How to Reuse Toroids

I am trying to re use some toroids that I salvaged from a broken computer PSU and motherboard. I have some electronics experience however this is mainly with digital circuits. I am reusing the toroids by removing the original windings and hand winding the several coils required by the circuit. I have noticed that while the one largest (2.2cm diameter) core works exceptionally well, all other cores salvaged from the devices work poorly. My goal is driving a nixie tube. While the "successful" core yields about 180 volts AC the rest can barely top 60v output (all have similar numbers of windings, roughly 150 turns on the primary, 5tap5 on the secondary, 10 on the feedback).

Clearly there is some level of variation between how the toroids perform. Is there any easy way to work out which cores will work? Can anybody explain which factor(s) are likely to be the cause? Visually the cores appear to be the same, looking like they are all made of soft iron.

For reference, I am using the toroids to make a Royer oscillator - which does appear to be working, as my source voltage is 5V.

Any help appreciated,

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Kiteman6 years ago
They're actually ceramic.

They can break if dropped on a hard surface, so maybe they get cracked on removal?

Possibly this could be detected with a multimeter? Prod around on the surface of the toroid, and watch the resistance?
andy (author)  Kiteman6 years ago
Hi Kiteman,
Thanks for the input. Since I'm afraid of breaking my project, I can't measure the "working toroid", however measurements of others has shown that most are indeed very high resistance (presumably ceramic?) and some are low resistance.

I forgot to mention, most have a painted surface (which is easily scraped off)... When scraping the surface of one of the painted toroids I was surprised to find not a dull grey but bright silver metal (presumably silicon-steel?).

After a bit of wikipedia research I think that I am after (for my application) a "low reluctance" core - which would be silicon-steel or powdered iron.

I will keep investigating. As far as I can tell none of the toroids are cracked (the thin layer of paint would not hold them together if they were) also I am normally very gentle when disassembling. The X that I was looking for seems as you say to be the material of the core.

Thanks for the frankly brilliant idea of using surface resistance to work out the toroid material - I'd have never thought of it.

Many thanks,
Kiteman andy6 years ago
You're welcome.
andy (author)  Kiteman6 years ago
Just finished a test with one of the low surface resistance (low reluctance) cores - it has ~100 turns primary. It's kicking out ~200 volts (93v average rectified) i.e. it's a successful test.

In other words this is a good way of identifying core material (reluctance by proxy of electrical resistance). For reference;
  • "Powder" cores have roughly >2kohms resistance over 5mm and are dark grey
  • Silicon steel cores are shiny and have <1kohm resistance over 5mm
  • Ceramic cores have very high resistance (my multimeter tops out at 5MOhms and it did not register at 1mm distance) and are dark grey (visually indistinguishable from powder cores).

P.s. I may do an instructable on salvaging and reusing these cores, pending new broken hardware to extract them from.
Kiteman andy6 years ago

Kiteman did a good thing?
Who'da thunk it?
Hey, I'll do the thinnin around here..... (a before your time reference, I am sure) :-D
andy (author)  Lithium Rain6 years ago
Sorry, what? I don't understand...maybe I'm missing something.

Kiteman andy6 years ago
It's a friendly undermining of my self-confidence.

Haha, I'm giving Kiteman a friendly jab - I'm saying "Who would have thought Kiteman woul do a good thing?" (it is, of course, entirely in jest). :)