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Can FBT be used for Electrostatics?

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paris

Mon Aug 02 2010, 03:07AM

Registered Member #3042 Joined: Wed Jul 28 2010, 12:36AM
Location:
Posts: 121

Hey People,
6 months ago I made a PS copied off a Kenwood kac716 car stereo ,4700uF, 2 x irf540 job ,for plugging coils into ,it has terminals so I can readily shove anything in and go. Centre tapped or not.
Its TL494 at 178khz but Ive got hold of a FBT ,wound 5 turns and plugged into ONE mosfet only so thats 89khz.

What Im wondering is ,
1 would it be easy enough to smooth the HV op to get DC ?
2 Should I aim for lower freq pulse ?
3 Is fullwave mazilli type the way to go ?
4 Can it be treated similar to a LV op filter circuit layout ,using HV components ?

I want to play around with e.s. without the trans internal arcing etc of being open ,my guess would be a HV "reservoir"
cap ....such a thing.. maybe shunt cap and diode
Any suggestions would be wicked ,Thanks , MORE POWER TO YA'S

Mattski

Mon Aug 02 2010, 06:20AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

1 would it be easy enough to smooth the HV op to get DC ?
Yep, it's pretty easy. You just need to pick a big enough filter capacitor. First you have to pick an acceptable voltage ripple in your output, for example 5-10% of your output voltage, and what is the maximum current draw where you want to satisfy that ripple specification. That tells you how big of a capacitor you need to filter the output:

2 Should I aim for lower freq pulse ?
As long as the core isn't heating up from high frequency losses I'd leave it. A higher frequency means you can get away with a smaller filter capacitor (see above link).

3 Is fullwave mazilli type the way to go ?
I'm not familiar personally with all the flyback driver topologies.

4 Can it be treated similar to a LV op filter circuit layout ,using HV components ?
Yes, the circuit principles are all the same, you just have to be sure you don't have arcing between any components, and depending on voltage you'll have some loss to corona.

I want to play around with e.s. without the trans internal arcing etc of being open ,my guess would be a HV "reservoir"
cap ....such a thing.. maybe shunt cap and diode
Sorry, I'm not sure what you mean here.

paris

Tue Aug 03 2010, 06:59AM

Registered Member #3042 Joined: Wed Jul 28 2010, 12:36AM
Location:
Posts: 121

Hey Thanks Mattski,
Ive been curious about the filtering bit. A HV "filter" cap and a choke may be the go , Ill have a crack and see what happens.
Ive had a sniff around and apparently these Lifter guys are up to similar stuff ,didnt know what one was till now,
how bads that .
Thanks again

Dr. Dark Current

Tue Aug 03 2010, 08:12AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

with such a high frequency the output will drop a lot under load (because of leakage inductance) and depending on the transformer used you might get just little voltage out (if the frequency is above the the system's resonant frequency). For single mosfet I'd use somthing like 20-40kHz.

paris

Wed Aug 04 2010, 02:59AM

Registered Member #3042 Joined: Wed Jul 28 2010, 12:36AM
Location:
Posts: 121

FBT = NEC #47105422 , Im lucky enough that the local scap metal guy keeps FBT , mic caps and diodes for me so Ive got a good supply ,not bad ...free .Yay

The single mosfet isnt triggered by feedback coil so its not a resonant circuit as far as I can see , but the 20 khz range sounds more practical ,its closer to stanard TV 16khz.
My understanding is that the FBT uses single transistor to take advantage of the energy transfer and the air gap.
But if its powered by 2 transistors then that goes out the window and it then relies on winding ratio instead.

I like the idea of the fundamental design so Ill stick to single input and follow all the imfo available on the forum.
Very interesting stuff going on and lots of talent .

Antonio

Sun Aug 08 2010, 10:59PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Most flyback transformers have an internal rectifier and a storage capacitor, so the output is DC. In most cases they are designed to operate at about 15 kHz. More than this in some CRT monitors.
It's easy to obtain about 25 kV, possibly more, what is enough for most electrostatic experiments.

radiotech

Mon Aug 09 2010, 12:27AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The FBT generates HV by di/dt collapse of the field, but most important of all, while the current is building it is also, by connection to the yoke deflecting the CRT beam in the scan. When the field collapses, and the HV pulse is created, the beam is pulled back, similarly by the yoke/fbt circuit. It takes 63 uSec to sweep and 13 to get back. In order to do this a 35 uSec conduction time is needed in the HO device.
If you booted the FBT with a 16kHz square wave at about 20 watts like the TV, but sans yoke, damper recovery and HV load you likely would flash it over.

Antonio

Mon Aug 09 2010, 01:10AM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

You can operate a FBT as a regular transformer, but with the regular input coil used the voltage gain will be of about 20 only. With an external coil with less turns it's possible to increase this, but the internal diode may break down due to too high reverse voltage in this mode of operation.

paris

Mon Aug 09 2010, 02:08AM

Registered Member #3042 Joined: Wed Jul 28 2010, 12:36AM
Location:
Posts: 121

G'day
The basis of my theory is... DC cant get thru a cap so why not build a HV cap and see what happens ?
If DC cant get thru cap ,then all thats left is static field..... correct? Uc = .5 C V ^2
The thing with the FBT is that it has 1/2 wave out put no matter what you throw at the input youve got the single diode at the end of the chain.
They reakon the CRT is smoothing the pulse with approx 500pf , but being 1/2 wave theres gotta be humungus ripple,surely , there must be dead time in there too.

Im no expert even on a good day but there has got to be current smoothing and all I can see is maybe the coil around the CRT ,(not the degauss coil).
Ive heard the human body generates 10kv up wards of static ,so I reakon the FBT can deliver on that one easy but I dont want to get internal arcs and stuff it as an open coil.
Im curious if the internal cap has to do with the winding capacitance.

My intention would be to make a tin foil cap with air gap wide enough to see effects , having 2 plates ,I cant imagine there being any thrust like the lifters .
I havent seen any solid state e.s machines theyre all rotary like Wimhurst etc so Im having a crack at it myself
..Thanks for imfo guys

Antonio

Mon Aug 09 2010, 03:11PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

As I said, most flyback transformers have an internal diode (actually several) and an internal capacitor. The capacitance is in the few nF range. The output is DC with a small ripple, because there is a bleeder resistor and the focus potentiometers too as load. Look in Google images for "flyback transformer schematic" and you will obtain some examples. Of course you can add more capacitance if you want. Just be careful because you will not like the shock from 1 nF of capacitance charged to 25 kV...
An electrostatic generator generates high voltage by a different mechanism, but the end product is the same.

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