transformer secondary coupling

[Jamie M]

Hi,

If the secondary of a push-pull ferrite transformer requires significant
HV clearance from the ferrite core will there still be enough coupling
for it to operate efficiently assuming the primary is wrapped right
onto the core? Also is it possible to use a simple ferrite rod as a
transformer at least somewhat efficiently? For making a secondary with
high clearance a ferrite rod is a lot easier than using a toroid or
core. I was thinking of a 1" diameter, 6" long ferrite rod, with a
single layer secondary 3" diameter and 6" long.

cheers,
Jamie

 

[Tim Williams]

Ferrite, or ferroxcube, or some company, published a single page of

EXACTLY what you're asking. The chart shows EFFECTIVE permeability of
a rod versus length/diameter ratios [plus rod's permeability, which
once more than a certain amount has little impact]


Not exactly; OP is talking about a transformer. Rods make poor transformers
due to the low inductivity and k. If you put another rod on the other side,
and some flat pieces to bridge across them and make a connected magnetic
path, you'll do alright. Such construction looks very much like the average
flyback transformer core.

The page you're thinking of may very well be,
http://myweb.msoe.edu/williamstm/Images/Rod_Core_Pressman_Billings_Morey.png
I snipped it from a book online by Pressman, Billings and Morey; evidently
they got it from Micrometals, but I don't recall finding it on their website
(it's probably buried in their literature somewhere).

Note also that the inductivity (uH/t^2) varies with position, so that turns
towards the center have the highest, and turns at the exact end have about
half the inductivity. These figures are the average over a winding covering
the entire core. The turns on the end contribute little, so they are often
omitted (which likewise raises the apparent average), as can be seen in most
commercial parts. Those turns probably have higher eddy current losses,
too, further lowering the Q.

Tim

 

[Tim Williams]

Has reasonable efficiency if you're doing a resonant converter. You may
even need to add leakage inductance (assuming you start with a closed loop
core, like a flyback transformer core). Tricky part is calculating how
much.

An open rod makes a terrible transformer, but it can still be usable.
Before deflection was integrated, the oldest prototype TV sets used a (air
cored) Tesla coil, or rod-cored construction, to generate HV from an
oscillator. Later on, the whole deal (HV, focus, deflection, isolated
filament voltages, auxiliary power and sync signals) was integrated, and the
flyback transformer we know today has changed little since.

Tim

 

[Jamie M]

In general, yes.


Also is it possible to use a simple ferrite rod as a


There will be a lot of pri-sec leakage inductance, so the secondary,
as a voltage source, will be soft, relatively high impedance. If you
only need a small output current, it might work. You could resonate
one or both windings just for fun.

How much output voltage and current do you need? AC or DC load?

Hi,

It is probably 50kV and 50mA max DC load.

cheers,
Jamie

 

[Phil Allison]

"Jamie Moron "

It is probably 50kV and 50mA max DC load.

** You musty be utterly INSANE !!!

Piss off imbecile.

 

[Wimpie]

El 05-05-12 23:10, Jamie M escribió:

Hi,

If the secondary of a push-pull ferrite transformer requires significant
HV clearance from the ferrite core will there still be enough coupling
for it to operate efficiently assuming the primary is wrapped right
onto the core? Also is it possible to use a simple ferrite rod as a
transformer at least somewhat efficiently? For making a secondary with
high clearance a ferrite rod is a lot easier than using a toroid or
core. I was thinking of a 1" diameter, 6" long ferrite rod, with a
single layer secondary 3" diameter and 6" long.

cheers,
Jamie

increasing the diameter of the secundary winding does reduce the
coupling, but doesn't reduce the sec to sec coupling (for you push
pull stage).

Try to figure out whether the prim or sec leakage inductance can be of
use (for example current limiting).

You will very likely run into partial discharge and/or corona
discharge problems.

What type of circuit are you using (current type or voltage type push
pull)?

Regarding the output is at 50kV at 50 mA, or 50kV no load, 50mA short
circuit?

