more noob questions - transformers

[[email protected]]

Ok ... try not to laugh when i ask... tho i only have a few questions
on designing a home made transformer.. And of course i dont want to
burn my house down.. so lets start from there.

I would like to make my own hv transformer and thought i would start
by pulling a apart a regular 12 volt ac / dc 12 volt 1.5 amp
transformer and removing the rectification.

From what i see of ac to ac transformers on design is that mains

come into and go directly to the core however i would think there is
some kind of resistor before or hiden around the primary windings.
First questions is this dont make alot a sense to me as it is
esentially a short curcuit. However if it does actually work this
way then the windings are wrapped so that itself is a good resistor
to precent the short circuit. however that the main reason why I am
writing all this.
So my question is if there is a resistor what should it be. ( or how
can i calculate what it shoudl be) . Now i know there is a billion
formulas for making good transformers so that why i want to start
from a premade dc to and remove the rectification so i can run it
into another transformer to step it back up again. this shoudl
limit what i shoudl have to calculate for resistors if needed.

. this is again why i like to know why it works that way from the
mains as i will have to pass the secondary winding thru the primary
of the home wound step up transformer which again would be a short
circuit .. or at least what looks to be. i will put fuses ( and i
would expect resistors ) between the 2 transformers to prevent too
much current on my hv winding .
I plan on striping another laminated core . or at least the secondary
windings on a second core to wind my prefered step up

now if i use a 110 ->12vdc transformer and its a 1.5 amp . I will
choose my second core to be rated around 1 amp and my hv amperage
should end up in small ma rating for light usage without burn out.

I reviewed many diferent transformer designs and all they ever give
is is a direct input from the mains to the windings or there just
way to complicated as there beyond the scope of anythign basic.

im looking for such a basic design it just keeps the same frequency
so that why i thougth stripping the rectificatin should do from a dc
supply. I can figure out the insulation needed for the layers on my
core to prevent it from shorting out to the core or itself due to the
hv so lets not worry about that either.

If anyone could please let me know if this should really be more
complicated if it really is that simple.
Im looking simplest as possible design that will not blow up when I
plug it into the mains.

I appreciate any imput reguarding this. I have some HV experience with
whimhurst and those types of machines however not much when it comes
to current limiting. Im certianly not going to make something without
making sure i understand the safty parts of it first.

Thanks,

Caltus

 

[tempus fugit]

Ok ... try not to laugh when i ask... tho i only have a few questions
on designing a home made transformer.. And of course i dont want to
burn my house down.. so lets start from there.

I would like to make my own hv transformer and thought i would start
by pulling a apart a regular 12 volt ac / dc 12 volt 1.5 amp
transformer and removing the rectification.

come into and go directly to the core however i would think there is
some kind of resistor before or hiden around the primary windings.
First questions is this dont make alot a sense to me as it is
esentially a short curcuit. However if it does actually work this
way then the windings are wrapped so that itself is a good resistor
to precent the short circuit. however that the main reason why I am
writing all this.
So my question is if there is a resistor what should it be. ( or how
can i calculate what it shoudl be) . Now i know there is a billion
formulas for making good transformers so that why i want to start
from a premade dc to and remove the rectification so i can run it
into another transformer to step it back up again. this shoudl
limit what i shoudl have to calculate for resistors if needed.

. this is again why i like to know why it works that way from the
mains as i will have to pass the secondary winding thru the primary
of the home wound step up transformer which again would be a short
circuit .. or at least what looks to be. i will put fuses ( and i
would expect resistors ) between the 2 transformers to prevent too
much current on my hv winding .
I plan on striping another laminated core . or at least the secondary
windings on a second core to wind my prefered step up

now if i use a 110 ->12vdc transformer and its a 1.5 amp . I will
choose my second core to be rated around 1 amp and my hv amperage
should end up in small ma rating for light usage without burn out.

I reviewed many diferent transformer designs and all they ever give
is is a direct input from the mains to the windings or there just
way to complicated as there beyond the scope of anythign basic.

im looking for such a basic design it just keeps the same frequency
so that why i thougth stripping the rectificatin should do from a dc
supply. I can figure out the insulation needed for the layers on my
core to prevent it from shorting out to the core or itself due to the
hv so lets not worry about that either.

If anyone could please let me know if this should really be more
complicated if it really is that simple.
Im looking simplest as possible design that will not blow up when I
plug it into the mains.

I appreciate any imput reguarding this. I have some HV experience with
whimhurst and those types of machines however not much when it comes
to current limiting. Im certianly not going to make something without
making sure i understand the safty parts of it first.

