{"id":5836,"date":"2013-01-17T09:28:26","date_gmt":"2013-01-17T17:28:26","guid":{"rendered":"http:\/\/rustybolt.info\/wordpress\/?p=5836"},"modified":"2013-01-19T10:45:39","modified_gmt":"2013-01-19T18:45:39","slug":"2013-01-17-questions-about-joule-thief-transistors","status":"publish","type":"post","link":"https:\/\/rustybolt.info\/wordpress\/?p=5836","title":{"rendered":"2013-01-17 Questions About Joule Thief Transistors"},"content":{"rendered":"

This is a continuation of my earlier Questions about Joule Thief..<\/em><\/a> blog.\u00a0 Paul, who is from the U.K., said he is using the BCY51, BFY51 and 2N1711 transistors, and wanted to know if I thought they were suitable for a Joule Thief.<\/p>\n

\"\"<\/a>The information I could get online about the BCY51 was scanty, but what little I saw seemed to indicate that it was made for high radio frequency use and cannot handle the JT current of over 100mA, therefore I would say that it is not suitable for a Joule Thief, however poorly it may seem to work.\u00a0 According to the datasheet, the BFY51 is a general purpose transistor that can handle a full amp.\u00a0 The Philips datasheet did not show any graphs.\u00a0 The hFE<\/sub> at 10V and 150 mA is given as 40 minimum.\u00a0 That’s much lower than the typical 150 or so current gain of a PN2222A or 2N4401.\u00a0 The Vce(sat) at Ib=15 mA, Ic=150 mA is 0.350 V maximum.\u00a0 That is higher (not as good as) the PN2222A, which is 0.3VDC.\u00a0 Because it can handle the current but has the lower gain and higher Vce(sat), I would put this on the lower end of suitability for a JT.\u00a0 It is going to need a lower resistance than the 1000 ohms of the typical JT.<\/p>\n

The Last transistor Paul used was the 2N1711, and I have a few of those.\u00a0 According to the ST datasheet, the 2N1711’s Vce(sat) at Ic=150mA, Ic=15mA is typically 0.5 V, maximum 1.5V.\u00a0 Oof!\u00a0 That last figure is the same as the supply voltage!!\u00a0 The JT transistor must conduct well over 100 mA in order to charge up the coil during its on time.\u00a0 Even if the 2N1711 is at the typical 0.5V, one third of the supply voltage will be wasted across the transistor as heat. \u00a0 I took one 2N1711 and soldered it into a Joule Thief which already had a BC337-25 in it.\u00a0 The BC337-25 was drawing 88 ma from the 1.5V supply, which is what I would expect from a Joule Thief putting out nearly 20 mA to the LED.\u00a0 When I put the 2N1711 into this circuit, the supply current dropped down to 64 mA.\u00a0 To get the supply current up to 86 mA, I had to drop the resistor from 1k down to 470 ohms.\u00a0 The lower current gain of the 2N1711 is what made the difference.\u00a0 I don’t have a current sensing resistor in series with the LED, but from the many other JTs that I’ve\u00a0 experimented with, I already can predict that the LED current will be only a dozen or so milliamps instead of the 18 to 20 it was getting from the BC337.<\/p>\n

To me the choice is obvious.\u00a0 The 2N1711 and other transistors he has experimented with are packaged in a metal can, which costs many times as much as a BC337-25 in a plastic package.\u00a0 The metal package is not needed; the heat dissipated by the transistor (when it’s the proper transistor with a low Vce(sat)) is minimal and it won’t get warm.\u00a0 And the BC337-25 makes a very good choice for a JT transistor.\u00a0 The BC337-25 is available from Farnell for the miserly sum of 2.50 to 3 Pounds for 100.\u00a0 The 2N1711 will cost you 40 Pounds for a hundred.\u00a0 Ouch!<\/p>\n

If you want to see an excellent JT transistor, try the ZTX1048A, from Zetex in the U.K.\u00a0 The gain at a half amp is typically 450, and the Vce(sat) is an incredibly low 0.027 volt!\u00a0 These are more expensive, but can drive several LEDs.\u00a0 Here’s a link to the Farnell catalog<\/a>.<\/p>\n

