Lab power supply from microwave transformer project.

[Damien]

Hey guys I'm at a standstill with my project.

I've been playing around with really basic electronics for years but thought I should take it to the next step, decided to make myself a big battery charger for charging a bank of batteries while we're camping and use our 240v gennie but have it so it's a far away from camp site that we can't hear it.
So I got started with the transformer out of a microwave. Rewound it for 18v ac, then put a bridge rectifier in, used an lm317 transistor with a rheostat to regulate and vary the voltage. Problem is, it's only good for 1 amp. I want like ...50 amps..
I just know someone out there knows how because I've seen it done on youtube but they never post a vid on how they did it.
If someone knows can they please let me in on the secret?

Here's a vid of what I have so far. -

Cheers! -

Damien.

 

[Damien]

Seven minutes 20 seconds into this tutorial the guy talks about using mosfets in parallel to increase current capacity in a circuit. To make my circuit variable though, do I put a variable resistor from gate to source on all of them or only on one and somehow drive the power mosfets from the control mosfet? Just wondering if it is possible to minimize soldering by having one mosfet as the controller and the rest as the drivers or if I must wire my rheostat with all of them in parallel.

 

[BobK]

You really don't want a linear regulator for 50A. If it is dropping 2V, it would have to dissipate 100W. There are switching regulator controller ICs that will control an external MOSFET. This is what you should look into.

Bob

 

[davenn]

So I got started with the transformer out of a microwave. Rewound it for 18v ac, then put a bridge rectifier in, used an lm317 transistor with a rheostat to regulate and vary the voltage. Problem is, it's only good for 1 amp. I want like ...50 amps..

and apart from the very relevant comments from BobK

the chances are that you wont get 50 amps from that transformer anyway
50A @ 12V is going to be a pretty bulky transformer ( large and heavy) not the small
physical size of the usual microwave transformer

 

[Damien]

Well, as much as I can get from this transformer then, even if I have to rewind the secondary with thicker wire to make small tunings. So tell me more about this switching regulator controller please? Thanks for your help guys. =)

 

[davenn]

So tell me more about this switching regulator controller please? Thanks for your help guys. =)

google boost and buck regulators and there are 100's of results

look for ones in the realm of your interest

 

[davenn]

Well, as much as I can get from this transformer then, even if I have to rewind the secondary with thicker wire to make small tunings.

You have a bit of a lack of understanding on how transformers work

increasing the thickness of the wire would increase the current, but at the cost of room for less turns on the transformer, so therefore less output voltage but the same amount of power

these are not actual figures, but just to demonstrate the principle
and we are referring to the secondary winding !
so say 100 turns of 18SWG gave 20V @ 1A = 20W
50 turns of 9SWG ( twice the wire thickness, but room for only 50 turns) is going to give 10V @ 2 Amps = 20W

now apart from what happens on the secondary side
The final amount of power out is also determined by the construction of the primary side
# of turns, their thickness, the size of the transformer core etc etc

NOTE ... I could hear your transformer buzzing quite badly in your video --- that's really a bad sign

The 12V, 50A transformer, I had for my big PSU some years ago, weighted some 10 - 12 kgs (> 22 lbs)

This is why these days people use switchmode power supplies, that you find in computers and just about every other piece of electronics these days that is powered from the mains supply. They are smaller, lighter and much more efficient than linear power supplies.

here's ne excellent example of a variable voltage high current lab supply .....
http://www.jaycar.com.au/productView.asp?ID=MP3090&form=CAT2&SUBCATID=999#3


I have used a lot of these ones in manufacturing I have done over recent years .....
http://www.jaycar.com.au/productView.asp?ID=MP3118&form=CAT2&SUBCATID=999#12

nominally 13.8VDC@18A, adjustable from ~ 11VDC to 14.5VDC

No its not 50 A, you will struggle to easily find 12VDC at ~ 50 Amps and when you do they are likely to be quite expensive, expect > AU$600


Dave

 

[Damien]

Weird...I thought the number of turns on the secondary was directly proportional to the voltage put out by the secondary. Since I'm only using crappy thin wire which leaves a little spare room in there I could get something with less insulation and more conductive material and it would give me more current. Any who, have a lot to learn I realize this! =)

Apparently texas instruments sells a control module called " A simple switch" Anyone familiar with this?

What do you think the best way of scaling this up would be and at what cost?

I hope this gets easier haha.