Thanks,

Caltus

There is no resistor in a transformer. There are 2 coils. The one that the
mains comes in contact with is called the primary. The secondary does not
make any contact at all with the primary, and therefore not the mains
either. They are 2 separate coils wrapped around a piece of metal (iron I
think). Since the mains voltage is AC, and therefore moving rapidly from one
direction to the other, it induces a current into the secondary. The voltage
you get out of the secondary depends on the ratio of the # of turns in the
primary to the # of turns in the secondary. So if you have 10x as many turns
in the primary than the secondary, there will be 12v (assuming 120v at the
primary) on the secondary. As far as choosing the type of wire or number of
wraps, that's a little out of my league, so I'll leave that to teh more
experienced in the group.

As always, be very careful when working with mains voltages......

 

[[email protected]]

Thanks for your reply. Thats the way it looked to me without a
resistor and i just wanted to make sure as primary going into the
mains still is not clear to me why it dont short out.
If thats the case does transformers come some power all the time?
they must. And if what im doiung is essentially chaining a few
together . i would think they even comsune more power without a load?
or is it the other way. because there is no load it prevents
current on secondary as the equal and opposite fields react as a
balanced resistor until the current is used? Is that right? if this
is the case the ammount of efficency loss in transformer i would think
would be measureable to the constant power drain of a transformer with
no load?
Like i said im a noob to this so I just want to make sure i
understand this the best i can befire i start with this.
and again thanks for your help.

Caltus

 

[John Fields]

Ok ... try not to laugh when i ask... tho i only have a few questions
on designing a home made transformer.. And of course i dont want to
burn my house down.. so lets start from there.

I would like to make my own hv transformer and thought i would start
by pulling a apart a regular 12 volt ac / dc 12 volt 1.5 amp
transformer and removing the rectification.

---
First off, what you have is _not_ a transformer, it's an AC to DC
converter or, simply, a "power supply"

The transformer is the device in the power supply with two leads
connected to the mains terminals and the other leads connected
either to four rectifiers (two leads) or to two rectifiers and to
the output common terminal (three leads)
---

From what i see of ac to ac transformers on design is that mains
come into and go directly to the core however i would think there is
some kind of resistor before or hiden around the primary windings.

---
No resistor is needed because the inductive reactance of the
primary winding (with an unloaded secondary) will limit the amount
of current in the primary to what is needed to magnetize the core
and supply the various winding and core losses.
---

im looking for such a basic design it just keeps the same frequency
so that why i thougth stripping the rectificatin should do from a dc
supply. I can figure out the insulation needed for the layers on my
core to prevent it from shorting out to the core or itself due to the
hv so lets not worry about that either.

If anyone could please let me know if this should really be more
complicated if it really is that simple.
Im looking simplest as possible design that will not blow up when I
plug it into the mains.

---
The simplest possible way to get what you want would be for you to
unwind the primary and count the number of turns as you unwind it.
Record that number so you won't lose it.

The output voltage of the transformer can be described by:


Ep Ns
Es = -------
Np


Where: Es is the voltage induced across the secondary,
Ns is the number of turns on the secondary,
Ep is the voltage impressed across the primary, and
Np is the number of turns on the primary.

so, rearranging to solve for the number of turns on the primary and
assuming you found 100 turns on the secondary:


Ep Ns 120V * 100t 12000 Vt
Np = ------- = ------------- = ---------- = 1000 turns
Es 12V 12V


Now, rearranging to solve for the number of turns required on the
secondary for any secondary voltage desired:


Es Np
Ns = -------
Ep


Assuming that you want 1000 volts out of the secondary:


Es Np 1000V * 1000t 1E6 Vt
Ns = ------- = --------------- = -------- = 8333 turns
Ep 120V 1.2E2V


So now you have the problem that you've got to get about 83 times
more turns on the secondary than you took off.

Maybe it'll work, maybe not. Let's try to find out.

A transformer is a device that transfers _power_ from the primary to
the secondary, so if your transformer is rated to supply 1.5 amps at
12V, then that's:


P = IE


where: P is power in watts,
I is current in amperes, and
E is voltage in volts,


so:


P = IE = 12V * 1.5A = 18 watts.


Rearranging to determine how much current your new secondary can
supply:

P
I = ---
E

and for a 1000V secondary, that's:


P 18W
I = --- = ------- = 0.018 ampere
E 1000V


OK. Now we have to determine what wire size to use and whether it'll
fit in the space the old secondary used to fit into. Assuming about
750 circular mils per ampere, your old wire was about #20, with a
diameter of about 0.032". At the same current density, your new
wire would have an area of:


0.018A * 750CM
A2 = Ik = ---------------- = 13.5 circular mils,
1A


Which fits in somewhere between #38AWG with an area of 15.72
circular mils and #39 with an area of 12.47 circular mils.

However, the enamel (and you'll need double or triple thickness
because of the high voltage requirement)covering the wire will add
to the area drastically, (remember that area goes with the _square_
of the radius) making me think you won't be able to do it.

