---Tutorial under development---

LOPT preparation tutorial --Volume {A}-- (Introduction through step 2-11 )

Last Action: Jan 23 2020


Scope of series:

Central topics:
This tutorial is one in a (prospective) series with focus upon low ripple, medium current/power PSU solutions for radiography and other modest photon energy (≤ 2 MeV) particle accelerator applications - and, to a lesser extent, exploration of certain 'current intensive' electrostatic phenomena (e.g. electret photography, electret formation, electrokinesis, establishment of electrostatically 'cordoned' (atmospheric) pressure gradients, high volume ion-flow systems, etc...

Peripheral and extraneous topics:
While everyone is welcome to view and post questions (within the confines of this site's TOS/UA) - Please be advised that those having interests confined to the following topics/applications will not likely find this series particularly useful (owing to the availability of significantly less 'labor-intensive' solutions to same):

--'Pedestrian' and 'Quasi'/'Peri'-EHT applications:
(e.g. CRTs, PMTs/Image tubes, Gas discharge devices, lasers, ionization air purifiers, TC primary drivers, GM tubes, etc...)
--Low current demonstration topics (e.g. 'Lifters', 'Jacob's Ladders', 'Plasma Spheres', 'Kirlian photography', 'Ion Motors', etc...)
--Capacitor discharge demonstration: (e.g. 'Quarter Shrinkers'/'Can Crushers', 'Rail/Coil Guns', 'EMP generators', electrical weapons, etc...)

Please be advised that site policy prohibits my assistance with most inquiries coming under the latter category...

Scope of this tutorial

Provision of a detailed, 'step by step' procedure for modification of 'heavy duty'
AC LOPTs for 'serious' EHT application/research...

Said procedure having the following principal goals:
-- Improvement of insulation integrity via removal of auxiliary windings coaxial to the EHT winding.
-- Enhancement of versatility via electrical 'floating' of the EHT winding.
-- Appropriate adjustment of reluctance gaps.

The desirability of LOPTs:
Of the commonly available magnetic components applicable to low through medium power EHT production, the LOPT offers the advantages of relatively high power intermittent operation (several hundred to >1kW per transformer) and useful continuous power operation (to Ca. 600 Watts each transformer under forced air cooling). Moreover, LOPTs' amenability to resonant (and, hence, sinusoidal) drive permits development of systems productive of highly desirable mean output EMFs (≈70% peak) as opposed to the rather low (< 1% through 5% 'spike' EMF) characteristic of flyback and similar pulse driven/energy storage topologies. Additionally, the inherent output symmetry attending resonant drive topologies facilitates implementation of effective external EMF 'multiplication' schemes.

Prominent among the LOPT's disadvantages are the common expedient of integral rectification/EMF multiplication circuitry and 'end point' grounding schemes --- While the former objection is readily addressed via judicious transformer selection, the latter requires minor 'reconstruction' - said procedure being the subject of this tutorial.

A few words regarding the unsuitability of DSTs and their ilk:
1) Integral components electrically unsuited to general application.
2) Practical impossibility of modification for improved EHT winding to core insulation.

Re: point 1 (above) Although asymmetrical drive may permit peak forward (and hence output) EMFs well in excess of the internal rectifiers' PIVs - such is, of course, inapplicable to 'filtered' or otherwise capacitively loaded schemes sans additional external rectifiers - moreover, 'integral rectification' greatly complicates cascading of multiple LOPTs for increased EMF and/or development of balanced output schemes...

Re: Point 2 (above)
'DC' LOPTs are constructed as indivisible units sans 'margin' for modification -- moreover (in the case of DSTs) the rectifiers are 'distributed' throughout the winding...

