source of electricity so it keeps burning. This was discovered a long time ago when electronic computers used thousands of electron tubes. If they did not turn off the heaters the tubes didn’t burn out very often.

So guess what might happen if we unplugged and plugged the oven in every day? It might cost us from 200 to 400 dollars per year, that’s what!

Here’s why I am almost certain I am correct. David must have had someone plug a dormant microwave oven into a power measuring meter. I can calculate backwards from the $2.80 per year to 0.767 cents per day. Using an electricity price of 10 cents/kilowatt-hour (Alberta unregulated price, not an Ontario subsidized price) I can calculate the dormant oven is consuming 3.2 watts. That is the amount of power consumed by a typical electron-tube heater (for example, 6.3 volts times 0.5 amperes).

The final nail in the coffin: the typical LED consumes from 30 to 60 milliwatts of energy, or .03 to .06 watts, not 3.2 watts. (See Wikipedia, for example.) Sixty milliwatts of energy translates to about 5 cents worth of electricity per year in Alberta.