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 17:54:22 -0700
Subject: Re: [EE] Microwave Oven Transformers
Thread-Topic: [EE] Microwave Oven Transformers
Thread-Index:
 AQHWWTDTdgLEYlLB80CMUa8aGORcYqkGGn2AgALBCRuAAAiKAIAABfHzgAAGgQCAAARH4YAAEa8AgAAJWtGABN8wPoAAE3VigAAK7ACAAAbCXQ==
Message-ID:
 <BYAPR02MB4055752702D7D60A89818638DC7A0@BYAPR02MB4055.namprd02.prod.outlook.com>
References:
 <BYAPR02MB4055907B481B158D04EA8552DC600@BYAPR02MB4055.namprd02.prod.outlook.com>
	<alpine.BSF.2.21.9999.2007151808010.43517@nickel.allotrope.net>
	<BYAPR02MB4055E993D92A5B73457E55EEDC7E0@BYAPR02MB4055.namprd02.prod.outlook.com>
	<CANqFfZCNx_VAVqziCevfruVYzPFd2YVWb2E-FbGRyvT_-Ev2VA@mail.gmail.com>
	<BYAPR02MB4055A81DBE35DFFFA66AC00FDC7E0@BYAPR02MB4055.namprd02.prod.outlook.com>
	<CAHD638iaeM-+k1_W1XZyzJDFLR+yHmFFFOw0r_SVk3Y=Pnbeaw@mail.gmail.com>
	<BYAPR02MB40556713C12321B212CCCD54DC7E0@BYAPR02MB4055.namprd02.prod.outlook.com>
	<BYAPR02MB40550FF5F13FD537C6A57E1BDC7D0@BYAPR02MB4055.namprd02.prod.outlook.com>
	<BYAPR02MB405571C12DAC49383E9ED1FFDC7D0@BYAPR02MB4055.namprd02.prod.outlook.com>,
	<7KntORXqSOGO8YQ99hAv9tCO6K_H7EGuugktp-Cl_ybkvAVcSLvWHLjxGa2rut6_YLHdoAiin8x5Y0eHpA6_cj2kCu9cZ2KkzidwKatn5KU=@protonmail.com>
In-Reply-To:
 <7KntORXqSOGO8YQ99hAv9tCO6K_H7EGuugktp-Cl_ybkvAVcSLvWHLjxGa2rut6_YLHdoAiin8x5Y0eHpA6_cj2kCu9cZ2KkzidwKatn5KU=@protonmail.com>
Reply-To: Microcontroller discussion list - Public. <piclist@mit.edu>
Accept-Language: en-US
Content-Language: en-US
X-MS-Exchange-Organization-SenderIdResult: Pass
X-MS-Exchange-Organization-PRD: mit.edu
X-MS-TNEF-Correlator:
acceptlanguage: en-US
received-spf: Pass (protection.outlook.com: domain of outlook.com designates
	40.92.42.60 as permitted sender) receiver=protection.outlook.com;
	client-ip=40.92.42.60;	helo=NAM10-MW2-obe.outbound.protection.outlook.com; 
dkim-signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=outlook.com;
	s=selector1;
	h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck;
	bh=HwI2pLJyUDTf9ntGN6OGOX5IZymnqFil3VpLpeJEhwM=;
	b=CUv4oxGN6MCqUUVUGUZ9aW2JACR5BFc0Efl54pChaBd+YXW310b+zKDgdSJ926qsXfSCRABK4/476/ALjz7qKyUiHQ3Hp6pkvXswz8sqP4JwnnH1DZW8rXWjONOLxg9sD4fkvdkmSYzZVvkXEV+z+rhG5ZA1g81e5w2BP3mHuj55L3MS6dSJpMZtGZEqEHyOZVRaZi8GwzXhFsqQwqlGmaec+9TNs7FEImxFV5hEnwZYq8zqcOJoHTC50iuBXlg9txkdkxXDs0TSABuMm3pZSAA1iwf9POnb+u4a8vglTtdLq6BYonIcjinIPtWuJr/HDg9pDrbtMS1iMO+H4tr/ZQ==
authentication-results: spf=pass (sender IP is 40.92.42.60)
	smtp.mailfrom=outlook.com; mit.edu; dkim=pass (signature was verified)
	header.d=outlook.com; mit.edu;	dmarc=pass action=none
 header.from=outlook.com; 
errors-to: piclist-bounces@mit.edu
list-id: "Microcontroller discussion list - Public." <piclist.mit.edu>
list-post: <mailto:piclist@mit.edu>
x-beenthere: piclist@mit.edu
x-mailman-version: 2.1.6
x-topics: [EE]
x-mime-autoconverted: from quoted-printable to 8bit by PCH.mit.edu id
	06J0sRmm029060
Content-Type: text/plain; charset="iso-2022-jp"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0

Many many years ago I learned a lot rewinding power transformers, using the=
 Radiotron Designer's Handbook (thanks RCA Australia) as a reference.