LOPT Selection:
The most desirable transformers are those intended for direct production of full (AC) EHT potential for application to external 'non-multiplicative' rectification pursuant to development of accelerating potentials for large CRTs -- Contrary to 'conventional wisdom' such units are yet readily available from long-established electronics supply houses (whom, in my experience, are only too happy to wave 'minimum purchase' and 'wholesale only' policies at the prospect of disposal of what they regard as 'dead stock') --- That said, one should expect to size their purchase so as to offset the supplier's 'invoicing costs'... Prices ranging from $20 (USD) per transformer (in reasonable quantities) are not uncommon...

The following images are offered as an aid to recognition of the desired form factor:

Assorted LOPTs typical of the desired geometry:

Detail - Axial view:

Detail - Transverse view:

The modification procedure:
--Please do not commence disassembly of the transformer prior to reading and comprehension of the entire tutorial...

Required materials:
Note: the quantities/dimensions cited below represent those necessary for completion of the modification - Minimum purchase requirements (while reasonable) will, in most cases, be significantly greater...

Acetone -- QTY: 946ml ‘tin’
Acrylic based hot-melt compound – Example: Surebonder Acrylstik - QTY: 100g
CPVC tubing (3/4” Nom. id {CTS}) --- QTY 120mm (lineal)
⇒Elastic bands (latex) --- QTY ≈ 40g assorted sizes (≈20mm -- ≈70mm unloaded diameter)
Electronics grade RTV Silicone -- Example: GC 19-155 – QTY: 90 ml
Polyolefin heat shrink sleeves (3/4" untreated id) -- Example: Gardner Bender HST-750 – QTY: Three untreated 4” {102mm} sleeves
Polypropylene pharmaceutical vial ( ~33mm Dia, ~64mm Len.)
PTFE sheet stock – Example: http://www.eplastics.com/Plastic/ptfe_sheet -- QTY: 100mm x 60mm rectangle of 0.093” {2.35mm} stock and 40mm x 20mm rectangle of 0.015” {0.381mm} stock

#4 solder lug (‘inside-tooth’ fastener surface)
#4 { ≈7/64” ≈3mm} bit
#4 nut and bolt
#6 {0.83mm} burr or bit

Recommended implements:
Note 1: The reader is assumed to be in possession of standard electronics/basic ‘hand’ - tools...
Note 2:
The reader is advised that 'workarounds'/'alternates' are possible for the following - please let us know should you have any questions...

⇒Dremel Moto-Tool or Equivalent
⇒Locking forceps
⇒Micrometer (Vernier)
Range = 0-25mm / Res=1um {metric} -OR- Range = 0-1" / Res= 1/10,000" {imperial}
⇒Nitrile Exam Gloves
or 'X-ACTO' set
⇒Sims Uterine Curette (#3 sharp, fenestrated) -- Example: Sklar 90-5423
⇒Stainless Picks (non-magnetic)
⇒Tweezers (‘splinter’)
Welder's Gloves (Supple varieties preferred for maximum dexterity)

=== Part 1 -- Disassembly of the LOPT unit ===

Step 1-1) Identify construction style of LOPT to be optimized.

Identify construction style via the description most closely matching the reader's device in the following...

Full EMF AC LOPTs feature one of two general construction styles as outlined below:

Style 1) Features a single piece frame ('clamp') -- the rails of which maintain compression on the core via a single linear ligature fashioned of a metallic strap or bolt extending from frame rail to frame rail across the top of the upper form leg 'facet' or 'furrow'.

Example of style 1

Style 2) Features two independent metallic frame pieces OR a metallic half-frame and a backing plate/board -- such assemblies are bound by a single "U- bolt"/"U-strap" or two linear bolts/straps extending along the outer aspects of each horizontal form leg...

Examples of style 2

View of LOPT from 'closed' side of "U-bolt" ligature

View of LOPT from 'open' side of "U-bolt" Ligature

View of LOPT from 'closed' side of "U-strap" Ligature

View of LOPT from 'open' side of "U-strap" Ligature

Step 1-2) Removal of form frame ('clamp')

Remove any tape, looped conductors, ground leads, etc. ensnaring/bonding the frame to the form.

