I used an older Joule Thief that I pulled out of the pile to make an adjustable power Joule Thief. \u00a0I changed the transistor to a SS9018, put a 1 ohm resistor in series with the cathode of the white LED to allow measuring the LED current. \u00a0I left a 470 Ohm resistor between the base of the transistor and the feedback winding – it can be zero ohms. \u00a0Between the other end of the feedback winding and the positive I put a 470 K trim pot to allow adjustment of the brightness. \u00a0From this end of the feedback winding to negative I put A 150 Picofarad \u00a0capacitor. \u00a0Without this low value capacitor the LED will not light at higher trim pot settings and lower brightness levels.<\/p>\n
The transistor is a low current preamplifier and has very high current gain, up to a thousand, so the resistance of the trim pot has to be high to allow the brightness to be adjusted to low values. Still the trim pot has point where it is very touchy and just a slight amount of movement causes a large difference in brightness. \u00a0This seems to indicate that there are two ‘modes’ of operation, and it tries to jump from one mode to the other at this touchy spot when adjusted. \u00a0This may have to do with interaction between the capacitor and coil.<\/p>\n
The 470 K trim pot allows adjustment from about 50 milliamps battery current down to less than 1 milliamp. \u00a0Correspondingly the LED brightness goes from high to low as it’s adjusted. \u00a0If a point is found where the LED is stable, the resistance of the pot can be measured and then the pot can be removed and be replaced with a fixed resistor.<\/p>\n
I monitored the input and output current and adjusted it for about two milliamps of input current and about 1 milliamp to the LED. The trim pot resistance was about 143k. I connected this circuit to a AAA cell on a table in my bedroom and watched it during the night. At only 1 milliamp, the LED was bright enough to serve as a good night light. \u00a0Assuming 2000 mAh for a AA cell, the JT should run for 1000 hours or about 6 weeks at 2mA. \u00a0But we all know that JTs seem to run forever as the cell is depleted and the LED brightness drops.<\/p>\n
Update -This has been running for a few weeks with a cheap carbon Zinc battery. It makes a good night light. \u00a0I have found that these very low power JTs look brightest to the eye when the LED is an ultrabright green. \u00a0The human eye is most sensitive in the yellow to green wavelengths. \u00a0Also its most likely that a white LED is losing some efficiency when it converts blue light to white. \u00a0So the green LED is probably more efficient.<\/p>\n","protected":false},"excerpt":{"rendered":"
I used an older Joule Thief that I pulled out of the pile to make an adjustable power Joule Thief. \u00a0I changed the transistor to a SS9018, put a 1 ohm resistor in series with the cathode of the white LED to allow measuring the LED current. \u00a0I left a 470 Ohm resistor between the <\/p>\n (Read More…)<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[],"class_list":["post-9330","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\/9330","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=9330"}],"version-history":[{"count":15,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/9330\/revisions"}],"predecessor-version":[{"id":9332,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/9330\/revisions\/9332"}],"wp:attachment":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=9330"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=9330"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=9330"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}