• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Opt laminations isolation

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Actually it is not paint but an oxyde layer (mat gray).
It must have been quite a job to remove that completely; when so the quality of the OPT will be pretty much impaired (eddy currents).
When you just removed the (usually black) paint from the sides of the complete built up transformer I would not bother too much.
 
Last edited:
Disabled Account
Joined 2010
I think,

It depends on where and how much can you post a picture?

Then it will be possible to give a better answer.

As above the oxide coating insulates the laminations from each other so its depends on where you have damaged the Tx..

Regards
M. Gregg
 
Thanks Pieter and Gregg :)

I have a picture but i cant upload it from the phone, as soon i arrive home i will upload from the desktop.

Again, thank you :)


Here it is...


What do you think about it?

Rui
 

Attachments

  • foto.jpg
    foto.jpg
    167.6 KB · Views: 146
This shouldn't be a problem, as you shorted out only a few laminations on the outer edge of the core. No high magnetic flux there, and no large area damaged.

Greetings,
Andreas

EDIT: Remember that some lower quality transformer manufacturers weld the laminations together in two straight lines to save the money for screws and nuts. This shorts all laminations at these two places. Still work ok, though this is not recommended practice if you're building state-of-the-art output transformers.
 
Last edited:
Actually, the welding on transformers such as microwaves is designed to create small eddy currents for safety reasons and RF issues, not to save money on screws.

This practice would be disastrous for audio transformers.

I would strongly suggest you carefully take a knife along the cracks where you have ground them together and shorted them,
and scrap out those cracks just about an 1/8" or less,
and while you are doing this, work in some High Voltage (10,000 volt) insulating lacquer to correct your mistake.

Also, instead of grinding with such a coarse grit, switch to something a hell of a lot finer, like emery paper for finishing jewelry, and wipe down all dust with a damp (but almost dry) cloth.

Somebody warned that some rust-paints may contain iron oxide and be conductive, so I would look for a latex or acrylic primer meant for metal (to seal and prevent rust).
Make sure your primer/paint is non-conductive,
and use it sparingly.
Rather let the High Voltage lacquer get in the cracks between the laminations.

If you can't get lacquer, you might substitute a real mineral oil, which is what some laminations are baked in.
 
If you can't get lacquer, you might substitute a real mineral oil, which is what some laminations are baked in.

Thank you nazaroo.
I don't think i will find 10.000 volt lacquer, the mineral oil approach seems to be the only alternative.


Its not a major concern...However Is this the only damage IE you haven't done the same on the other side?

M. Gregg.... :eek: Yes it have, much smaller though, between two laminations.



When i think in all the care i had with this project wasted in one minute of pure distraction :mad::mad::mad:

Thank you all.
 
Disabled Account
Joined 2010
:eek: Yes it have, much smaller though, between two laminations.

When i think in all the care i had with this project wasted in one minute of pure distraction :mad::mad::mad:

Thank you all.
Leave the two laminations alone..you might make the situation worse..

If I was going to do anything I would polish with a very very fine abrasive only in the damaged area...untill I could see the lines of the laminations..then Coat with a varnish ordinary polyurathane, Waterbased is not a good Idea or better still is PCB lacquer..Don't make the damage any bigger...You could just leave it alone. It will be OK..but as the saying goes if it isnt broken don't fix it... :)

Regards
M. Gregg
 
Last edited:
Actually, the welding on transformers such as microwaves is designed to create small eddy currents for safety reasons and RF issues, not to save money on screws.
This practice would be disastrous for audio transformers.

Actually seen this on small power transformers as well as on cheap OPTs, I don't know if there are practical reasons for it in special applications. The ones I saw where for cost cutting reasons. 'Disastrous' sounds a bit harsh, as all it does is increasing the iron losses, i.e. the power dissipated in the core, heating it up.

Can you explain in more detail how eddy currents and increased core losses help in the RF range?

Trying to repair the small damage shown in the picture will most probably increase the number of shorted laminations, as M Gregg wrote above, better to leave it just like it is.

Greetings,
Andreas
 
Actually seen this on small power transformers as well as on cheap OPTs, I don't know if there are practical reasons for it in special applications. The ones I saw where for cost cutting reasons. 'Disastrous' sounds a bit harsh, as all it does is increasing the iron losses, i.e. the power dissipated in the core, heating it up.

Can you explain in more detail how eddy currents and increased core losses help in the RF range?