2) Free each end of the frame ligature{s}

3A) In the case of cylindrical bolt{s} or a cylindrical shaft 'U-Bolt':
Remove backing fasteners (i.e. nut{s}/burs{s},etc...) then carefully withdraw the bolt{s} from the assembly.

3B) In the case of independent strap{s}, or a 'U-Strap': Straighten and/or unsolder each fastening 'tab' then splay the frame rails such that the frame 'ears' clear the ligature 'tabs' --Or-- if so constructed, remove the backing plate/PCB from the ligature ends ('tabs').

4) Remove the transformer (i.e. form and winding assembly) from the framing/clamping/mounting components -- please retain the frame and all associated hardware!

Image of transformer following removal from a single strap-ligature frame:

Annotated image of assembly features referenced in following steps:

Step 1-3) Procedure for removal of strap ligature{s} from LOPT core/winding assembly:

Meticulously straighten and deburr exposed segments of the frame ligature{s}.

2) Carefully 'work' the frame ligature{s} free of the adhesive, then withdraw it/them from the space intervening the form and winding (and, hence, from the assembly) -- Under no circumstances apply counter-thrust or torque to the EHT winding!

Ligature partially withdrawn from assembly.

Ligature (in foreground) fully withdrawn from assembly.

Note: Inasmuch as same will be removed and discarded in later steps, damage to the external auxiliary windings (i.e. those on the form leg opposite the EHT winding) or coaxial auxiliary windings immediately adjacent to the core/form leg is of no consequence -- Care must be exercised, however, to avoid damage to the brittle ferrite form/core via excessive force or mechanical shock... Also - avoid handling the assembly by the EHT winding!

Step 1-4) Removal of external auxiliary windings from 'lower leg':

Using an appropriate scalpel, 'X-Acto knife' or similar implement, linearly section the full length of the external Aux winding assembly. --- Again, the referenced winding resides on the form leg opposite that carrying the EHT winding!!!

2) Peel, crumble and/or break the 'external' Aux winding assembly off of the lower form leg.

Step 1-5) Separation of form pieces

Caution: In performance of the following tasks, it is of paramount importance that neither torques nor linear forces are applied to the EHT tyre! -- Moderate forces may be applied to the auxiliary shell and moderate compressive force may be applied to the ferrite form longitudinal to the EHT winding 'leg' only!


Please note that task #1 is with reference structures coaxial to the EHT winding

1) Endeavor to remove (or loosen) any shims intervening the form and inside surface of the auxiliary sleeve -- While damage to the auxiliary sleeve or winding is of no consequence, care must, nonetheless, be exercised to avoid breakage of the form!

Image of wooden shims (indicated by pick)

2) Using a #3 sharp fenestrated uterine curette, or similar non-ferromagnetic stainless steel instrument, carefully 'clean' the lower form leg of superfluous epoxy -- Note: Use of razor blades or other ferromagnetic or 'soft' metallic instruments will likely 'foul' the ferrite via metallic transfer to same!

3) Submerge the lower leg of the form (prepared as per task #2 above) in acetone for a period of ≈ 90 minutes at 7°C (45°F) through 33°C (90°F) - Note: the EHT winding assembly must not be submerged, splashed or otherwise wetted with acetone! - nor should acetone be allowed to condense on same! Owing to the flammability of ketones, said procedure is best preformed in open air or in an open container 'under' an OSHA (or jurisdiction-equivalent authority) certified fume hood (caution! - this does not include 'range-hoods', 'range fireplaces', 'walk-in fireplaces' or their ilk!) -- Moreover, a closed container is not recommended owing to liability to condensation...

4) Upon removal of the lower form leg from the acetone bath, drive the reluctance spacer out of the lower form joint using an appropriate feeler gauge leaf --- Note that during and following removal of the reluctance spacer/adhesive from the lower form joint, application of even moderate constrictive or distensive forces to the form will result in significant cantilevered forces at the upper form joint with (almost certain) breakage of one or both form pieces.

