on winding a transformer

What's with the RMS? Your primary drive is 12 Volts peak to peak. Your secondary is 100 Volts peak to peak. The turns ratio is 12:100. It's not a matter of length but rather number of turns.

Be sure to drive the transformer through a capacitor to keep ALL DC out of it.

The number of turns depends on the area of the core and the maximum flux density, Bm for the particular core. The core material must not have too much loss at 10kHz.

You need to know something about the core, it's size and material.

secondary should be 50V p-p... or am I missing something here? +/-

ok will do. But how do these guys get away without the cap?

from

What kind of cores are commonly found in pc power supplies? that's where I got it...

thanks

Michael

" Note that the transformer, T1, is a special type which includes an air gap in its core (among other things) to provide the inductive characteristics needed for operation in flyback mode. "

..... Phil

of

Phil above has it right. That gif is a flyback design. The primary with the air gapped core forms an inductor designed to handle the DC component.

Interestingly some torroids can work in this mode, most cannot. The ones that can have a powdered iron core. The minute spacings between the iron grains form the equivalent of an air gap.

Again, you have to know what your core is to determine how to use it. I know you got it from a power supply but that doesn't mean much without some knowledge of the circuit.

It is possible to measure the core if you have a scope and a few components by winding a test winding on it then measuring the B-H curve. Then, by applying DC, determine how the curve responds and what happens to the saturation characteristics.

But, that's part of a course in magnetics and probably a bit beyond the scope of what you are trying to do.

I tell you to add a capacitor to the primary to keep DC out of the core. That is the safest thing to do, but your drive circuit has to both source and sink current. You can't do this with a single FET. You'll probably have to use a half bridge or what amounts to a two switch forward configuration.

I think the turns ratio of 12:100 is correct. Remember you will have +/- 6 volts on the primary and +/- 50 volts center taped on the secondary. That's

This sounds a lot like the same project that was discussed by the OP at length March 22 ("Need a big transformer") and March 31 ("transformers in parallel"). If the OP simply needs +/- 24 VDC at 2 (or even 5) amps, there are easier and better ways to get that than tinkering with parts from an old PC power supply. Unless you need something with exotic specifications, you can always buy it for less than you can build it (unless your time is worth nothing or it is mostly for learning). And for learning, it is best to build basic circuits until you understand them, and there is also a lot you can do by just running simulations. Much better and safer than blowing up parts in a breadboard.

I bought a 200 watt dual output +/- 43V 2A switcher originally designed for an audio amp for about $22 on eBay, and it is a well-made device that will meet my needs if I ever decide to build a high-power op-amp or audio amplifier.

Paul

Yep, same project, and learning is the main objective (along with powering an LM3886 someday).

I already built a boost power supply with a 555 switching the same inductor, pulsing at 100 Hz. It provided about 100VDC (!!), powered a

I've learned loads since the boost converter project, and I'm still learning.

I'm using a 12V battery as a starting point - safer than a first homemade switcher powered by the mains.

Thanks,

Michael

Totally agreed.

Do you still have the item number ? I'd be intruiged to just look at that.

Graham

Flyback converters are a completely different fish from forward converters.

Power Integrations (powerint.com ?) has a wealth of application notes obviously targeted at their own parts yet which are incredibly useful for learning about switching supplies.

Graham

Probably similar to this one:

Cheers,

Michael

It was still on my watch list, and I just got this:

I bought some other items from them. A good vendor.

Paul

Thanks for the info. That is a cheap find on eBay. I'll keep it in mind, though I think +/- 43V would shorten the lifetime of an LM3886 (not to mention my brother-in-law's speakers).

I'll experiment on the lower power end to minimize possible damage (and to avoid inadvertent in-house pyrotechnics).

Michael