Long ago I built DC-DC converters using a toroid made with tape wound core. The cores were about 2 inches O.D. and we used regular 12 AWG solid insulated house wire for the primary windings. Even so, these cores vibrated a lot at several hundred Hz, making a screeching noise.
I saw some very small tape wound toroid cores for a dollar each from Surplus Sales of Nebraska. They have the part number 50B12-1D printed on them and Surplus Sales adds ICH- to the front of that for their Item number. The cores are 0.56 inch O.D. They were made by Magnetics. A link to the data sheet is in the advert.
I cut two lengths of 30 AWG wire each a foot (30 cm) long, and started winding them bifilar, together at the same time. As I wound, I measured the inductance with an LC meter. The inductance increased with each turn, until I got to 190 uH, but I still had enough wire for another turn, so I wound a thirteenth turn. The inductance jumped from 190 uH up to 10 millihenrys! So I removed that turn and it went back down to 190 uH. I thought that was really odd. I wound the thirteenth turn again and it went back up to 10 mH again. Strange.
Oh, well. I left it that way and soldered the rest of the parts to it to make a conventional Joule Thief. I used a 5 mm white LED, 1k resistor, BC338 NPN transistor and a 10 uF capacitor from plus to minus to bypass the ripple from the JT. The LED lit up brightly at 1.5 volts, and the frequency was 100.6 kHz.
According to the data sheet, the -1D suffix means the core is wound with 0.001 inch thick Square Permalloy 80 tape. The case is nylon.
These tape wound cores work great for a Joule Thief
Update May 27 – I bought a few toroid cores from China, and since the packages are in Chinese and they send no invoice or any documents, I really can’t tell who they were. But the cores are about 7/8 inch or 22 mm O. D. and are painted green. I wound one with 13 turns and I got almost 500 uH. So they are high mu, and should work good for a Joule Thief.
I decided to look for more, so my search came across some green cores on eBay. I ordered ten of these smaller ones, and they cost only 99 cents, shipping included. I think they are 9 mm O. D. I’m taking a risk, because they say nothing about the permeability, the cores could be made out of baked clay and painted green. I would expect that at this low price. I’ll just have to wait until June to find out. I received this in an email order confirmation, but I don’t know Chinese.
Seller:
深圳市艾姆诗数码科技有限公司
digital.mart899@gmail.com