Of special concern in this regard:
A) Do not attempt to expand the gap while removing the spacer! -- The leaf thickness must be less than the gap width. Maintenance of a parallel relationship between leaf and form piece ends (i.e. gap 'walls') is imperative!

B) Following removal of the spacer, take care to avoid application of constrictive/distensive forces or piece-to-piece torsion to the form! -- The safest handling method is manipulation of the assembly from one 'side' (i.e. by one form piece) only!

Following clearance of the lower form joint of its reluctance spacer and adhesive, continued half-to-half form tenure owes to the upper form joint adhesive (at the center of the EHT winding assembly) - and, to lesser extent, adhesion of each form piece to the form sleeve of said assembly.

5) Using a 'heat gun', play a stream of hot (≈200°C /400°F) air in a swiftly alternating, sweeping pattern upon each 'form rail' taking great care to avoid directing said stream upon the EHT winding tyre! - In this fashion gradually and evenly increase the temperature of each 'rail' to ≈ 100°C / 200°F.

6) Wearing "welder's gloves" grasp form rails (each to a hand) then apply torque (not to exceed 5.5 N⋅cm!) axial to the upper form leg.

A) If/when form-rail to form-rail angular motion is evident, continue rotation until the pieces rotate freely relative to each other -- If necessary, the form sleeve may be used as a 'counter torque' but under no circumstances apply force of any kind to the EHT tyre!

B) In the event that relative angular motion/displacement is not discernible following 30 seconds' application of torque, repeat Task #5 (above) but heat the rails to ≈ 150°C /260°F then promptly repeat task #6 (above).

c) In the event that, having begun to move, sustained rotation becomes impossible prior to 'defeat' of the adhesive, repeat steps 5 and 6 (above) exactly as presented.

7) Carefully 'work' each form piece free of the form sleeve -- Again, avoid application of thrust or torque to the EHT winding tyre!

8) Thoroughly clean the form pieces of adhesive residue via 30 minute's immersion in acetone followed by scrubbing with an acetone wetted cotton cloth.

=== Part 2 -- modification and reassembly of LOPT unit ===

Here we will consider four (coaxial) features of the EHT winding assembly:

To Wit:
(Peripheral to central).
1) The EHT 'tyre' (comprised of the EHT winding proper, potting material and bunting).
2) The Auxiliary shell (a paperboard or polymer 'collar' intervening the central aspect of the EHT winding and the peripheral aspect of the Aux winding...
3) The Aux Winding{s}.
4) The form sleeve (intervening the central aspect of the Aux winding and the ferrite form).

Please note said features in the image below:

Image of unmodified EHT winding assembly.

Step 2-1) Locate the lug carrying the EHT return:
The EHT return generally emerges with one or more AUX leads from the peripheral circumference of the AUX winding whence they are tied to a shell lug...

In the case of the demonstration transformer, said lug occupies the 2-o'clock postilion -- The image below shows said leads emerging from the Aux winding immediately below the CCW edge of the lug (as indicated by the ball-point pen) -- of special significance is the extreme peripheral position of the break-out point.

Image of the bundle containing the EHT return.

Step 2-2) Separate and identify the EHT return:

Carefully unsolder all leads from the lug identified in Step 1 (above).
2) 'Untwist' and otherwise separate said leads.
3) Using a resistance indicator, check each lead for continuity to the EHT output contact (i.e. 'output tower') --- the return lead will show a resistance of between 400Ω and 1000Ω -- All other leads should show infinite resistance.

In this case three leads were tied to the lug -- the resistance examination identified the 38 AWG lead as the EHT return (said lead indicated by the pen in the attached image).

Image of separated EHT return --- Note that the associated lug was broken out of the way.