 

[Fred Abse]

Hi,

If the secondary of a push-pull ferrite transformer requires significant
HV clearance from the ferrite core will there still be enough coupling for
it to operate efficiently assuming the primary is wrapped right onto the
core? Also is it possible to use a simple ferrite rod as a transformer at
least somewhat efficiently? For making a secondary with high clearance a
ferrite rod is a lot easier than using a toroid or core. I was thinking
of a 1" diameter, 6" long ferrite rod, with a single layer secondary 3"
diameter and 6" long.

Sounds like a six inch air gap, and a field that'll couple to anything
that happens to be passing.

 

[Jamie M]

El 05-05-12 23:10, Jamie M escribió:

increasing the diameter of the secundary winding does reduce the
coupling, but doesn't reduce the sec to sec coupling (for you push pull
stage).

Try to figure out whether the prim or sec leakage inductance can be of
use (for example current limiting).

You will very likely run into partial discharge and/or corona discharge
problems.

What type of circuit are you using (current type or voltage type push
pull)?

Regarding the output is at 50kV at 50 mA, or 50kV no load, 50mA short
circuit?

Hi,

I don't know the exact required current, 50kV at 50mA is an upper
rough estimate of the power required.

cheers,
Jamie

 

[Jasen Betts]

Hi,

If the secondary of a push-pull ferrite transformer requires significant
HV clearance from the ferrite core will there still be enough coupling
for it to operate efficiently assuming the primary is wrapped right
onto the core?

as long as it gets close enough pass through the hole it should work
as a transformer.

Also is it possible to use a simple ferrite rod as a
transformer at least somewhat efficiently?

for very low values of "somewhat"

For making a secondary with
high clearance a ferrite rod is a lot easier than using a toroid or
core. I was thinking of a 1" diameter, 6" long ferrite rod, with a
single layer secondary 3" diameter and 6" long.

look at the high voltage transformers used in old CRT displays,
it's a square ring with HV winding on one side ans LV on the other.

 

[Jamie M]

"Jamie Moron"




** You musty be utterly INSANE !!!

Piss off imbecile.

Another project worthy of the "Phil Allison seal of approval"

 

[Jamie M]

as long as it gets close enough pass through the hole it should work
as a transformer.


for very low values of "somewhat"


look at the high voltage transformers used in old CRT displays,
it's a square ring with HV winding on one side ans LV on the other.

Hi,

For a custom ferrite core prototype, how well would packed ferrite
powder work compared to a solid ferrite core? I guess the inductance
would be lower due to the larger distributed air gap. I was thinking
of making a wood or plastic form to hold the ferrite powder and also
act as the primary to secondary insulation.

cheers,
Jamie

 

[Phil Allison]

"Jon Elson"
Jamie Moron :
Phil Allison

Well, but he's right! You certainly cannot do 2.5 KV with a
little ferrite rod. And, 50 kV will require some large separation
of windings.

** That was not my reasoning.

The OP is clearly a clueless, trolling idiot with a death wish.

His motives for wanting such a supply are deeply suspicious and he has not
one tiny clue of the safety or design issues involved in such high voltages.

Shame here on anyone who assists his lunacy.


..... Phil

 

[Jamie M]

"Jon Elson"
Jamie Moron :
Phil Allison


** That was not my reasoning.

The OP is clearly a clueless, trolling idiot with a death wish.

His motives for wanting such a supply are deeply suspicious and he has not
one tiny clue of the safety or design issues involved in such high voltages.

Shame here on anyone who assists his lunacy.

Hi Phil,

I've made a handwound tesla coil before so try to be careful around
HV. The purpose of the power supply is for an electron beam
melter/welder.

cheers,
Jamie

 

[Jamie M]

You could take several single winding 4 connection autotransformers
and 'stack' them. Say with 5, each one would only 'see' 10kV each.

Hi,

Thanks that is interesting and could allow using smaller standard
ferrite cores instead of a custom large toroid core. How would the
efficiency of five stacked autotransformers compare to a single
transformer? I'm not sure about driving the autotransformers though, I
guess there would need to be extra floating HV supplies to drive them.

cheers,
Jamie

 

[Phil Allison]

Jamie Moron :

I've made a handwound tesla coil before so try to be careful around
HV.

