From: Bob Blick <bobblick@outlook.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
Sender: "piclist-bounces@mit.edu" <piclist-bounces@mit.edu>
Date: Sat, 18 Jul 2020 16:33:02 -0700
Subject: Re: [EE] Microwave Oven Transformers
Thread-Topic: [EE] Microwave Oven Transformers
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By the way, none of this is unexpected. The flux density in a transformer c=
ore is highest with no load, but microwave ovens are always running at full=
 load, so they get along with fewer turns of wire and smaller cores than th=
ey would otherwise. They also don't interleave the laminations, which proba=
bly allows them to get away with an even smaller core at the expense of poo=
rer regulation, which they also don't need.

Adding 20 percent to the primary (about 20 more turns in this case) to the =
primary would probably allow it to work lots better at 120V. But at the exp=
ense of less room for a secondary winding, and less power.

The moral of the story is that a microwave oven transformer works pretty we=
ll in a microwave oven.

________________________________________
From: piclist-bounces@mit.edu <piclist-bounces@mit.edu> on behalf of Bob Bl=
ick=20
Sent: Saturday, July 18, 2020 3:44 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Microwave Oven Transformers

Just a quick update.

Cutting apart the transformer was easy. Hacksawing along the two welds a li=
ttle more than a millimeter deep was enough.

Making my own flat wire is a bit challenging. I experimented on a 1 meter l=
ength. Stretching 12AWG solid wire to make it straight is hard. I will use =
a lever or other human invention. Keeping four 8-meter wires parallel while=
 taping will be challenging. I may tack-solder it at intervals along its le=
ngth. Spiral wrapping the Kapton tape seems to be easier than lengthwise.

In the meantime, a friend gave me a transformer he had stripped the seconda=
ry from(in anticipation of making a spot welder) and then abandoned. I deci=
ded to do some tests just for giggles. It is a 120V 60Hz model, from an ove=
n rated either 900 or 1000 watts. I connected it to a Variac and a low-qual=
ity AC volt/ammeter on the primary, wound 5 turns of 12AWG for a secondary,=
 and put an RMS voltmeter and oscilloscope on it.

Applying up to about 90 volts, primary current was 0.8A and I got something=
 resembling a sine wave on the secondary, about 4.2V.

By the time I got to 120V the primary current was 5.8A, there was minor ann=
oying hum and the output waveform looked rather stressed-out 5.3V. See atta=
ched photo. 2V/division.

The transformer still had its shunt blocks(search the internet and you will=
 see what they look like and where they go). Removing them and retracing my=
 steps gave me similar results at lower voltages but less voltage compressi=
on and 6.8A input current with 120V in. At 117V the current was 5.8A and ou=
tput voltage was 5.27V

Very little heat was produced, it appears that most power was reactive.

More fun to follow some other day. I need to fix my chainsaw and do other i=
mportant chores.

Bob

--=20
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