Step 2-3) Removal of the form sleeve:

Please note:
Far and away the most significant challenges attending this procedure owe to the 'gossamer' nature of #38 and #40 AWG conductors... From this point onward it will behoove the reader to remain mindful of the EHT return whenever handling the assembly - while a severed or broken return lead is not necessarily 'fatal to the cause' - such will greatly complicate matters via the requirement of 'teaseling out' the end of a #40 or even #42 (AWG) lead from the lacquered winding!

Carefully dress the return lead out of 'harm's way'.
2) Twist, pry and/or break the form sleeve out from the center of the Aux winding.

Image of EHT winding following removal of the form sleeve. Note the EHT return at ~ "2 O'clock"

Step 2-4) Remove the AUX winding:


While 'unraveling' the Aux winding, apply counter-thrust only to the shell --
as opposed to the tyre!
While debriding residue from the 'central gap' following removal of the Aux Shell - great care must be exercised is damage to the central layer{s} of the EHT winding to be avoided!

1) Cut all leads attached to all remaining lugs.
2) Using a surgical pick or similar instrument, pry a few 'layers' of the Aux winding into the center such that they may be grasped with forceps.
Image of Aux winding prepared for removal.

3) Grasp the the loosened layers and begin withdrawing ('unraveling') the Aux winding -- Taking great care to avoid application of tension to the return lead as the periphery is approached.

Image of Aux winding in an intermediate stage of removal.


Note: The EHT return will typically be 'laminated' between the penultimate and ultimate layers of the auxiliary winding bunting.

4) Cautiously break (not tear!) the bunting material away from the return lead.

In the image below you may observe the end of the EHT return (as per the pen) and its emergence from the bunting in the gap intervening the shell and the deflected bunting medial to the remaining lug at 4 O'clock

Image of EHT return embedded in peripheral Aux winding bunting.

Image of EHT winding following complete removal of Aux winding

Step 2-5) Remove the AUX Shell:


Carefully break, crumble and/or fold the auxiliary shell into the 'central gap' via symmetrical application of force to the distal aspects of the former at quadrature to the plane of the return's emergence.

Under no circumstances attempt to pry the shell from the interstices!!! Doing so will almost certainly damage the EHT winding beyond repair!

2) having thus loosened the shell, carefully remove the fragments being ever mindful of the return...

Images of the EHT winding following removal of the auxiliary shell...




Image of EHT winding and debris following removal of the form sleeve, Aux winding, and Aux shell:

Note: Should the reader's EHT winding be potted in silicone (as opposed to merely 'jacketed' as in the case of the demonstration transformer) it will be desirable to arrive at this juncture sans penetration of the potting material -- Should a breach occur, prompt repair via application of electronics grade RTV is advisable...

Step 2-6) Preparation of The Dummy Shell:

Note: Owing to the 'brittleness' of 'Purell' grade polypropylene, use of a sharp instrument is recommended!

Excise the base of the polypropylene pharmaceutical vial.
2) Section the (resulting) cylinder longitudinal to its axis.
3) Temporarily mount the dummy shell on the ferrite form -- Verify end clearance ≥ 5mm (form pieces in firm apposition). If end clearance < 5 mm, trim the dummy shell to 'compliance' but try to keep end clearance < 1 cm.

Image of prepared dummy shell

Image of correctly 'trimmed' dummy shell on leg of LOPT ferrite form:

Step 2-7) Prepare Return Lug:
Form a #4 solder lug such that the 'tab' is at a right angle to the plane of the fastener surface.
2) Drill a #66 (~0.83mm) hole through the lug immediately above the bend.

Image of properly prepared lug -- Note the small (#66) hole near the fastener angle.