** Thought you must be one of THOSE NUT CASES !!

The purpose of the power supply is for an electron beam
melter/welder.

** You must be utterly INSANE !!!

Piss off - bloody imbecile !!!!!!!!!!!!!!!!!!

 

[Jamie M]

No need for 'extras'

simply drive the coil at GND potential and the voltage multiplies up
to 50kV
using four autotransfomers with four connections like such:
d-
.
c
.
.
.
.
b
.
a


first transformer GND a, drive b. at d you have 10kV

2nd transformer connect a2 to c1 and b2 to d1, at d2 you have 20kV

3rd transformer connect a3 to c2 and b3 to d2, at d3 you have 30kV

and so on, until at d5 you have 50kV

*IF* you build the structure properly, you will even have controlled
gradients that will handle the inevitable discharges WITHOUT
catastrophic failures. Just envision how a diode stack is constructed
using rings.

If the efficiency of your transfomer is 98%, then you have an overall
efficiency of 90% only 150watts, HOT!

99% you have 95% and only 75 Watts still HOT
can you build your transformers to have 99.5% efficiency? then you're
talking about 37 watts, not so bad.

Remember therule of thumb for heat in still air
1 C rise for every watt being dissipated over 100 sqin.

If your structure has 100 sqin of 'good' surface, we're talking 40C
rise above ambient, not bad, but not good either.

Oh, did you want DC at 50kV to supply 1500w? then good luck. Can't
even imagine the diode stack to do that! From memory diode stacks are
lucky to be 70-80% efficient. but it is doable. You can stack
commercial DC supplies *IF* you do it carefully.

or make your own, probably a 7 ring stack would compromise well.

The efficiency goes to hell as you try to use unmatched components and
MUST add balancing circuitry else a discharge will....just say it's
dramatic.

To increase efficiency, you could add some complexity, drive high
voltage components to act like diodes, that exhibit extremely low Von.
But we're talking complexity.

Hi,

Thanks, would vacuum tube diodes work at 10kHz+ and 50kV/50mA?

I am using a vacuum chamber so maybe could put thermionic diodes
in a full bridge configuration right in it?!

cheers,
Jamie

 

[Fred Abse]

Thanks, would vacuum tube diodes work at 10kHz+ and 50kV/50mA?

Early color TVs used a thermionic diode as a half-wave rectifier with
half-sine pulses >25kV, about 20 microseconds wide at a PRF of 15.75kHz.
About a milliamp DC out. Directly heated filament, supplied from a
well-insulated few turns loosely around a flyback transformer limb.

I doubt anybody makes them, now. Somebody like Perkin Elmer might still
make similar diodes to special order, at megabux.

If you only want a few, swap meets might be the best bet. I can't recall
the tube number, but I bet Mike (Terrell) can.

 

[Tauno Voipio]

Hi,

Thanks, would vacuum tube diodes work at 10kHz+ and 50kV/50mA?

I am using a vacuum chamber so maybe could put thermionic diodes
in a full bridge configuration right in it?!

cheers,
Jamie

How are you thinking to avoid being grilled by X-rays? Even the tube
rectifiers generate an appreciable amount of X-rays at those voltage
and power levels.

 

[Spehro Pefhany]

How are you thinking to avoid being grilled by X-rays? Even the tube
rectifiers generate an appreciable amount of X-rays at those voltage
and power levels.

In the old color TV sets, the diodes were in a metal box, both to keep
the 30kV away and to contain the X-rays.

 

[amdx]

Hi Phil,

I've made a handwound tesla coil before so try to be careful around
HV. The purpose of the power supply is for an electron beam
melter/welder.

cheers,
Jamie

How about taking the easy route and get some *old style neon sign
transformers. I think a standard rating was 15kv at 30ma, so it may take
series/parallel combo to get to 50kv at 50ma. I'll let you pick the
rectifiers.
I ran a couple in series one time and had sparks coming out of the
ground, yikes!
Mikek