Early on in my JT experiments I experimented with a pot (potentiometer) in place of a resistor, which was used as an adjustable resistor and allowed me to optimize that particular JT.\u00a0 After the optimum setting was determined, the pot could be removed and replaced with a regular resistor.\u00a0 It seemed that the pot was confined to use just for experimenting.\u00a0 But at the same time I realized that using that same pot, a JT could be made to have adjustable brightness.\u00a0 One problem was that the size of the pot was as big or bigger than the rest of the JT circuit, and pots are more expensive than a simple resistor.\u00a0 One other thing I noticed: it seemed that no one else left the pot in for daily use.<\/p>\n
Then I just came across this circuit<\/strong><\/a> that does use a pot to adjust the brightness.\u00a0 Good to see someone else decided to make use of this pot for every day use. But why did the author contradict the design by adding a 39R resistor and the second BC547 transistor to limit the current?\u00a0 He could have done the same thing with the 330 ohm resistor by using its value to limit the maximum current. The 330 ohms is low, it should be closer to 1000 ohms as used in a typical JT.<\/p>\n The BC337 is a very good choice for the JT transistor, but it has a self-imposed limit on the amount of current it can handle in a JT circuit.\u00a0 Once the 330 ohm resistor has been set to give a maximum LED current of 20mA, then the second transistor is no longer needed. And along with that, the high speed diode, 100u filter cap and 39R resistor can go away.\u00a0 All that’s really needed are the three LEDs across the BC337.<\/p>\n Oh, and another one of my humble opinions.\u00a0 The high speed diode and 100u filter capacitor are not necessary and wasteful of power.\u00a0 The 0.8V or more voltage drop across the diode, multiplied by 20 milliamps, equals 16 milliwatts of wasted power that could be better put to use making light in the LED.\u00a0 When you think of the word diode, remember that the LED is already a light emitting diode<\/strong>.\u00a0 So why would you want to add something you already have?<\/p>\n Last but not least, the three LEDs are connected in series.\u00a0 With each (white) LED having a voltage drop of 3.3 volts, there is about ten volts total across these LEDs when they’re lit up brightly.\u00a0 A normal 1 LED JT has to boost the voltage a bit more than twice (3.3V \/ 1.5V), but this JT has to boost the voltage 6.7 times.\u00a0 This puts a much higher demand on the coil, which should be low loss and have low wire resistance.\u00a0 The 35 turn primary winding should not be made of thin wire, it should have a resistance 1\/4 ohm or less.<\/p>\n Needs More Power<\/strong>\u00a0 Also, you must remember that this JT will not be able to light up the LEDs to full brightness.\u00a0 Let’s do some simple math to see why.\u00a0 The three LEDs require 10V total, at 20 milliamps for full brightness.\u00a0 That is 200 milliwatts, or three times the power of a typical one LED JT.\u00a0 Therefore the input current from the battery must be three times higher – a typical JT uses 80 milliamps, so this JT needs 240 milliamps.\u00a0 The problem is that even as good as the BC337 is, it can’t handle that much current.\u00a0 What this JT needs is either three transistors or a transistor that can handle three times the current.\u00a0 Typical high current transistors are the KSD5041, ZTX651 or NTE11.\u00a0 Or put three BC337s in parallel.\u00a0 Just connect the emitters ans collectors of the three transistors together, and connect each base to its own 1k resistor, with the other ends of these resistors going to the feedback winding.<\/p>\n The coil has a feedback winding with 20 turns, which reduces the negative voltage that the transistor’s base to emitter junction has to handle, which is specified at 5V maximum in the datasheets.\u00a0 But 20\/35 times 10V is 5.7V, which is still somewhat over the maximum.\u00a0 The feedback winding should have a few turns less, say about 17 or 16.<\/p>\n Obnoxious<\/strong>\u00a0 I noticed whenever my mouse pointer went over one of the highlighted, underlined words in the text, it shot up a big advert for something totally unrelated to that word.\u00a0 This is really obnoxious and it’s a shame that the blogger\/blogspot (owned by Google) has to stoop so low to get people to see their ads.\u00a0 I hope they end up in the same situation as AOL: no one wants to put up with this pestilence.<\/p>\n Back to experimenting…<\/p>\n","protected":false},"excerpt":{"rendered":" Early on in my JT experiments I experimented with a pot (potentiometer) in place of a resistor, which was used as an adjustable resistor and allowed me to optimize that particular JT.\u00a0 After the optimum setting was determined, the pot could be removed and replaced with a regular resistor.\u00a0 It seemed that the pot was <\/p>\n (Read More…)<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5,4,12],"tags":[],"class_list":["post-324","post","type-post","status-publish","format-standard","hentry","category-flashlight","category-joule-thief-smps-dc-dc","category-led"],"_links":{"self":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/324","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=324"}],"version-history":[{"count":9,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/324\/revisions"}],"predecessor-version":[{"id":1575,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/324\/revisions\/1575"}],"wp:attachment":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=324"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=324"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=324"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}