I built this up to see how well the blue LEDs I got recently would work on the DC output of a JT. I put three pairs of two in series on a perf board and soldered them to the air wired JT.
I knew that the Vf of two blue LEDs was going to be more than 6V, so I put a zener diode across the output as protection in case I happened to disconnect the LEDs. I had a 5.6V zener, but I didn’t think that that voltage was high enough. I added a regular diode in series with the zener, to boost the voltage somewhat. The voltage measured 5.75V, which was still well below the 6.6V that I thought the voltage should be. So I added a second diode in series, and the voltage went up to 5.79V. That still wasn’t enough so I added a third diode, but that brought the voltage up to 5.8V. I knew that I had reached the point of diminishing returns, and adding another diode wouldn’t do anything more. Theoretically, each diode should have at least 0.6V drop, which should give at least 5.6 + 0.6 + 0.6 + 0.6 or 7.4V total. I measured the drop across one diode and got 0.42V, so apparently all of the JT’s current is going to the LEDs and almost nothing is going through the zener and diodes.
Looking at the Blue LED Vf
I took one of the individual blue LEDs and connected it in series with a current limiting resistor and to a power supply with a milliammeter, and a DVM across the LED to measure the Vf (forward voltage drop). I adjusted the PS for the current through the LED and got the following results for the Vf:
2.5 mA Vf = 2.90 V
3.0 mA Vf = 2.93 V
4.2 mA Vf = 3.0 V
10 mA Vf = 3.19 V
15 mA Vf = 3.28 V
20 mA Vf = 3.37 V
I’m getting 5.8V output from the JT, and there are two LEDs in series, so the Vf of each LED is about 2.9V. The above table would indicate that the LEDs are getting about 2.5 mA per LED. For three pairs that’s about 7.5 mA total. Not a lot for the usual JT, but because of the higher 5.8V voltage and 6 LEDs, means there’s less power available for each LED. Multiplying 0.0075 A by 5.8V gives 43 milliwatts. One uncertainty I have is that I used a fresh 1.5V AA cell for the JT measurements, so I’m not sure what the actual supply voltage was.
I have another brainstorm. Replace the three series diodes with a LED. Why not? I got a whole bunch of blue ones, sooo…. BRB.
I replaced the three diodes with a blue LED. When I powered it up, the blue LED barely glowed, indicating that almost all of the power is going to the LED array, and only a very small fraction of a milliamp is going through the zener and blue LED.
Back to experimenting…
