# my last transformer post, I promise

Discussion in 'Hobby Electronics' started by gav, Sep 21, 2003.

1. ### gavGuest

hello again,
I've been searching and trying to understand as much as I can with my
limited electrical experience but I really do have to ask some more

Aim:
I plan on experimenting with winding my own toroidal power transformer.
I require a 1:2 step up at 20V 150hz supply aiming at 150-300VA or so.

Method:
I have found a general ballpark equation to calculate primary turns for an
EI type transformer which goes like this:
is N = (E · 10^8)/ (4.44·f·A·B),

N= number of pri turns
E= supply voltage
f= frequency
A= Cross sectional area of core (I think in square inches)
B=max flux lines = rough figure of 60,000

Using a toroidal core with say a CSA of 1sq" it gives me 50 turns for the
primary.

Is this equation correct and should it work for a toroidal?

Is the value of B appropriate for say a Mains 300VA toroidal GOSS (Phil)
core off the shelf at Jaycar?

I think I understand that too few turns on the primary will cause saturation
and large currents. Is there a severe disadvantage to putting a few too
many turns on the primary apart from increased copper losses? I only ask
because someone knowledgeable may be able to advise me X turns on the
primary will work and I'll be able to forget about the theory for now.

Does it have to be more precise than this. I don't mind the thing burning up
if it's inefficient when I put a load on it. Just means I'll have to
re-wind.

If anyone can help me answer these questions, or point me somewhere that
can I would be very grateful.

Thankyou
Gavin.

2. ### Mark HarrissGuest

AAAAAAHHHHRRRRRRRGGGGGG!!!!

Just Kidding Gav!!

mostly in the 1970's editions.

4. ### Phil AllisonGuest

** A 50 Hz toroidal needs at least 4.5 turns per volt for 1 sq inch of
core. So for 150 Hz only 1.5 turns per volt are needed. For 20 volts that
means only 30 turns .

Max B is 1.1 Teslas for the grade of GOSS normally used in 50 Hz
toroidals. ( B can go as high as 2 Teslas when exotic materials are used.)

** No - a few extra will not hurt.

Your problem may be to get enough volume of copper into a mere 30
urns - for reasonable efficiency the space available for windings must be
well filled, half the space with the primary and half with secondaries.

Probably you need to wind several layers of 16 G wire for the primary
and then parallel connect them - same goes for the secondary. I hope you
have strong fingers.

............ Phil

5. ### Peter HowardGuest

Can't help you Gav, I know as much as you, but here's a dumb question for
the group.
How DO they wind toroidal transformers or chokes commercially? I know it
can't be rows of strong fingered and patient ladies winding the turns on by
hand. I have wound the occasional hf toroid by hand and can't even begin to
imagine what sort of machine would pass a tiny shuttle loaded with a tiny
reel of wire through the hole and around the outside time and time again.
Anyone ever seen such a machine in action?

Peter

6. ### John CrightonGuest

Hello Gavin,
what is that you are ultimately wishing to do. I remember
something about running a transformer from a car alternator
for some reason. I can't remember, tell me again.
What equipment do you wish to operate in your car?
Just Interested.
Regards,
John Crighton
Hornsby

7. ### Phil AllisonGuest

** A doof doof machine of course !!!

.......... Phil

8. ### John CrightonGuest

Hello Phil,
I have no experience in these things. I thought
they operated off plain ordinary 12 Volts D.C.
from the car battery.
Does Gavin wish to operate a special amplifier
from a higher DC supply voltage? How High?
How much current?
Regards,
John Crighton
Hornsby

9. ### Phil AllisonGuest

** Sensible ones do.

** Yep.

How High?

** +/- 60 volts.

** 4 amps DC average per rail.

.......... Phil

10. ### John CrightonGuest

Hello Phil,
cripes, that is awkward!
A DC to DC converter would be neat, but to supply
plus and minus 60 Volts at 4 amps would be a big
project in itself. A good one though.

Getting back to the transformer.
How was that going to work with an alternator?
A second alternator modified for single phase
output, maybe? Sounds interesting!
Regards,
John Crighton
Hornsby

11. ### Phil AllisonGuest

"John Crighton" <

** Well, that is HIS problem.

He is the sort of "individualist" who thinks it is worth a go.

Buckley must be his patron.

............. Phil

12. ### Mark HarrissGuest

I asked a guy at Harbuch that question, he said he can't
explain it but that it's really obvious once you see a machine.

13. ### gavGuest

HAHA,

John, Phil has pretty much summed it up
..
Second alternator, modified for single phase AC output.
A Tranny to do 1:2 step up.
Rectified and fed back to modified regulator circuit to hold at 60V
Use this as your power supply for any amp module from jaycar requiring 60V
rails.

It would be cheaper to go buy a normal car amp. But not as fun.

bye,
gavin

14. ### John CrightonGuest

Hello Gavin,
how about a second dedicated electrical power system of 24V.
The second 24 V alternator (used one from wreckers) charges
up two 12 volt batteries in series tucked away somewhere.
Now you would have to find or build an amplifier that ran off 24V.
but you would have more DC power at your disposal.
The car engine not running wouldn't be a problem either.

High power 12 volt amplifiers are readily available second
hand from cash converters or wherever. Maybe half a dozen
amplifiers with half a dozen speaker would give lots of
noise for the least effort, running off a second dedicated
12 volt electrical system off course.

You could build your own amplifiers using individual
components or try the high power audio module types.
That would be fun!

Just a thought.
Regards,
John Crighton
Hornsby

15. ### gavGuest

Hello Gavin,
I've gone too far to turn back now.
Truck alternator is the next upgrade though. Bet they put out some power.
gavin.