{"id":171,"date":"2011-12-03T07:21:26","date_gmt":"2011-12-03T15:21:26","guid":{"rendered":"http:\/\/rustybolt.info\/wordpress\/?p=171"},"modified":"2012-09-15T02:55:00","modified_gmt":"2012-09-15T09:55:00","slug":"2011-12-03-low-voltage-joule-thief-tn0702-mosfet","status":"publish","type":"post","link":"https:\/\/rustybolt.info\/wordpress\/?p=171","title":{"rendered":"2011-12-03 Low Voltage Joule Thief – TN0702 MOSFET"},"content":{"rendered":"

<\/strong>\"\"<\/a>\"\"<\/a>In Jan 2009<\/strong> I collaborated with Quantsuff<\/a> and we tried different designs for a low voltage Joule Thief.\u00a0 The conventional silicon BJT Joule Thief (b<\/span>ipolar j<\/span>unction t<\/span>ransistor) has a base to emitter forward voltage of about 0.55 to 0.6 volts, so if the supply is below this voltage it will not start.\u00a0 But once it gets started, it will run to below 0.5V, maybe down to 0.35V.\u00a0 We can change the transistor to a germanium, which is an ancient 1950s transistor technology no longer made.\u00a0 The\u00a0germanium’s forward voltage is about 0.2 to 0.25 volts, so the germanium BJT Joule Thief will start up at half the voltage of a silicon BJT.\u00a0 But as I said, these are old transistors no longer made and difficult to obtain, and the reasonably priced ones can’t handle a large amount of current and power.<\/p>\n

One idea QS had was to use a switch to charge a capacitor when the JT was off.\u00a0 Then when the switch was turned on, the capacitor’s voltage was added to the supply voltage, and the JT would start up.\u00a0 The supply voltage could be as low as 0.3 volts, and the added voltage would be 0.6V, enough to start the JT.\u00a0 Cool.\u00a0 But someone had to switch the switch every time it had to be powered up.\u00a0 About that time solar LED garden lights became cheap and experimenters like Bill Sherman were buying them, opening them up and experimenting with the parts.\u00a0 If you built solar garden lights with a half volt solar cell and the switch and capacitor to start them, you would have to walk around to every garden light to switch the switch to turn it on.\u00a0 Obviously this was not a very practical solution.<\/p>\n

The Law of Diminishing Returns<\/strong>\u00a0 Another problem at low voltages was getting enough power to light the LED reasonably bright.\u00a0 Let’s say we have a JT that puts out enough current to the LED, say 10 milliamps, with the 1.5V cell supplying a current of 50 milliamps. The 1.5V times the 50 milliamps is 75 milliwatts drawn from the cell.\u00a0 The JT is boosting the 1.5V cell up to about 3 volts across the LED, or about doubling the voltage.<\/p>\n

We want to run the JT at half that voltage, 0.75 volts, but we still want the LED current to be 10 milliamps.\u00a0 We lower the supply voltage down to 0.75 volts, but the current from the cell and the current to the LED drop dramatically.\u00a0 We have to do something to bring the LED current up to 10 mA, so we replace the 1k resistor that is used in the conventional JT with a lower value.\u00a0 This helps some.\u00a0 We look at what the JT needs<\/em> to do the same job that it was doing at 1.5V.\u00a0 When the supply voltage is cut in half, the supply current needs to be doubled in order for the JT to draw the same amount of power from the supply.\u00a0 Therefore the supply has to see a load that is one quarter the resistance<\/em>. Yes, the voltage will be half, and the current will be double, so the load has to be 1\/4<\/em> the resistance in order to draw the same power.\u00a0 Also, the JT has to boost the voltage from 0.75 volts up to 3 volts, or four times higher<\/em>.<\/p>\n

