I was on eBay and saw some small green ferrite toroid cores, ten for 99 cents including shipping. I figured that since I had bought some larger green cores a few weeks ago and liked them, I couldn’t go wrong buying these since they were less than ten cents apiece.
They were sent from an address in China.
From: 3 FLOOR 31 BUILDING, FUMING INDUSTRIAL PARK PINGFU TOWN LONGGANG DISTRICT SHENZEN
zip: 518111 tel: 13428933145
This is the same address that the large cores were shipped from last month. Last month my email from PayPal showed the seller’s name in Chinese, which I can’t read.
I received them this weekend and put one on my LC meter. I put a short piece of wire through the core and measured it, and like the larger cores, it measured about 1.3 uH. Then I wound about a foot of wire (about 18 turns) on it, and I measured 1.5 millihenrys. I wound another core with ten turns of 24 AWG solid insulated telephone wire and it measured 520 uH, which is a good value for a Joule Thief. There is still enough room to wind a second feedback winding on it. It would be okay to use 5 or 6 turns to give more than 100 uH, enough for a Joule Thief.