I think there's nothing wrong with repurposing MOTs(microwave oven transfor=
mers), the ultimate performance limitation is that the laminations are not =
interleaved. But that makes them easier to modify, so that's easily excused=
.. Accept the fact that you will never achieve perfect regulation. If you cu=
t the core apart, clamp it super tight when you use it(or when you re-weld =
it, TIG with no filler or silicon/aluminum bronze filler would be best, wor=
k hot and fast).

If I was looking at it with an early 20th century eye, I would rate an MOT =
at 2/3 the oven's rated output. IOW, a 900W output microwave oven can yield=
 a good 600W transformer. Add an extra winding in series with the primary, =
20 to 25 percent of the existing primary. Expect less regulation(more drop =
under load) than a high quality transformer. If you are OK with that, you s=
hould be quite happy.

>From my earlier experience winding transformers, don't expect to get too m=
uch, don't cheat on turns-per-volt, and you will have a fine transformer. A=
fter you're happy with it, dip in some type of glue. Leftover alkyd or oil =
paint, floor finish, epoxy, etc. Not linseed oil.

As far as capacitors, I am not an expert, but I've been happy with MLCC cap=
s in power applications as long as the voltage rating is very conservative.=
 The Piclist has real experts who may chime in, listen to them.

Cheerful regards, Bob

________________________________________
From: piclist-bounces@mit.edu <piclist-bounces@mit.edu> on behalf of madsci=
entistatlarge=20
Sent: Saturday, July 18, 2020 5:02 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Microwave Oven Transformers

Thanks for that.  I've been considering converting one to an isolation tran=
sformer and was guessing roughly 20 more primary turns would allow shunt re=
moval (based on the cross section of the magnetic shunt compared to the cor=
e).

It wouldn't be good for the same VA since it would saturate faster but woul=
d hopefully work fine with a small variac.  Is it the shunts and their satu=
ration that limits the power when in the circuit it was designed for?  I kn=
ow they are there to reduce magnetizing current when first switched on, but=
 I'll be running it through the variac first and turning it up slowly.

I want to play with offline switching power supplies and they are much more=
 dangerous without isolation obviously.  On that note, I see people are rec=
ommending safety glasses.  Makes sense with electrolytic caps, But I was wo=
ndering if it also stemmed from the ceramic caps.  Just how do you find the=
 ripple current rating for an MLCC?  I know when people over clock computer=
s the ceramic caps on memory cards can fail.  I've only seen one ceramic ca=
p fail and it was in a high peak current circuit (not monolythic and it fai=
led "open").  I'd already decided that plexiglass and goggles would be requ=
ired.  I've seen smaller electrolytics fail (well, when I was younger I'd h=
ooked them up to a power cord on a long extension cord and plugged it in a =
couple of times).  I also know tantalums can fail even more violently, I've=
 been told some are equivalent to blasting caps!


Sent with ProtonMail Secure Email.

=1B$B!>!>!>!>!>!>!>=1B(B Original Message =1B$B!>!>!>!>!>!>!>=1B(B
On Saturday, July 18, 2020 5:33 PM, Bob Blick wrote:

> By the way, none of this is unexpected. The flux density in a transformer=
 core is highest with no load, but microwave ovens are always running at fu=
ll load, so they get along with fewer turns of wire and smaller cores than =
they would otherwise. They also don't interleave the laminations, which pro=
bably allows them to get away with an even smaller core at the expense of p=
oorer regulation, which they also don't need.
>
> Adding 20 percent to the primary (about 20 more turns in this case) to th=
e primary would probably allow it to work lots better at 120V. But at the e=
xpense of less room for a secondary winding, and less power.
>
> The moral of the story is that a microwave oven transformer works pretty =
well in a microwave oven.
>
> From: piclist-bounces@mit.edu piclist-bounces@mit.edu on behalf of Bob Bl=
ick
>
> 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 =
little more than a millimeter deep was enough.
>
> Making my own flat wire is a bit challenging. I experimented on a 1 meter=
 length. Stretching 12AWG solid wire to make it straight is hard. I will us=
e a lever or other human invention. Keeping four 8-meter wires parallel whi=
le taping will be challenging. I may tack-solder it at intervals along its =
length. Spiral wrapping the Kapton tape seems to be easier than lengthwise.
>
> In the meantime, a friend gave me a transformer he had stripped the secon=
dary from(in anticipation of making a spot welder) and then abandoned. I de=
cided to do some tests just for giggles. It is a 120V 60Hz model, from an o=
ven rated either 900 or 1000 watts. I connected it to a Variac and a low-qu=
ality AC volt/ammeter on the primary, wound 5 turns of 12AWG for a secondar=
y, and put an RMS voltmeter and oscilloscope on it.
>
> Applying up to about 90 volts, primary current was 0.8A and I got somethi=
ng 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 a=
nnoying hum and the output waveform looked rather stressed-out 5.3V. See at=
tached photo. 2V/division.
>
> The transformer still had its shunt blocks(search the internet and you wi=
ll 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 compres=
sion and 6.8A input current with 120V in. At 117V the current was 5.8A and =
output 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=
 important chores.
>
> Bob
>
> -
--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.