{"id":4018,"date":"2012-09-04T05:41:33","date_gmt":"2012-09-04T12:41:33","guid":{"rendered":"http:\/\/rustybolt.info\/wordpress\/?p=4018"},"modified":"2013-02-27T05:26:57","modified_gmt":"2013-02-27T13:26:57","slug":"2012-09-04","status":"publish","type":"post","link":"https:\/\/rustybolt.info\/wordpress\/?p=4018","title":{"rendered":"2012-09-04 V Booster, NPN-NPN (often mislabeled a Joule Thief)"},"content":{"rendered":"

\"\"<\/a>Quantsuff<\/a> gave me permission to post his schematic of the two NPN transistor voltage booster, often mislabeled a Joule Thief.\u00a0 This uses an NPN transistor to drive the output transistor, which is also NPN.\u00a0 The commonest two transistor V booster uses a PNP to drive the NPN output transistor, and for good reason, as I explain below.<\/p>\n

I should explain why I know a bit about this circuit.\u00a0 I first started using the single transistor voltage booster, later known as the Joule Thief, more than ten years ago but due to my inexperience with the very high current demand on the transistor, I didn’t get the performance that I was expecting.\u00a0 So I changed to the two transistor designs, including this one.\u00a0 I got better performance from these so I used the two transistor design to build my first batch of single cell LED lights.<\/p>\n

Demanding<\/strong> – There are at least two criteria that must be met to get this circuit to put out enough current to the LED.\u00a0 First off the output transistor Q1 must be able to switch high enough current.\u00a0 When this transistor is fully on, the collector voltage should be less than 1\/4 volt.\u00a0 Then the resistor R1 supplies the current to cause Q1 to saturate, so this resistor must be low enough to allow that high current.\u00a0 Remember that the supply voltage is only 1.5V, and the base to emitter voltage is at least 0.6V (probably more like 0.7 or maybe more), so the voltage across R1 is less than 0.9V.\u00a0 I used a BC337 for Q1 and I used a 470 ohm resistor to get the Q1 base bias current up high enough to cause Q1 to fully saturate.\u00a0 But my calculations show that it might be 100 ohms or even less.<\/p>\n

Once we get Q1 to saturate, we then need to get it turned off.\u00a0 In order to do this, Q2 must shunt the base current of Q1 to ground.\u00a0 Q1 must turn fully on and drop its collector voltage to less than 0.6V, which is the base voltage of Q1.\u00a0 So R2 has to be low enough to cause Q2 to fully turn on and saturate.\u00a0 Thus, R2 must be less than a certain value depending on the current gain of Q2.\u00a0 I put a 100k pot there and adjusted it lower and higher.\u00a0 At the higher settings the collector voltage of Q2 \/ base voltage of Q1 was too high and the circuit put out less light and drew excessive current.<\/p>\n

Some experimenters love to put adjustable resistors AKA pots in there and see what the circuit does.\u00a0 A good designer will calculate the resistance value assuming minimum supply voltage and minimum gain for the transistors, and then use that value in the circuit.\u00a0 But there’s not much fun then, is there?\u00a0 One other point.\u00a0 The circuit must have enough gain to start oscillating when powered up.\u00a0 The gain of Q2 must be high enough to get it oscillating.<\/p>\n

\"\"<\/a>The PNP – NPN version of this circuit (see photo) is different in that when Q2 turns on, its collector current turns on Q1’s base, and therefore the Q1 base current is not determined by the resistance.\u00a0 Instead it is determined by how hard Q2 is turned on, and that could be tens of milliamps.<\/p>\n

Back to experimenting…<\/p>\n","protected":false},"excerpt":{"rendered":"

Quantsuff gave me permission to post his schematic of the two NPN transistor voltage booster, often mislabeled a Joule Thief.\u00a0 This uses an NPN transistor to drive the output transistor, which is also NPN.\u00a0 The commonest two transistor V booster uses a PNP to drive the NPN output transistor, and for good reason, as I <\/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-4018","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\/4018","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=4018"}],"version-history":[{"count":20,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/4018\/revisions"}],"predecessor-version":[{"id":4020,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/4018\/revisions\/4020"}],"wp:attachment":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4018"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4018"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4018"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}