Paul got me thinking about the Joule Thief with low battery current.\u00a0 In his nineteenth JT he used the very high performance Fairchild KSD5041, same as the 2SD5041 (Japanese pinout – the center pin is the collector).\u00a0 He thought it was his brightest, and that could very well be.\u00a0 His theory is that the coil should be wound with the maximum number of turns of fine wire.<\/p>\n
My assumptions are based on the theory that less is more, less turns allows more current and since the stored energy is equal to the current squared, the lower winding resistance means higher peak<\/em> current and therefore lower losses,\u00a0 \u00a0 I decided to make up this proof of theory JT using the KSD5041 and the following parts.<\/p>\n Toroid core = YJ41003TC<\/a>, core O.D. = 3\/8″ or 9mm.\u00a0 This is a high permeability core, which, with very few turns, gives the optimum inductance of 100 uH or more. Both windings are\u00a0 7 turns of solid wire, the primary winding 24 AWG with plastic insulation, the feedback winding 24 AWG enameled.\u00a0 Each winding measured 120 uH.<\/p>\n LED = 5mm Blue, with a 1 ohm resistor in series to measure the current.<\/p>\n The resistor was a 4.7k with a 50k trimpot in series.\u00a0 I found that a 68 pF capacitor across both resistors kept the LED lit even at the higher pot settings, and looked brighter than when I used a 1000 pF.\u00a0 The LED would not light at the higher setting without the capacitor.\u00a0 These were connected between the coil and the base lead of the transistor.<\/p>\n I put a 33 uF bypass capacitor across the plus and minus terminals.\u00a0 I built the circuit on a 1-1\/2 by 2 inch piece of scrap wood.\u00a0 I pounded some 5\/8 inch brass brads into the wood for terminals and wrapped and soldered the wire leads to them.<\/p>\n I set the supply at 1.5V and adjusted the trimpot to give 25 mA supply current.\u00a0 I measured the voltage across the 1 ohm resistor and found that there was 7.2 mA flowing through the LED.<\/p>\n I removed power and measured the resistors and found the total to be 14.7 k.<\/p>\n I calculated the efficiency at (.007A * 3.3V) \/ (1.5V * .025A) = 61.6 percent.\u00a0 This is one of the best, if not the best efficiency that I’ve measured for a conventional Joule Thief.\u00a0 A typical JT is around 50%, sometimes 55%.\u00a0 But I have seldom seen one go above 60 percent.<\/p>\n I can adjust the trimpot and get the LED to light from very bright at minimum setting to almost dark at maximum setting.\u00a0 This gives complete control over the battery current, which can make the battery last a lot longer, with some sacrifice in LED brightness.<\/p>\n This shows that it’s possible to get excellent efficiency and performance at low current from a Joule Thief using a high performance transistor.<\/p>\n Update Apr 30 – I experimented with the value of the 68 pF resistor bypass capacitor.\u00a0 I connected an adjustable capacitor in parallel with the 68 pF, and adjusted it until the LED current peaked at a very broad peak.\u00a0 I removed the capacitor and measured it and it was about 130 pF.\u00a0 So I found a 120 pF ceramic disk capacitor and soldered it across the 68 pF, for a total of about 190 pF.\u00a0 I checked the LED current and it was almost 8 mA.\u00a0 But the supply current had also gone up slightly.\u00a0 So I readjusted the trimpot to set the supply current at 25 mA.\u00a0 The LED current was then about 7.8 mA, so the JT circuit was slightly more efficient than it was in the original test above. The total resistance then measured 14.85k.<\/p>\n I’m doing a few more measurements, but the circuit has been optimized.<\/p>\n","protected":false},"excerpt":{"rendered":" Paul got me thinking about the Joule Thief with low battery current.\u00a0 In his nineteenth JT he used the very high performance Fairchild KSD5041, same as the 2SD5041 (Japanese pinout – the center pin is the collector).\u00a0 He thought it was his brightest, and that could very well be.\u00a0 His theory is that the coil <\/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-7193","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\/7193","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=7193"}],"version-history":[{"count":14,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/7193\/revisions"}],"predecessor-version":[{"id":7223,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/7193\/revisions\/7223"}],"wp:attachment":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7193"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7193"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7193"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}