I used a lot of PN2222A transistors in my Blue Blinkies for Xmas decorations; they’re good for experimenting and available at Radio Shack stores.\u00a0 I noticed that they don’t seem to be in Farnell’s catalog.\u00a0 The 2N4401 is another one available at Radio Shack stores.\u00a0 The BC337-25 and BC337-40 are great for JTs – probably the best buy for the money at 5 cents apiece.\u00a0 The -40 will act like a racehorse in a JT and you have to say “Whoa, Nellie!” by increasing the resistance to 3.3k or more, lest the LED get too bright and burn out.\u00a0 The complimentary (opposite polarity PNP) to it is the BC327-25.\u00a0 The overseas company futurlec.com has components in small quantities and the shipment I got a few weeks ago took a couple weeks to arrive, but then some delay might have been because of the Xmas holidays.\u00a0 However I did have a problem (see my other blog<\/a>), and they finally rectified it by sending me replacements which arrived a few days ago.<\/p>\n

Paul asked about<\/p>\n

The transistors that Paul said he has, for their day, were very good.\u00a0 But they were the ‘jack of all trades’ type which didn’t have any outstanding attributes.\u00a0 They were designed with few compromises.\u00a0 The transistors of today, such as the ZTX1048A, are more specialized, and have a few outstanding attributes, such as the Vce(sat), and high gain.\u00a0 But in order to get those, they had to sacrifice some other place.\u00a0 One sacrifice is the ZTX can only handle 17.5v maximum, less than half of what those earlier transistors’ maximum.\u00a0 There are other sacrifices, too.\u00a0 The laws of physics can’t be ignored, and the transistors have to obey just like everything else.\u00a0 One thing that has improved is the manufacturing process, so that newer transistors don’t have to have the greater margins of safety that older ones had.\u00a0 For instance, the equipment lets the designers control the process to closer tolerances, which allows them to make the transistors so that the junctions are closer together, thus giving higher gain.\u00a0 And the same applies to the chemistry and temperatures.\u00a0 One thing that bothers me, though, is that Fairchild or Philips may sell a transistor to us for five cents and still make money, but the ZTXs sell for nearly twenty times that much, yet they’re all made from the same silicon and similar processes.\u00a0 I guess it’s a question of supply and demand.<\/p>\n

One important point.\u00a0 You don’t need a ZTX1048A for a Joule Thief with a single LED.\u00a0 This transistor is so powerful that you want to use it for a large number of regular 5mm LEDs or for a 1 to 3 watt LED.\u00a0 I tell others to use the BC337-25 because it matches up well with a single 5mm LED.\u00a0 If you use its higher gain version, the BC337-40, you will get more than the required 20 mA for full brightness with 1 LED; instead you can use it to power two or three 5mm LEDs.\u00a0 The ZTX can power many more than that, so don’t buy them if all you want is to power 1 or 2 LEDs.<\/p>\n

You don’t need to order a minimum at some stores.\u00a0 I got some of my transistors from eBay in smaller quantities.\u00a0 But I have also had some bad experiences with eBay sellers.\u00a0 Almost all of the commodity priced components there are new and may even come on tape, so you know they’re not used.\u00a0 But some vendors sell certain things, especially LEDs, at good prices, and they don’t tell you (or may not know themselves) that these are factory rejects with flaws such as LEDs with air bubbles in them.\u00a0 For experimenting, this isn’t so much of a problem, but if you’re building something like a flashlight, you want a tight beam of light, not something that’s diffused by air bubbles.<\/p>\n","protected":false},"excerpt":{"rendered":"

This is a continuation of my earlier Questions about Joule Thief.. blog.\u00a0 Paul, who is from the U.K., said he is using the BCY51, BFY51 and 2N1711 transistors, and wanted to know if I thought they were suitable for a Joule Thief. The information I could get online about the BCY51 was scanty, but what <\/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-5836","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\/5836","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=5836"}],"version-history":[{"count":20,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/5836\/revisions"}],"predecessor-version":[{"id":5838,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/5836\/revisions\/5838"}],"wp:attachment":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5836"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5836"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5836"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}