# Rewinding microwave oven transformer

T

#### [email protected]

Jan 1, 1970
0
I am thinking of rewinding an old microwave oven transformer to drive a
car amp to produce some bass in my bedroom. The amp will continuously
draw about 30A and will draw a surge current of 150A. I was thinking of
using copper wire to rewind the transformer but getting copper wire in
the required thickness was going to be a problem not to mention working
with the stuff. What I thought I would do is use the wire from some old
car jumper cables (with the insulation left on) and rewind the
transformer using the jumper cables. Is there any problem with using
multicore cable to rewind a transformer? I am aware of the fact that
the insulation might melt...

E

#### ehsjr

Jan 1, 1970
0
I am thinking of rewinding an old microwave oven transformer to drive a
car amp to produce some bass in my bedroom. The amp will continuously
draw about 30A and will draw a surge current of 150A. I was thinking of
using copper wire to rewind the transformer but getting copper wire in
the required thickness was going to be a problem not to mention working
with the stuff. What I thought I would do is use the wire from some old
car jumper cables (with the insulation left on) and rewind the
transformer using the jumper cables. Is there any problem with using
multicore cable to rewind a transformer? I am aware of the fact that
the insulation might melt...

If it's a car amp, it is setup for 12V DC.
A transformer won't provide DC.

Ed

A

#### Art

Jan 1, 1970
0
May be easier to just buy a commercial 12VDC Supply, there are some nice
ones that will put out way over the required 150 Amp constant power. May
cost you a quid or two tho!!

J

#### Jamie

Jan 1, 1970
0
I am thinking of rewinding an old microwave oven transformer to drive a
car amp to produce some bass in my bedroom. The amp will continuously
draw about 30A and will draw a surge current of 150A. I was thinking of
using copper wire to rewind the transformer but getting copper wire in
the required thickness was going to be a problem not to mention working
with the stuff. What I thought I would do is use the wire from some old
car jumper cables (with the insulation left on) and rewind the
transformer using the jumper cables. Is there any problem with using
multicore cable to rewind a transformer? I am aware of the fact that
the insulation might melt...
Hmm, ?

T

#### [email protected]

Jan 1, 1970
0
I understand that a car amp needs dc to opperate. I am going to use the
diodes from an old alternator to get the required dc voltage. It would
be much more easy for me to just buy a large power supply but short of
an arc welder i don't think I will be able to find one that can supply
100A Also I don't want to pay anything for it. I allready have the
transformer, the diodes and the jumper cables But I don't want to wire
it all up until I know if it is ok to use multicore cable to rewind a
transformer.

P

#### Paul Burridge

Jan 1, 1970
0
I understand that a car amp needs dc to opperate. I am going to use the
diodes from an old alternator to get the required dc voltage. It would
be much more easy for me to just buy a large power supply but short of
an arc welder i don't think I will be able to find one that can supply
100A Also I don't want to pay anything for it. I allready have the
transformer, the diodes and the jumper cables But I don't want to wire
it all up until I know if it is ok to use multicore cable to rewind a
transformer.

Not unless you get off on the stench of burning plastic! Your idea's a
non-starter.

J

#### Jasen Betts

Jan 1, 1970
0
I am thinking of rewinding an old microwave oven transformer to drive a
car amp to produce some bass in my bedroom. The amp will continuously
draw about 30A and will draw a surge current of 150A. I was thinking of
using copper wire to rewind the transformer but getting copper wire in
the required thickness was going to be a problem not to mention working
with the stuff. What I thought I would do is use the wire from some old
car jumper cables (with the insulation left on) and rewind the
transformer using the jumper cables. Is there any problem with using
multicore cable to rewind a transformer? I am aware of the fact that
the insulation might melt...

No problems with multicore cable in a transformer, the only reason solid is
used is that it's easier to insulate and cheeper.
You must have some pretty flimsy jumper leads there if you're expectring
to get 10-20 turns on the transformer.

microwave oven transformers aren't built for continuous running without a
cooling fan blowing over them, they can be modified by adding some extra
turns to the primary,

Don't forget to fuse it, something like that could start a fire.

Bye.
Jasen

R

#### Rich Grise

Jan 1, 1970
0
I am thinking of rewinding an old microwave oven transformer to drive a
car amp to produce some bass in my bedroom. The amp will continuously
draw about 30A and will draw a surge current of 150A. I was thinking of
using copper wire to rewind the transformer but getting copper wire in
the required thickness was going to be a problem not to mention working
with the stuff. What I thought I would do is use the wire from some old
car jumper cables (with the insulation left on) and rewind the
transformer using the jumper cables. Is there any problem with using
multicore cable to rewind a transformer? I am aware of the fact that
the insulation might melt...

The problem isn't using stranded wire - in fact, in those "make a spot
welder with your MOT" articles, they recommend welder cable, which is
stranded.

The problem is a MOT won't give you 100A either, at lest not for very
friggin' long. And they're rated for intermittent duty, and _that_'s
with forced air cooling!

