This Youtube video of a 7 Year Joule Thief from Xee2 AKA Xee2vids is similar to my low power Joule Thief I recently blogged. I recently built this very low power Joule Thief. The notes in the video were difficult to read, but this is what I could make out.
Notes:
1 – 1″ diameter toroid (Electronics Goldmine 5 for $1)
2 – Green LED is easily seen but not full brightness
3 – Needs capacitor but 100 pF works better than 0.01 uF.
4 – Frequency is about 10 kHz.
5 – Current, frequency and LED brightness are about the same if capacitor put in parallel with resistor.
6 – Should light LED for over 7 years on 2000 mAh AA cell. ( 2000 / .03 / 24 / 365 = 7.6 )
My comments on the notes on the schematic:
Regarding Note 1 – The 5 for $1 toroids from Electronic Goldmine have been sold out for quite a while and since they were surplus, it is very unlikely that they will ever be back in stock. I suggest that you try the toroids available from surplussales.com. They have a range of toroids available, from less than fifty cents apiece. I bought some of their ICH T231212T cores, which are only a very small 1/4 inch outside diameter. But they will work fine for this low power JT. The price is reasonable ($.25 apiece), and they say they still have a large quantity available.
Regarding Note 2 – The green LED is very visible to the normal eye because it is in the middle of the color spectrum. Remember that the total input power to this circuit is 1.5V multiplied by 0.00003 amp, which is 0.000045 watt or 45 millionths of a watt. That is extremely low power, and it’s amazing that your eye can see the LED lit up with such low power. It is best to use a super bright LED and one that is easier to see, such as green, yellow or aqua color.
Regarding Note 3 -I put a variable capacitor in the circuit and tuned it for the maximum brightness. The peak was very broad, so I used a 47 pF capacitor. But the resistor was less than 2 Megs, and each circuit will probably take a different value of capacitor depending on the coil, transistor, resistor and LED. In this case, 100 pF is a good choice.
Regarding Note 4 – As with the capacitor, the frequency will vary depending on the coil, transistor, resistor and LED. Mine was 29 kHz, but 10 kHz (in the video) is a reasonable value.
Regarding Note 5 – I put the capacitor in parallel with the resistor. But (as shown in the video) connecting one end to negative should not make any difference to the circuit.
Regarding note 6 – The AA cell will probably lose some of its capacity over the years just from old age. But one thing Joule Thiefs have is that as the cell voltage drops, the current is reduced, which extends the run time. One thing is certain: no one is going to wait 7 years to find out!
About the transistor
I used the BC337-25, which is a very good transistor for a full power Joule Thief, and capable of handling more current than the MPSA06 in the video. However, this JT is only using a third of a milliamp, so the transistor is not being pushed to its limits. I think even the lowly 2N3904 or BC547 would work okay in this circuit. What we are looking for is good current gain at very low currents. If you change the transistor, it might change the brightness and/or current consumption, so check to see if your circuit is okay with both.
Update Aug 9 – I wound a Goldmine “5 for $1” toroid with 20 turns and measured the inductance. It was 5.95 millihenrys, high for the typical Joule Thief (typically .5 mH or less), but it’s not an issue with this low power JT.
Back to experimenting…