Greetings,
Andreas

Again, the reason microwave transformers are welded is to prevent a disaster if the microwave is turned on with nothing in the oven.

The microwave transformer is a specially engineered CONSTANT CURRENT DEVICE, not a normal transformer:

"These transformers are designed with as little copper as possible. The primary for 115 VAC is typically only 120 turns of thick wire - thus about 1 turn per volt input and output (this is about 1/4th as many turns as in a "normal" power transformer. (It's usually possible to count the primary turns by examining how it is wound - no disassembly required!) So there would be about 3 turns for the magnetron filament and 2080 turns for the high voltage winding for the transformer mentioned above.



The reason they can get away with so few turns is that it operates fully loaded about 90 percent of the time but is still on the hairy edge of core saturation. The HV components are actually matched to the HV transformer characteristics.



Performance will suffer if the uF value of a replacement HV capacitor is not close to that of the original.


There is also generally a "magnetic shunt" in the core of the transformer. This provides some current limiting, possibly to compensate for various magnetron load conditions. However, it's not enough to provide any reduction in the likelihood of electrocution should you come in contact with the HV winding!

High voltage transformer

(From: John De Armond.) "The transformer goes by several names, depending on where you are. Variable reluctance, leakage flux, stray flux, etc. It is exactly the same construction and operating principle as a neon transformer, some kinds of HID light ballasts and some series streetlight constant current transformers.
The core is an almost standard "E" core (or "H" core if you prefer) with one exception. The center leg has an air gap. The windings are on the end legs of the "E" instead of the center leg.
There are two magnetic paths around the core for the field set up by the primary to travel. Around the periphery and across the secondary and around the center leg and across the air gap. The field that travels along the center leg does not cross the secondary and induces no voltage.
With no load applied, the bulk of the field travels the peripheral, very much lower reluctance solid iron path, inducing full secondary voltage proportional to the turns ratio. As current flows in the secondary, counter-MMF raises the reluctance of the peripheral path so that some of the flux travels through the center leg. With less flux traveling around the periphery and cutting across the secondary, the secondary voltage drops as the current remains about the same. At the limit, if the secondary is shorted, the peripheral path has so much reluctance that most of the flux travels the center leg and across the air gap. The same current as before flows through the secondary but at zero volts.
When the dimensions of the core and gap are set up correctly, the transformer behaves as an almost perfect constant current device. That is, the secondary voltage varies as necessary to keep the same current flowing through a varying load. Just what the doctor ordered to keep the magnetron happy.
The secondary current can be increased by opening up the air gap. This raises the reluctance of that path and forces more field through the secondary leg. Closing the gap has the opposite effect.
The center leg is often called the magnetic shunt and frequently it is a separate piece of laminated iron stuck between the coils and TIG welded in place. It is a common trick for Tesla Coilers to open up a neon transformer and either knock out the shunt entirely or grind it down to open the air gap. This modification causes the transformer to output much more current than it is designed for - for a little while, at least :) The same thing works with microwave oven transformers (MOT).
This design in a microwave oven is a vital part of keeping the magnetron anode current within spec. The magnetron is electrically a diode. A diode that isn't emission-limited would draw destructive current if not externally limited. With this design, the filament can be heated good and hot for long life and not have the tube run away. The design also is vital for protecting the magnetron from potentially damaging conditions such as operating the oven empty, arcing, etc.
It's popular to use several MOTs to build an arc welder. This works quite well specifically because these transformers are constant-current devices - exactly the characteristic stick welding needs. If they were conventional transformers, the first time the rod touched the work and shorted the secondary, fault current would flow and the breaker would trip or blue smoke would leak out.
Along similar lines, one can cut off the high voltage secondary and replace it with a suitable number of turns of heavy wire, connect a bridge rectifier and have a nice constant current battery charger. Select the turns carefully and it'll do the bulk/absorption stages of the smart 3 stage charging algorithm."
 
Nazaroo,

thanks for the nice explanation, didn't know these details about 'constant-current transformers' before!

Greetings,
Andreas

My pleasure.
One of the most common dangerous mistakes people try,
is to try to use a Microwave transformer for a HV supply.

This a worthless exercise but can also be LETHAL.

Another common mistake is to try to read the voltage on HV transformer.
That also can be lethal, and will usually destroy a voltmeter.

Max ratings for most voltmeters is 700 volts.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.