![\"DSC_0464S3\"](\"http:\/\/rustybolt.info\/wordpress\/wp-content\/uploads\/2013\/08\/DSC_0464S3.jpg\")
<\/a>I started out with a run-of-the-mill Joule Thief having a coil, a BC337-25 transistor, a 1k resistor and a blue LED.\u00a0 The coil was a T231212T core only a quarter of an inch (6.4 mm) O.D., with four windings of 7 inch lengths of 30 AWG enameled wire wound quadrifilar, with three of the four windings connected in parallel for the primary winding.\u00a0 The blue LED lit up very brightly, and the supply current was the typical 60 or so milliamps.\u00a0 In order to reduce the LED brightness and the supply current, I chose to increase the 1k resistor.\u00a0 I added a 100k pot in series to allow me to adjust the brightness.\u00a0 As I adjusted this pot from 0 to 100k, the brightness dropped a lot, but the LED was still putting out quite a bit of light.<\/p>\nI decided I would try a higher resistance, so I put a 51k in series with the pot, and as I adjusted the pot to a resistance above 110k, the LED went dark. \u00a0\u00a0 Well, I figured that the problem was being caused by the loss of drive from the feedback winding, which also had to go through this 110k or more resistor.\u00a0 I clipped a small capacitor across the resistors so that it was in parallel with the total 151k of the pot and 51k resistor, but it was still in series with the 1k resistor.\u00a0 The capacitor was a .0047 uF or 4.7 nF.\u00a0 The LED lit up again, but not brightly.\u00a0 So now that I had the LED working, I could again increase the total resistance, so I put a 470k in series with the 150k and put the capacitor across the 470k and 150k, but leaving the 1k resistor still in series.\u00a0 The LED still stayed lit, so I measured the supply current, and it was only 60 microamps, which was very low.<\/p>\n
I removed the 4.7 nF capacitor and replaced it with a 470 pF, which was 1\/10 the capacitance of the 4.7 nF.\u00a0 I couldn’t tell if the LED was brighter, so I put the 4.7 nF across the 470 pf, and the LED got slightly dimmer.\u00a0 Weird.\u00a0 This meant to me that there was some sensitivity to the size of the capacitor, so I removed both caps, and connected a variable capacitor that could be adjusted from 10 to 150 pF.\u00a0 When I adjusted this capacitor I found that there was a point where the LED lit up brightest.\u00a0 So I left the capacitor at that spot, and disconnected it and measured its capacitance, and found that it was 37 pF.\u00a0 But this peak was very broad, so I got a 47 pF capacitor from the spare parts box and soldered it in, and the LED lit up not very brightly, but it was clearly visible.\u00a0 I measured\u00a0 the supply current, and it was 310 microamps, or slightly less than a third of a milliamp.\u00a0 That is very low power: 1.5V times 0.00031 amp is about 0.000465 Watt, or 465 microwatts, not even a half milliwatt.\u00a0 Comparing that to the usual 120 milliwatts for the Joule Thief, it was about 250 times lower in power.\u00a0 Wow, I now had a very low power\u00a0 Joule Thief!<\/p>\n
You might think why did I leave the 1k resistor in there.\u00a0 Well, I connected a jumper across the 1k, and I couldn’t see any change in brightness of the LED.\u00a0 So I figured that it didn’t make any difference.\u00a0 All these resistors added up to 620k, so I replaced them with a single 1 meg resistor.\u00a0 I measured the supply current, and it was 240 microamps.\u00a0 The frequency was 29 kHz.<\/p>\n
When I put the 150k in parallel with the 1 Meg, the supply current jumped up to 1.7 milliamps and the LED got a lot brighter.\u00a0 I figure that with the 1 Meg resistor and a quarter milliamp battery current, a fresh alkaline AA cell running 24 hours a day should last\u00a0 for several months.\u00a0 With the 150k resistor, the battery should last for about two months.\u00a0 But this assumes that the battery current will remain the same during that time.\u00a0 We all know, from our Joule Thief experiments, that the battery current tapers off as the battery voltage drops, so the LED doesn’t go out, it just gets dimmer and dimmer.\u00a0 So in these cases, the LED could still remain lit for weeks more.<\/p>\n
I left the blue LED pointing up toward the ceiling and the battery connected, and with the battery current at a quarter of a milliamp I can clearly see the spot of blue light on the ceiling when the lights are out.\u00a0 That’s not bad for a half a milliwatt of power.<\/p>\n
Conclusion<\/h3>\n
Using the conventional Joule Thief with a resistor of a much higher value, and a small capacitor in parallel with it, the experimenter can control the battery current down to a fraction of a milliamp and still have a LED\u00a0 that is bright enough to see clearly.\u00a0 The battery lifetime will be greatly extended, and the LED can still put out enough light to be useful.\u00a0 By using a 1 meg pot in series with a 1k resistor to limit the maximum current, and the 47 pF capacitor across them, the experimenter can make a Joule Thief that is adjustable from very low light up to full brightness, and anywhere in between.\u00a0 The pot should be a logarithmic taper audio pot to give better control at the brightest end.\u00a0 And the left or lower resistance end of the pot should be connected to the coil winding.<\/p>\n