Step 2-8) Installation of the dummy shell:

Image of EHT winding at this juncture - note return lead:

Furl the dummy shell, then position it in the central gap from the side of the winding opposite the emergence of the return lead and rotated such that the 'cut' faces opposite the output 'tower'.
2) Using a stylus, scribe, etc - mark the dummy shell at the point 5mm (back) from the edge lateral to emergence of the return.
3) Carefully remove the dummy shell.
4) Using a 7/64” {≈3mm} drill bit, bore through the dummy shell at the point marked in task #2.
5) Liberally coat the dummy shell and gap surfaces with electronics grade RTV. -- Avoid application of RTV to the return lead!
6) Reinstall the shell as described in task #1, taking care to position the cut opposite to the output 'tower', and the bolt hole lateral to the emergence of the return lead.
7) Mount the EHT winding/dummy shell assembly on the ferrite form.
8) Adjust the (linear) position of the winding tyre on the shell such that the former is equidistant from the form piece angles while the end of the shell emerging from the 'return lead side' of the winding tyre extends as far as possible from the winding sans contact with the form piece.
9) Remove the form from the assembly taking care not to alter the relative position of the shell/winding.
10) Manually expand the shell as much as possible -- smooth and 'touch-up' the silicone... --- Again keep the return lead free of adhesive!
11) Allow RTV to cure for at least 72 hours at temps of 7.2°C (45°F) through 15.5°C (60°F) -OR- for at least 48 hours at temps >15.5°C (60°F)

Step 2-9) Installation of the Lug and connection of the return:
Carefully insert the #4 bolt from inside the shell.
2) Secure the lug to the top of the shell as shown in the images below.
3) Solder-tin the end of the return lead.
4) Insert ~ 2 mm of the return lead into the #66 hole near the base of the lug then secure it with solder.

Note: It is imperative that the return lead is neither trimmed nor glued at this point! - Please loop any slack as shown in the images.

Images of the assembly; Dummy shell & Lug installed, return connected:


5) Verify that the DC resistance (lug to output tower connector) -- is that measured in step #2-2, task#3

Step 2-10) Prepare the CPVC Form sleeve:

1) Cut a length of 3/4" id (CTS) CPVC tubing equal to the total length of the form-leg on which it will be placed (In the case of this demonstration 8.5 cm) See the two images immediately below...

Image illustrating desired sleeve dimension

Image of form and adjacent CPVC section cut as per task #1

2) Cut 'shoulder stirrups' into the CPVC Sleeve:
Note: the stirrups must be sufficiently deep as to allow a slight amount of end-clearance when mounted upon the form (such that form piece apposition is assured).

Image of form pieces and 'prepared' CPVC sleeve

Image of form and CPVC sleeve in an intermediate state of assembly

Image of (temporary) fully assembled form and CPVC assembly

Step 2-11) Permanently install CPVC form sleeve in EHT winding assembly:

Image of components referenced in this step:
(L to R) CPVC sleeve, EHT winding assembly, Form [core] pieces

Assemble EHT winding, CPVC sleeve and form as shown (below).
2) Using PTFE shims, center EHT winding assembly about CPVC sleeve (both sides).
Note: While the adhesive does not bond PTFE it is, nonetheless, essential that the shims be so formed as to preclude (interlocked) captivity in the former!

Images of mounted and centered EHT winding as seen from 'EHT return side':



4) Using acrylic-based 'hot-melt' adhesive, tack the CPVC sleeve to the polypropylene shell at several points each side.
5) Remove form pieces.
6) Fill the space intervening the CPVC sleeve and the PP shell with acrylic-based 'hot-melt' adhesive taking care not to disturb the relative positions of said structures!
7) Allow the adhesive at least 60 Minutes to cool/cure.
8) Withdraw the shims.

Image of assembly immediately following withdrawal of shims as seen from EHT return side.

9) Fill the 'shim gaps' with acrylic-based 'hot-melt' adhesive.

Image of EHT winding assembly following completed installation of CPVC sleeve.

Identification of components/materials referenced in steps #12 through #15 (inclusive):
1) PTFE insulator.
2) EHT Winding assembly.
3) Form pieces.
4) LOPT Frame.
5) Polyolefin (heat shrink) sleeves.
6) Elastic cushions.
7) Frame ligature.
8) Form standoffs.
9) Reluctance gap spacers (see text).


Continued on Volume {B}

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