I bought some ‘high current’ chokes from Jameco, P/N 642855, which are 390 uH. They’re bigger than average, and mount axially; in other words the leads are at opposite ends of the cylindrical package. They are made by winding relatively heavy copper wire onto a ferrite bobbin. The bobbin is then covered by a piece of heatshrink tubing (see photo).
Adding Second Coil
I added the second winding, about 5 feet of 30 AWG enameled wire. I cut off the heatshrink tubing cover and removed 46 inches (117 cm) of wire, and resoldered the end to the lead. The wire is the kind that the insulation melts away when the soldering iron is applied. With the wire removed the coil measured 147 uH, a good value for a JT.
I wound about 5 feet (150 cm) of 30 AWG (.25mm) enameled wire over the existing winding, which filled the bobbin up to about level with the ends. I could have used the extra grooves in the ends of the ferrite bobbin to hold this wire. But instead I ran both wires through a short piece of green wire insulation, and I wrapped the whole coil with about 4 inches of 3/4″ black electrical tape to hold everything in place.
The Joule Thief
I used a PN2222A NPN transistor, a 1k resistor and a red LED, which happened to be lying on the bench at that time. I tack soldered everything together to see how it would work. The red LED lights up very brightly and the JT draws about 125 mA at 1.5V, and runs at 33 kHz.
I named the JT after the song Love Potion Number Nine, which has the words “..it smelled like turpentine and looked like India ink..” because the choke looked like India ink due to the black heatshrink tubing. It didn’t smell like turpentine as I unwound the wire, but I could feel the wire and tell that the choke was covered with varnish after it was wound, and varnish smells like turpentine as it dries. These chokes are ‘closeout’ and will very likely not be restocked again after they are sold out.
After building this, I was thinking that it was a shame to waste such a low resistance coil on a regular JT, and I should convert it over to a Supercharged Joule Thief, which takes only a diode and a capacitor. I looked for some disk capacitors and found a package of 100 assorted disk capacitors that I bought from Radio Shack (cat# 272-801). I wrote on the back
85 out of 100 “assorted disc capacitors” were marked “561”. Measured with cap meter, none were above 560 pF. Most were 530 to 540, some below 500. Tolerance should be +0, -20%, so measure them before using.
That’s not much of an assortment when 85 percent of the ‘assortment’ is the same capacitor. So the adage “caveat emptor” applies, here: let the buyer be aware that the package may not be all that it seems.