This very low power technique could be applied to the twenty LED strings I recently blogged<\/a>.\u00a0 The light output is much lower but the battery life could be extended to a month or more.\u00a0 Try this very low power Joule Thief out and see what happens; you might be pleased with the results.<\/p>\n Back to experimenting…<\/p>\n Update Apr 17 – In an email, Paul said, “I note you used quad winding but with such low currents surely that is not needed”.\u00a0 With such low currents, that might hold true for the low current in the transistor, which gets pushed to its limit at low voltage and high current.\u00a0 But with the core windings, which have the resistance of copper wire, the losses don’t change, percentage wise, as the current goes lower.\u00a0 If you have 100 milliamps or 100 microamps current, the DC resistance doesn’t change, and still wastes the same percent of power, even though the amount may be very small.\u00a0 Another point is that at higher frequencies, the Skin Effect takes effect.\u00a0 That’s why Litz wire is better than solid conductor wire.\u00a0 So having three conductors instead of a single conductor gives more surface area and the skin effect has more surface to give better conduction.<\/p>\n I took a look at the waveform with the o’scope, and saw that the waveform is a much narrower pulse than the typical JT.\u00a0 It is somewhat lower amplitude, but a good part of the lower light output is from the lower duty cycle (on time) of the pulse.\u00a0 When I put a 22k in parallel with the 1 Meg, the pulse amplitude gets higher, but the pulse gets a lot wider, and the transistor stays turned on longer.\u00a0 The circuit has been running on a ‘heavy duty’ (not alkaline) cell for several weeks, and the cell voltage is 1.435 volts.\u00a0 Looks like it will run at least a month more on this cell.<\/p>\n Let’s assume, for the reason of eliminating it as a factor, that the pulse height didn’t change when the 22k was put in parallel.\u00a0 What we then have is a change only of the duty cycle; the on time of the 1 Meg is much lower than the 22k’s on time.\u00a0 But remember that the only time there are losses in the transistor is when it is switched on.\u00a0 Now the coil has losses in the resistance when the transistor is turned on and charging the coil.\u00a0 When the transistor is turned off and the coil is transferring its energy to the LED, there is current flowing, so I assume there is also loss in the coil.\u00a0 But the current is much greater during the on time which leads to the conclusion that most of the loss occurs during the on time.\u00a0 My point is that due to the high current during\u00a0 the on time, there is a justifiable reason to minimize the DC resistance of the coil’s primary winding so the losses will be minimal.<\/p>\n Update May 12 – I connected the low power JT up to a 50 Farad supercapacitor.\u00a0 The base resistor is a 470k resistor in series with a 1k resistor.\u00a0 There’s a 47 pF capacitor across the 470k only.\u00a0 I connected a fresh AA cell across the 50F cap, charging it up to 1.56 volts.\u00a0 I connected it to the JT, and set it aside.\u00a0 After 8 hours, the capacitor voltage was 1.137 volts, and the LED was still glowing, a bit weaker but still very visible.\u00a0 At 12 hours, the cap voltage was down to 1.05V, and the LED is still going, not as bright as it was, but still fully visible.\u00a0 At the 20 hour point, the voltage has dropped to 0.890 volts, and the LED is still lit, slightly dimmer than earlier.\u00a0 At the 24 hour point, the voltage is now at 0.865 volts, and the LED is getting dimmer, but still plainly visible.\u00a0 Next morning, thirty hours later, the voltage has dropped to 0.809 volts, and the LED is getting dimmer.\u00a0 And finally at 44 hours, the voltage has dropped to 0.704 volts, and the LED has dimmed to where it looks like it’s ready to go out – there’s very little light, just a faint glow.<\/p>\n Note:\u00a0 One thing that could be added is a switch to bypass part of the resistance and increase the brightness when it gets dim.\u00a0 This could also be a variable resistor or potentiometer, but just two switch settings should suffice.\u00a0 The switch could be a SPDT center off switch so the on\/off switch serves a dual purpose.\u00a0 The idea is to allow the user to ‘turn up’ the light when the capacitor has discharged.<\/p>\n A later blog about this is here<\/strong><\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":" A question a Youtuber brought up made me think about going to the opposite extreme from the one I normally pursue: trying to get a Joule Thief to run at very low power.\u00a0 Normally I would try to maximize both the light output and efficiency. I started out with a run-of-the-mill Joule Thief having a <\/p>\n (Read More…)<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[],"class_list":["post-7070","post","type-post","status-publish","format-standard","hentry","category-joule-thief-smps-dc-dc"],"_links":{"self":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/7070","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=7070"}],"version-history":[{"count":33,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/7070\/revisions"}],"predecessor-version":[{"id":7077,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/7070\/revisions\/7077"}],"wp:attachment":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7070"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7070"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7070"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}