Very very bad idea - you'd be better off to look at something like
surplus BMF transformers, or an industrial battery charger.

Good Luck!
Rich

B

#### Brian KP

Jan 1, 1970
0
If you would really like to use a car amplifier, I would suggest
getting an old car battery and an old car battery charger. The kind you
use to charge a dead battery inside your house, they plug into
household 120V outlets and continuously charge the car battery. (Lead
acids like being charged constantly). If you haven't lost me here is

More than likely, the car amp only uses 150A when it needs to drive a
really hard bass hit at full volume. Usually it is only using a
fraction of that. No transformer that isn't \$ is going to provide
that. However, hooking your amplifier to a car battery that is being
continuously charged can handle that. When the amp draws alot of power
the battery acts like a huge capacitor and provides it, otherwise most
of the power comes from the battery charger and a little from the
battery. Of course in the end the net charge into the battery must be
greater than the net discharge of the battery but the battery will be
recharged whenever you're not using the amplifier, which if your normal
will be at least 8 hours a day.

T

#### [email protected]

Jan 1, 1970
0
Thanks for all your help it has given me a few good ideas. I gave up on
the jumper cables as they where to large but I am going to try
rewinding the transformer with 80A car wire. I will let you know what
happens. The amp is only 300w so I don't think that the microwave
transformer will have any problem powering it.

T

#### [email protected]

Jan 1, 1970
0
I set it up as above and it appears to be working fine but after
running for 1/2 an hour or so the transformer gets very hot. I have
kept this under control with a couple of fans but I am wondering if it
is damaging to the transformer running it like this? What could I do to
get the heat down other than adding more fans? Would replacing the
magnetic shunts stop it heating up or does it need some form of current
limiting?

T

#### Tim Williams

Jan 1, 1970
0
I set it up as above and it appears to be working fine but after
running for 1/2 an hour or so the transformer gets very hot. I have
kept this under control with a couple of fans but I am wondering if it
is damaging to the transformer running it like this?

As long as temp stays below, say, 100°C, it should be fine.

Technically. Personally, I would try to improve that!
What could I do to
get the heat down other than adding more fans? Would replacing the
magnetic shunts stop it heating up or does it need some form of current
limiting?

Nah. Here's what you have to do. Look at the size of the primary winding's
wire. Find some laying around of the same size (I'm guessing 14 to 16AWG).
Put on about 20 turns (yes this will take up some of your secondary's space,
you'll have to live with that) and connect it in series with the primary.
Make sure to match winding direction, otherwise you'll overload it worse.

You'll need to add a few more secondary turns.

Alternately, Google "buck transformer" and drop primary volts by 10-20V or
so. Same idea, you need more secondary turns.

The problem is, MOTs are wound with as little copper as possible. This
overloads (saturates) the iron core, causing high peak currents in the
primary (the secondary load doesn't matter!). High currents means more heat
in the primary copper, causing it to heat up. A rather unhappy situation,
so by adding primary turns (or reducing the voltage), you give the iron core
an easier life and everything is happy in Transformer Land.

Yes, to fit more turns probably means smaller wire means less current
handling. That's the price you pay for electrical happiness.

Tim

J

#### John Popelish

Jan 1, 1970
0
I set it up as above and it appears to be working fine but after
running for 1/2 an hour or so the transformer gets very hot. I have
kept this under control with a couple of fans but I am wondering if it
is damaging to the transformer running it like this? What could I do to
get the heat down other than adding more fans? Would replacing the
magnetic shunts stop it heating up or does it need some form of current
limiting?
Removing the magnetic shunts and using some of that space to put some
additional primary turns would help a lot.

A way to effectively double the primary winding is to use two
identical transformers, but put the two primaries in series and the
secondaries in parallel.

T

#### [email protected]

Jan 1, 1970
0
Thanks for all the help. I think I will try putting two transformers
together with the primaries in series and the secondarys in parallel.
However I don't have two MOTS the same. Will it matter if they are a
little bit different? One is a bit smaller than the other.

J

#### Jasen Betts

Jan 1, 1970
0
I set it up as above and it appears to be working fine but after
running for 1/2 an hour or so the transformer gets very hot. I have
kept this under control with a couple of fans but I am wondering if it
is damaging to the transformer running it like this? What could I do to
get the heat down other than adding more fans? Would replacing the
magnetic shunts stop it heating up or does it need some form of current
limiting?

Add more turns to the primary, (this will reduce the current in the primary,
and therfore the heating...) and also the voltage in the secondary, so you
may need to add turns there too)

T

#### [email protected]

Jan 1, 1970
0
Thanks to everyone that posted replies. I found a similar transformer
to the one I all ready had at work and I set the two up with the
primaries in series and the secondary in parallel and with this setup
the transformers will happily power a 100w bulb continuously with only
a little heat output from one transformer and no heat output from the
other one. I think I could improve this by using two identical
transformers but I hardly ever see two identical microwaves.

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