2013-03-17 Ultra Low Voltage Joule Thief – More Experiments

My first ULVJT which used four JFETs in parallel was a success in that it put out light at supply voltages as low as 34 millivolts.  But it put out only a minuscule amount of light, and needed to be increased.  The next one I built had six 2SK170 JFETs in parallel. It used a bit more current than the first, but still not enough to brighten up the LED.  One of the Youtube posters said he put a ZTX1048A in the circuit to boost the current.  I added one of these to my circuit, and it helped somewhat but really wasn’t all  that much.

At the end of my blog I  talked about using a MOSFET, but that it needed quite a bit of power to drive the gate.  I decided to go with a much smaller MOSFET, the 2N7000.  This has less than 60 pF gate capacitance, compared to the thousand pF of a power MOSFET.  The low capacitance makes it much easier to drive with low power.

I tack soldered a 2N7000 MOSFET to the existing JFETs, source to source and drain to drain.  I connected the gate to the other gates through a 0.1 uF capacitor.  This is because the gate of a MOSFET must have a positive voltage on it, whereas the JFET needs a negative voltage.  I then connected a 470k resistor to the MOSFET gate to supply the positive voltage.  I connected the remaining end of the 470k to the center wiper arm of a 500k potentiometer.  One end of the pot was connected to ground and the negative of a fresh AAA cell.  The other end of the pot was connected to the positive of the AAA cell.  I now had an adjustable voltage source that could put from zero to 1.6V on the gate of the MOSFET.  Since there is no gate current, there will not be any voltage drop across the 470k resistor.

I turned the pot  down to zero and applied power to the circuit and the LED lit up when I advanced the supply voltage to 100 millivolts and decided this would be the supply voltage to do the tests.  The LED was lit but not brightly.  When I turned up the pot, all the way to its maximum, I could not see any difference in the brightness of the LED.  I shorted the MOSFET gate to the source to see if it was conducting current, and I saw no change in the LED, so the MOSFET was apparently not conducting current with 1.6 volts on its gate.

I needed more voltage than 1.6V, so I removed the AAA cell and clipped a power supply set at 3V onto the battery holder.  Now when I turned up the pot, the LED got considerably brighter.  Now I had the answer: the MOSFET needed more than 1.6V to function.

I had one obvious choice.  Just replace the AAA cell with a 3V lithium coin cell.  Easy peasy, and it will last forever since there was almost no current draw.  I could put a 470k resistor at the bottom end of the pot since I knew that the voltage would have to be higher than 1.5V, giving a total of 1 meg, so the battery current would be 3 microamps; so low that the battery will last its shelf life.

I got another brainstorm.  The circuit is already generating a small amount of current.  Why not just rectify the existing pulses, and use the resulting DC to furnish the gate bias?  I decided to use a voltage doubler.  That should give me the three volts easily.  I added a coupling capacitor, two 1N4148 diodes and a filter capacitor to rectify and filter the DC.  I thought that this would give enough voltage to do the job, but it didn’t,  It put out only 1.75VDC, just slightly more than the single AAA cell.  So I may have to add another diode to make it a tripler, and maybe replace the diodes with Schottky diodes to give less voltage drop.

I added a third diode and another capacitor to the doubler, and it made the gate bias voltage jump up above 3 VDC.  But when I put my DMM on it, I can see that the voltage sags from 3.3 or so volts down to about 3 volts.  The impedance of the tripler is so high that the 10 megohm impedance of the meter loads it down.

I set the supply at 100 millivolts as before.  With the tripler, the LED is brighter than it was before.  I would like to give an accurate amount but I can’t, all I can say is it is substantially brighter.  When I ground a point in the tripler and cause the gate bias to be zero, the LED dims substantially.  When I remove the ground, the LED lights up substantially brighter.

I noticed that the minimum supply voltage has changed from below 40 mV to about 45 mV, but as the voltage rises from 45 to 100 mV, the LED gets much brighter, much faster.  This 5 millivolts difference may be caused by the voltage  drop in the supply cables and wiring, due to the increased current draw with the MOSFET working.

Conclusion – The circuit as it is, with four JFETS, a single MOSFET and a few capacitors and diodes is much better than the original circuit with just the JFETs alone.  I am not using any gate bias battery, the circuit is stand alone, making its own gate bias for the 2N7000 MOSFET.  I’m not going to go any further than I have in developing this circuit.  But if I decided to go for it, I would remove a few of the JFETs and replace them with the MOSFETs.  The supply current and LED current would be increased even more.

Previously I had another very low voltage Joule Thief blog, but it used regular transistors.