# Transformer

Discussion in 'Electronic Basics' started by Jim, Nov 7, 2004.

1. ### JimGuest

Hi,

I would like to make a 800V DC power supply, which will be used to charge a
capacitor bank. The power supply will run of a 12V battery. To make the
power supply I am thinking of making an oscillator, which will generate a
12V AC signal. The 12V AC signal will then be fed to a transformer, which
will step the 12V AC signal upto 800V. I will then use diodes to convert
the 800V AC signal to approx. 800V DC.

I would like to make my own transformer (I can't find any suitable premade
transformers). I understand that the transformer will need a ratio of
800/12. But how do I determine how many turns I should have on the primary
coil? Also, how do I determine the best frequency for the transformer?
Should the frequency be as high as possible? And what about wire thickness,
how thick should it be? Do any calculators exist which will tell me all
this?

Can anyone point me in the right direction (websites or answers)?

Thanks.

2. ### John PopelishGuest

Do you have an idea of the current you will expect (or how big a
capacitor you need to charge in how much time)?

Do you need isolation between the 12 volt source and the 800 volt DC
output?

I suspect that a flyback converter may be good for this sort of thing,
since it handles big voltage variations in the output (like starting
with a fully discharged capacitor load that ramps from zero to 800
volts).

This type also reduces the turns ratio required, because it can
produce more than 12 volts across the primary.

An old TV or monitor flyback transformer (and built in high voltage
rectifier) might serve.

4. ### JimGuest

Sorry, I accidently hit the send button on my last message.

I don't really know what the current requirements are. The capacitor bank
is around 800V @ 700uF. The time it takes to charge is not really an issue.
If it takes a while then that is fine.

I do own a flyback transformer (from a computer monitor), but I would rather

I guess it would be better to make a transformer which can generate 200V and
then use a voltage quadruppler (diodes and caps). Hopefully the voltage

I don't know how many turns to use or which wire gauge to use. The best
frequency to use is a bit of a mystery as well. Surely some formulas (or
simulators) exist, which will allow me to work out what I need. How do
people normally go about working these things out?

5. ### John PopelishGuest

First they learn about the circuit configurations available and pick
one or two good candidates. (I mentioned the need for an ability to
handle loads that vary from nearly zero volts to 800 volts as an
example.) Then they study the design of the magnetic components
needed.

I suggest you study the flyback converter (or boost regulator) from
several sources, till you have a basic understanding of the concepts.
http://www.national.com/appinfo/power/files/f5.pdf

When this study has allowed you to have a specification for the
inductive components (inductance, current levels, magnetic energy
storage capability, etc.) you need to read up on the design of the
magnetics, themselves. Many core manufacturers have tutorials at
their web sites. Lots of the design formulas are in the back of the
catalog (page 163) from Fair-Rite:
http://www.fair-rite.com/fr_catalog-14thed_rev3.pdf
but you may need a lot more hand holding than this brief reference to
understand all the design process.

6. ### peterkenGuest

one of the nice things of resonant circuits using transformers is that they
can go pretty high on the secondary side.
try using your transformer in an oscillator circuit, OR

another way is using a 12V to 220V transformer (standard 220V to 12V but
using it reversed), and use a simple tripler-circuit to "amplify" the
secondary side.
regulating the 12V side voltage will regulate the 800V side then

tripler schematic (view using notepad and fixed font)
extending the tripler can be done by adding more of the same on the dots
(gives higher voltage then)
enlarging capacitors allows more current
choose a frequency for the 12V side giving you some "resonant upswing" in
the transformer (but don't overdo it or it breaks down...)
12V ac in and doubling the secondary side gets you about 600V already, so be

o--||--+------+--||--+.....--o V+
| | |
--- --- ---
ac in / \ \ / / \
--- --- ---
| | |
0------+--||--+------+.....--o V-

Hi,

I would like to make a 800V DC power supply, which will be used to charge a
capacitor bank. The power supply will run of a 12V battery. To make the
power supply I am thinking of making an oscillator, which will generate a
12V AC signal. The 12V AC signal will then be fed to a transformer, which
will step the 12V AC signal upto 800V. I will then use diodes to convert
the 800V AC signal to approx. 800V DC.

I would like to make my own transformer (I can't find any suitable premade
transformers). I understand that the transformer will need a ratio of
800/12. But how do I determine how many turns I should have on the primary
coil? Also, how do I determine the best frequency for the transformer?
Should the frequency be as high as possible? And what about wire thickness,
how thick should it be? Do any calculators exist which will tell me all
this?

Can anyone point me in the right direction (websites or answers)?

Thanks.

7. ### Steve EvansGuest

Yes, you need a copy of a book called the 'Coil Design and
pounds Sterling. The chapter on power transformers in conjunction with
the tables in the back cover *everything* (and I mean everything -
former, core type, turns, turns ratio, wire size, winding spacing; you
name it!) you need to know. You should be able to get this excellent
book even cheaper on e-bay. Good luck!

8. ### Fritz SchlunderGuest

Greetings Jim.

be quite lengthy. There are entire books dedicated to answering just some

That said, I highly recommend reading (and/or watching the narrated slides)
of the Magnetics Design Handbook that Texas Instruments is generous enough
to provide to the world for free. You can get the goods here:

http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM401014

handbook is pretty comprehensive and contains most of the answers to your
questions, you will likely need to read it more than once while
simultaneously supplementing that reading with other books (perhaps from
your local university's science library) and personal experimentation before

Good luck Mr. Jim.

9. ### Active8Guest

On Mon, 08 Nov 2004 19:16:08 -0500, John Popelish wrote:

What, you no lika http://genome.dna.tripod.com/ ?

Actually, that nat'l doc looks ok.
And free design/selector tools. Same for the controller chips -
Nat'l, TI, Maxim, LTI ... LTI - friggin' amazing company. A new
controller every month in EDN or wtf mag it is.