In a comment, Metis asked:
How can I reliably measure and/or control the output voltage of the JT? I’m currently trying to build a small power supply to recharge a Li-ion battery and I’d prefer to be able to keep the output voltage at (or rather below) a known level rather than invest in a buck converter to regulate it.
I don’t have an oscilloscope at the moment, trying to find one that’s affordable, but until then I won’t be able to measure it directly.
[When I wrote this, I didn’t read the above close enough. Voltage was what he was talking about. Skip to update for more.] You can control the JT power output by changing the resistor to a pot or variable resistor. The 1k resistor can be increased to 3.3k or much more, depending on the amount of light you want. I would try a 50k pot, with audio taper so it gives finer adjustment at low levels. But it will be maximum light at the lowest setting, which is at the counter-clockwise end. I have 100k audio pots with an on/off switch, which makes it convenient.
You also might want to consider using a module that’s made for this exact purpose. DealExtreme.com sells this type of module. It’s a PC board that’s the diameter of the cell that goes into a flashlight (torch). These come in various voltages and currents. The different power levels are controlled by quickly switching it off and on. Some of them are rated for connecting to a single 2.6V Li Ion cell, such as the 16450, or else two RCR123 rechargeable cells in series. This makes it convenient to use any of several voltages. you could also add it to a bike light that uses three AA cells in series. They are typically very efficient, much better than a Joule Thief. They use surface mount components and their cost is very inexpensive.
You also might consider getting the whole flashlight preassembled so all you have to do is mount it on the bike. Some have adjustable beams, which can give you enough beam width to cover the road. I bought one with the reflector that is bumpy, which spreads out the beam somewhat, and gives a bright spot in the middle. Mine is an Ultrafire, and uses a 16450 rechargeable cell.
Update Jan 3 – Upo closer reading, I see what the issue is. I have built and blogged a few DC to DC converters that regulate the output voltage. Here is one. Here is another.
This still applies to an SJT, no?
Also, is there any way to measure, or calculate, the output voltage? In lieu of an oscilloscope that is.
The output voltage across the LED is a spike that is about 4 to 5V peak for blue or white LEDs and lower for other colors. But this is dependent on the current through the LED, which depends on the rest of the JT, including the LED itself. The voltage is not what is important, it’s the light output of the LED. If you’re using a BC337-25 transistor, the LED current should be about 20 mA. This is measured across a 1 ohm resistor in series with the cathode of the LED, which should measure about 20 millivolts.
If you substitute a rechargeable battery in place of the LED, then the battery voltage will cause current to go back into the JT. You need a diode between the battery and the output. I would use a 1N5819 but a 1N4148 will work. The pulses can be filtered with a 100 uF or larger capacitor.