Custom transformer

I don't know if there is one unified source -- there's a bit of black art to the whole magnetics thing. The ARRL has some good publications if you want to build transformers at RF, I would expect that a good book on switching power supplies would devote a good bit of text to magnetics, but I don't know if there's One True Book.

In your case, yes, stacked steel laminations count as "iron core". You have a better chance of the thing working well if you took the core from an audio transformer rather than a power line transformer, but AFAIK laminated steel cores aren't the good choice for switching supplies -- the problem is eddy currents, which get worse the thicker the laminations are (who eddy is, and why his currents cause problems, is a mystery that you may be able to clear up with some Googling).

The number of turns you are using is alarming -- one of the sources of loss in a transformer is inter-winding capacitance; you may be running afoul of this. Checking the voltage ratio with a sweep source, and doing some leakage inductance and parasitic loss measurements, may illuminate some problems without resort to an O-scope.

But your best bet is probably to just chuck the thing and get some ferrite E-cores, and rewind the transformer. See if you can find the Amidon website and puzzle through what it has to say on the subject; you may find what you need there (and be able to buy some cores).

One wonders: all the time and effort you put into this, couldn't you just replace all the caps in the scope?

Steel laminations aren't great at 50kHz, you need a ferrite. Can be hacked out of an old power supply but make sure that it either does not have an air gap or that you can remove it (often it's just a slice of circuit board material or plastic). Hint: If there is only one power transistor it's most likelty a flyback switcher and the core will have an air gap.

Why do you need this to repair your scope? If you suspect bad caps an ESR meter alone may not tell. Sometimes they just loose capacitance or leak. If the scope is valuable just swapping out caps may be better.

google Amidon Associates.

Steve Roberts

Yep, that's where I get prototype quantities. With this many turns I'd use pot cores for easier winding:

Thanks for the links. I have some ferrite cores here that look a bit like what you linked above, I'm gonna try that. For the turn count, I just followed what's suggested in the article (400:20) but I've no idea how to determine that. I'll first google a bit further.

And as you suggested, I'll probably end up replacing all caps in the scope.

When using ferrite be aware of the material. Huge differences. Some (such as -43) is for MHz-range transformers, other material (such as -J or -77) for lower frequency power transformers. Unfortunately ferrite cores are not marked and you can't judge from the blackness of the material. If in doubt, measure it. for example by making a resonaht circuit which will reveal at least the Al value.

Well, I wired a ferrite transformer tonight and the ratio is really good now. I've no idea about the kind of ferrite it's made of but it's good enough at 50kHz it seems. I' now going to repair the scope so I can have a look at the signals :-)

Thanks to all for your great answers, very valuable.

David

Oh yeah! A young friend built an Elecraft K2 a few years back which didn't work. After he tore out his hair for a while, he asked me for help. There were a number of problems easy enough to find. But then, after I tore out _my_ hair for a while on the last problem, it finally dawned on me that the ferrite material for one of the coils was wrong - even though it was black (as specified in the construction manual). Elecraft sent a new toroid & I wound the coil. I saw no difference in the color of the two toroids, but the thing played perfectly with the new coil.

I needed your warning back then!

Ed

The other issue to watch out for are hair cracks (a.k.a. unwanted "air gaps"). Especially after an event like the "UPS chuck". Parcel leaves hand of UPS driver, sails 10ft in airborne configuration, lands on concrete front porch. *Thwack*. Seriously, I saw that with my own eyes.

Report it.

UPS here brings package to door, sits it down, rings doorbell, _then_ walks away. (The neighborhood _is_ safe.)

...Jim Thompson

If it breaks I will. This was paperwork, busted open but no damage. Thing is, when you call companies you easily spend 15-20 mins on the phone. BTDT, this moring, with a coffee maker mfg that had bungled our repair order.

This neighborhood is safe as well. But they often seem to be in a great hurry.

Like "Jim" who I was talking to the other day for an HP printer warranty repair (I remember when HP made reliable stuff). This particular "Jim" was in India. Of course the product was made in China.

$/hr for unionized staff (the "International Brotherhood of Teamsters" in this case) is high.

And he probably had you spell out your name, street, town, phone number, email address, shoe size and so on. The same stuff that you gave to "Jeanette" in India about 10 minutes earlier. Never mind that all this was already on the warranty registration you had sent in a couple months ago.

Yep. I prefer Fedex whenever that option is available. But often it isn't.

Ouch. A lot of years ago when I was in high school, I worked for an outfit named Hospital Photo Guild. We set up stations where the nurses would wheel in a bassinet with a newborn and photos of the baby were automatically taken. In some cases, we mailed the unexposed film to the hospitals & they mailed the exposed rolls back to us. In others, the hospitals were close enough for us to deliver & pick up the film.

Anyway, the boss was looking for a new/cheaper/better packaging of the film. I was one of three people who "tested" the packaging. We walked on it, dropped it and threw it against a concrete wall, trying to simulate what might happen in the post office. The boss was a smart guy - the packaging worked. :-)

Phone call. Me: "Hello Amidon? The toroids you sent me are broken. The packaging is horrible. I told you I wanted them properly packed." Amidon: "Oh, we thought you said properly *thwacked*."

Ed