 

[Damien]

*dives head first* haha.
This look ok for a rough draught? Probably the most complicated electrical circuit I have drawn before. I know I have some spelling errors in there like hf instead of uf on one of the caps.

 

[davenn]

Weird...I thought the number of turns on the secondary was directly proportional to the voltage put out by the secondary.

its is .... did you not read what I wrote ?

what part didn't you understand ?

 

[Damien]

its is .... did you not read what I wrote ?

what part didn't you understand ?

so say 100 turns of 18SWG gave 20V @ 1A = 20W <-- This part.

 

[davenn]

what specifically do you not understand ?

V x A = W .... did you not know that part ?

Dave

 

[Damien]

Hi Dravenn

I thought that the number of turns on the secondary was directly proportional to the voltage on the secondary if the turns on the primary had the same number of turns as the input voltage. Anotherwords, I thought that 20 turns on the secondary would equal 20 volts a.c on the secondary.
VXA=W ? Yep familiar with ohms law. =)

On another note, I've watched a few vids on switch mode power supplies and some of them have said y'know it's really complicated etc so I am a little nervous but I will give it a shot.

If that doesn't work out do you think an e.s.c before the transformer would work as a form of voltage adjustment for the linear power supply I have put together? I know this wouldn't be regulated though hmmm what to do.

Like this guys one -

 

[davenn]

Anotherwords, I thought that 20 turns on the secondary would equal 20 volts a.c on the secondary.

No, that's incorrect

here's one site giving all the maths and diagrams
http://www.sayedsaad.com/fundmental/21_TURNS AND VOLTAGE RATIOS .htm

and another....
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/tracir.html#c2

Wiki has some good basics as well .....
http://en.wikipedia.org/wiki/Transformer

you can see its not a totally simple thing to work out.

you shouldn't even be thinking about SMPS units till you have the basics sorted out

Dave

 

[Damien]

No worries Dave thanks for your help. =)

 

[davenn]

no probs .... its all in the fun of learning

have a good read through those links and ask any questions on things you are not sure of
There's sure to be someone who can help


Dave

 

[Damien]

That's Dave yep I might build some more simple transistor circuits first to familiarize myself more as this is all so new.

I ordered a 555 timer ic & some more mosfets & resistors to play around with blinking led's & possibly get an oscilloscope to have a look at what the waveform is doing in different circumstances.
Breadboard came in the mail today - looked much bigger in the pictures! =/ haha.

 

[davenn]

I ordered a 555 timer ic & some more mosfets & resistors to play around with blinking led's & possibly get an oscilloscope to have a look at what the waveform is doing in different circumstances.
Breadboard came in the mail today - looked much bigger in the pictures! =/ haha.

That's great
yes, get lots and lots of experience with low voltage stuff before getting in to mains voltage projects -
they call for lots of experience and a lot of care

use batteries or plugpacks for all your initial experiments --- uh huh, the usual breadboard isn't very big
and when using them use single core wires NOT multi core wires for the jumper leads

Dave

 

[Damien]

Oh thanks for the jumper lead trick that should make things easy. I was thinking of getting light guage multicore & just twisting them really thin & a little solder to stop the wires from bending as they're inserted into the breadboard.

Uhh here's the thing Dave I've been working with 240 for a few years now. You really think I should go back to low voltage stuff?

 

[davenn]

Uhh here's the thing Dave I've been working with 240 for a few years now. You really think I should go back to low voltage stuff?

Its just safety suggestions us guys always have to make, cuz we have no idea where you ( or anyone else) are at, what experience you have. So we start at the safest point and work from there till we can get an idea where a person is at

If you have been on forums long enough you get to see all types. And some prove just by what they say that they have absolutely no idea what they are doing and they are trying to play with mains or higher voltages.
As a result we have to close the thread for their and our safety. Imagine that several of us gave good info to some one inexperienced ( but we don't know that) and they kill themselves, burn the family home down, kill some one else because they were too inexperienced to follow instructions given.
In today's legal world, there's a good chance that if it was found out that some one on a forum had given that info ( regardless of how it was used) that we could be made libel for loss of life etc

So therefore, we have to, in general, treat everyone as a beginner till we can judge where they are at
People can write anything on a forum ... we don't really know if they have 2 days, 2 yrs or 20 years experience
There's no way that us guys can validate a persons claimed experience .... see the difficult position we are in ?

cheers
Dave