Also, I've assumed a perfect transformer with no core or copper
losses, and that makes it even more likely that you won't be able to
build a high voltage supply just by rewinding the secondary,
especially if (as indicated below) you're expecting to reach
voltages approaching those generated by Wimshurst machines

 

[Anders N.Vinje]

Thanks for your reply. Thats the way it looked to me without a
resistor and i just wanted to make sure as primary going into the
mains still is not clear to me why it dont short out.
If thats the case does transformers come some power all the time?
they must. And if what im doiung is essentially chaining a few
together . i would think they even comsune more power without a load?
or is it the other way. because there is no load it prevents
current on secondary as the equal and opposite fields react as a
balanced resistor until the current is used? Is that right? if this
is the case the ammount of efficency loss in transformer i would think
would be measureable to the constant power drain of a transformer with
no load?
Like i said im a noob to this so I just want to make sure i
understand this the best i can befire i start with this.
and again thanks for your help.

Caltus

Hello!
Th reason the coil or inductor does not short out is beacause the voltage i
AC. Inductors are like capacitors, they store energy. However real world
inductors do have some small resistance allso. In the inductor the energy
is converted to a magnetic field. So when you turn of the power, the
magnetic field collapses and is converted back to current. But very
importently the current is now going in the reverse direction! With a
capacitor the current will not change direction. It`s more like charging a
battery.
So when you have two inductor close to each other the magnetic field in
one(primary) will be converted a current in then other(secondary). But if
the secondary inductor has no load, no power will be used there.
But like i wrote before some of the power will be lost in the inductors
because of the internal resitance in the wires and thus generate heat. And
heat is the problem. When you buy a transformer it is rated in VA[Voltage x
Ampere]. You might think why not use Watt instead? That V*I too. True, but
only for resistance. For reactive elements like inductor and capacitors
there is a phaseshift between voltage and current. The "resitance" you see
in such a element is called impedance Z and is dependent on the frequency of
the AC signal.

So when you connect a reactive element in a circuit you must watch out
because the the current can become quite high. A typical reactive device is
a electrical motor or transformer. You often have to compensate this by
inserting a capacitor so the phaseshift is minimal. In big factories that
use alot of electrical engines this is requirement or the power company will
cut the power.

Hope this helped a litle.
If you want to know more about reactive elements and such you should take a
look at laplace transform of electrical circuits. Its much easier than using
diffrental equations

Good luck

Anders

 

[Bob Myers]

not short out is beacause the voltage i AC. Inductors are like capacitors,
they store energy. However real world inductors do have some small
resistance allso. In the inductor the energy is converted to a magnetic
field. So when you turn of the power, the magnetic field collapses and is
converted back to current. But very importently the current is now going
in the reverse direction! With a capacitor the current will not change
direction. It`s more like charging a battery.

Sorry, no. The current does NOT change direction - in fact, that is one
of the key things to learn about inductors. (The voltage ACROSS the
inductor is another story, however.)

Two simple rules:

- Capacitors oppose changes in voltage (i.e., a capacitor will "try" to
maintain the voltage across it at the same level despite changes in the
current through it), while...

- Inductors oppose changes in current.

If you try to interrupt the current flowing through a conductor, energy
will come back out of the magnetic field (where it has been stored)
in such a way as to try to keep the current going at the same level.
This will result in a (possibly very large) voltage spike across the
inductor, if that's what it required to try to keep the current at the
same level (as would be the case if you suddenly switched a large
resistance into a series connection with the inductor, or simply tried
to open the circuit with a switch - which is why switches that are
switching large inductive loads very often "arc over").

This is expressed by the equation:

V = L(di/dt)

If you aren't familiar with calculus, all this is saying is that the voltage
across an inductor is proportional to the time rate of the change of the
current. Note that if the change is negative, so is the resulting voltage.

So when you have two inductor close to each other the magnetic field in
one(primary) will be converted a current in then other(secondary). But if
the secondary inductor has no load, no power will be used there.

But like i wrote before some of the power will be lost in the inductors
because of the internal resitance in the wires and thus generate heat. And
heat is the problem. When you buy a transformer it is rated in VA[Voltage
x Ampere]. You might think why not use Watt instead? That V*I too. True,
but only for resistance. For reactive elements like inductor and
capacitors there is a phaseshift between voltage and current. The
"resitance" you see in such a element is called impedance Z and is
dependent on the frequency of the AC signal.

However, an ideal transformer with a purely resistive load does NOT present
a
reactive load at the primary, which may be a source of some confusion here.
Any
apparent reactance seen at the primary (and the resulting phase shift)
would come
from the characteristics of the load itself and from imperfect coupling
between
the primary and secondary windings (i.e., not all of the field from the
primary
winding "links" to the secondary, and so that portion of the field accounts
for
some inductance apparent from the primary side).

So when you connect a reactive element in a circuit you must watch out
because the the current can become quite high. A typical reactive device
is a electrical motor or transformer.

But again, a transformer per se is not NEARLY as reactive as you
might think it would be from just looking at the primary winding. Neither
is a motor. (Can you see why? Think about where the energy in the field
is going...).

Bob M.