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Old 24th July 2005, 08:34 AM   #21
mzzj is offline mzzj  Finland
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Quote:
Originally posted by rjon17469
[B

BTW, where do you typically go to get cores? [/B]
230W-400W AT/ATX power supplies are "easy" source for ferrites, iron toroids, hw caps, switches, PWM-modulators and so on. You have to figure out specs by yourself, but some simple maths+scope+func.gen. should do that. You can also reverse-engineer transformer specs, calculate operating freq from controller chip R/C combination and note how many turns of wire there is in primary.

Typically you can get broken ones for free when you find right place/person to ask. Dont use small electrolytics from these, they are almost always bad(and the reason why its broken)

For more serious projects I prefer ordering new stuff with specifications.
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Old 2nd August 2005, 02:26 AM   #22
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Hi all,

I see some very initiated posts here, and I have a pair of beginner's questions for you:

Eva wrote:

Quote:
- When winding transformers, pay great attention to output isolation. There should be three layers of isolating tape between primary side and secondary side windings. Magnet wires from different sides should never be in direct contact, there must be either isolating tape, proper plastic tubing or 3mm of air inbetween. There should be 5mm or more clearance between solder pins of different sides.
It is said in an artice on the smps.us web site that PTFE (teflon) tape is approved as isolation material in SMPS transformers. However, such tape is rather thin, with a thickness around 0.1 mm (at least the kind I have). How many layers of this would be required between primary and secondary in order to adhere to formal safety regulations?

Also, from the same article, it appears that the secondary windings should normally be closest to the core in SMPS transformers, with the primary wound on the outside.
Is this just to minimize wire length per turn for the side with the highest current, or is there some other reason?

Cheers,
Marcus
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Old 7th August 2005, 04:58 PM   #23
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any chance we could see EVA's schematic?
__________________
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Old 7th August 2005, 06:09 PM   #24
mzzj is offline mzzj  Finland
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Quote:
Originally posted by marcus66
Hi all,


It is said in an artice on the smps.us web site that PTFE (teflon) tape is approved as isolation material in SMPS transformers. However, such tape is rather thin, with a thickness around 0.1 mm (at least the kind I have). How many layers of this would be required between primary and secondary in order to adhere to formal safety regulations?

Marcus
Depends ultimately wich regulations and wich specific class you refer to. In sweden EN6xxx is probably right one to look for, then it depends if it is Class I(basic insulation) or II(double insulation/reinforged insulation) Sorry, I dont have any of those regulations at hand right now...

Same as with EMI-regulations about line harmonics, "hand tools" have real loose spec about them and television much more strict.
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Old 9th August 2005, 02:45 AM   #25
Eva is offline Eva  Spain
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Quote:
Originally posted by davesaudio
any chance we could see EVA's schematic?

Do you mean the schematic for that small unregulated SMPS?

Sorry, I have no schematic, I laid out that board from memory and then I tuned up component values. Anyway, the layout is rather self explaining and you also have component values so you can easily draw your own schematic.
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Old 12th August 2005, 04:09 AM   #26
blmn is offline blmn  Brazil
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I made this some time ago.

I donīt remember the specs and I used just components that I have in my workbench.

The transformer will go hot but it worked fine for few hundred miliamps.

I hope it helps.

Regards
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Old 19th August 2005, 02:40 PM   #27
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IMHO, if you don't want to pay full price for HF transformer cores, the only safe surplus source is a core that served as main transformer in another SMPS.

Old or broken ATX supplies are a good source, but I try to avoid the flyback types, since I don't want gaps in my cores.

I'm not sure what value there is to reverse engineer the windings on these cores, since I think you should strip and rewind anyways.
I've successfully stacked two identical E-cores together from ATX supplies, to emulate a larger core. This doubles the cross-section area (which nearly quadruples the power rating). I had to make my own bobbin, since the core center is no longer square, it's rectangular.

My core of choice, an EER-42, is not easy to come by, and is expensive, so I like to recycle whenever possible.

Adrian
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Old 19th August 2005, 02:56 PM   #28
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Default SMPS housekeeping voltage

Although all SMPS designs seem to derive the low-voltage controller supply from an auxilliary winding on the main XF, I find it more practical to power the controller from a separate, analog transformer power supply.

This can be very tiny since it only needs to supply 50mA or so. Many miniature wall cubes can provide this circuit ready-made.

Just because you've decided to go SMPS, doen't mean you can't still have the supporting circuitry run off analog.
I find this brings a benefit in reducing the complexity of the SMPS controller circuit, by transferring some of the work out-of-circuit, to a reliable venue. Because this also removes the off-mains start-up supply to the controller, You can easily end up removing 7-10 parts from the SMPS.

I'm not a pro at designing these, so the more parts I can remove from the SMPS controller to simplify design, the less troubleshooting I have to do.

The second, and greatest benefit, though, is removing the housekeeping voltage winding from the HF XF core. The extra core window space is welcome, either for additional secondary power, or to make-up for my imperfect and not-so-compact hand winding technique.

Cheers
Adrian
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Old 25th August 2005, 08:16 PM   #29
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Quote:
Originally posted by funberry

I'm not sure what value there is to reverse engineer the windings on these cores, since I think you should strip and rewind anyways.

Whats your method of seperating the two halves of the core?

I've tried paint thinner (not strong enough), boiling (cracked the core), and force (cracked the core completely)...

I've heard laquer thinner is strong enough, but I dont want to buy any unless I know it will work.



btw good advice on using a standard transformer for controller power (if you have a source for them).
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Old 25th August 2005, 09:27 PM   #30
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Hi thomas997

I don't separate the core.
I simply unwind all the original wire till I get to the bobbin, then rewind my wire with the core in place.

I first thought "this is going to be a monk's work", but it isn't; 30-40 minutes, and you're done, because you're dealing with so few turns.
As the core window gets narrower, passing adhesive tape through the windows becomes difficult, so I use a regular paper strip, later impregnated with enamel.
You need to correctly cut to length the wire you'll need for each winding before you wind.

If I want to "parallel" two cores, (square profile), I destroy the bobbin (since I can't use it anyways), usually by melting it in a toaster oven, then pulling the softened plastic off with large tweezers.

You'll be much happier if you let go of the idea of separating the core halves.

But if you insist on dissolving the glue that holds the thing together, you may want to try the following:

Acetone
Paint stripper
Trichloroethylene, trichloroethane
(paint thinner must--by your government's orders--be safe for all whining consummers, therefore, useless.)

let the core (without bobbin) soak in the liquid overnight in a small COVERED container, **outside of your house**.

Warning: ventilation, ventilation, ventilation; protect all children, pets, spouses, etc.
You do this at your own risk--I didn't tell you to do it.

Adrian
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