We look at the components: the supply, the transistor, the coil, the resistor, the wiring (the LED has to stay the same; it can’t have a lower forward voltage).\u00a0 The transistor has to conduct twice as much current at half the supply voltage, so it has four or more times the demand put on it.\u00a0 The coil has to handle twice the current at half the voltage, so the DC resistance of the primary winding must be reduced to 1\/4 to keep the losses down to the same level.\u00a0 This means the wire has to be much heavier to handle the higher current and lower voltage.\u00a0 And the coil has to boost the voltage twice as high as it did previously so it has a greater demand put on it.\u00a0 The supply has to supply twice the current, or 100 milliamps to the JT, so the demand has doubled on the battery.\u00a0 Also the resistor has to be reduced to 1\/4 (or more) because the transistor has to handle twice the collector current, which means at least twice the base bias current, and at half the supply voltage. And remember, we have only reduced the supply voltage to 0.75V, which is half of 1.5V.<\/p>\n

When we again reduce the supply voltage by half to 0.375 volts, the supply current again doubles to 200 milliamps.\u00a0 The demands again multiply exponentially, so that things that were insignificant before at 1.5V are now becoming really significant and can cause problems if not taken care of.\u00a0 An example is the size of the copper traces (‘wires’ or conductors) on the circuit board have to be much heavier to handle the greatly increased current.\u00a0 The bypass capacitors have to be much larger to handle the much higher currents.\u00a0 The length of the circuit board traces may become a problem because they add inductance, and may interact with the coil and capacitors.\u00a0 The resistor has to again be reduced by 1\/4, so it is now down to 62 ohms (1\/16th of its original 1000 ohm value).\u00a0 My whole point of this explanation is we are reaching a point where the Law of Diminishing Returns begins to eat away at our results, and it becomes much, much more difficult<\/em> to do what seemed relatively easy at 1.5 volts.<\/p>\n

MOSFETs<\/strong>\u00a0 I experimented with the MOSFET augmented Joule Thief using a 2N7000 and a 2N3904 BJT.\u00a0 It worked fine, but it would not start reliably below 0.6 volts.\u00a0 I brainstormed and came up with the idea that since the gate of a MOSFET does not need any current, only voltage, I could put a button cell in there and it would reduce the gate voltage to less than 1 volt.\u00a0 This worked well for the 2N7000 Joule Thief.\u00a0 But we really wanted to get the JT to start and run at voltages below 0.6V and also put out a reasonable amount of current, which the silicon and germanium JTs could not do at very low voltages.<\/p>\n

I ordered some TN0702 low gate threshold voltage MOSFETs.\u00a0 These are low power devices; they are not power MOSFETs.\u00a0 But one can put out enough current at low voltage to drive the LED of a Joule Thief to reasonably bright current.\u00a0 The first circuit I built with one was a MOSFET JT that did not need a resistor, and was the simplest Joule Thief possible, only three parts: a coil, a TN0702, and a LED (not counting the battery and on.off switch) (see Fig.1 of the schematic).\u00a0 I blogged this and it became one of the more popular of my blogs because the search engines gave it a lot of hits when people searched for low voltage Joule Thief.<\/p>\n

Considering the demands I pointed out above, the TN0702 worked as well as could be expected in the JT circuit I made (see the pictures).\u00a0 The extreme demands put on it at a supply voltage of only 0.2 volts made it impossible to supply as much LED current as a JT at 1.5V, but it did as much as it could, and I measured 0.5 milliamps LED current.\u00a0 That doesn’t seem like much but it was far greater than I had ever been able to do with a JT at a supply voltage of only 0.2 volts.\u00a0 The supply current was only 45 milliamps, but again that was much more than any germanium transistor was able to do (silicon transistors were useless – not able to go below a half volt).<\/p>\n

The article found here (.PDF)<\/a> explains the theory and construction of a DC-DC converter with an input of 350 millivolts (0.35V) and an output of 5 V for running a microprocessor or other device, meant to be supplied by a thermoelectric generator (TEG).\u00a0 TEGs are often made with Peltier junction devices, which are also used in small coolers that plug into the 12V car power.\u00a0 In this case the TEG was a thermopile.<\/p>\n

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

In Jan 2009 I collaborated with Quantsuff and we tried different designs for a low voltage Joule Thief.\u00a0 The conventional silicon BJT Joule Thief (bipolar junction transistor) has a base to emitter forward voltage of about 0.55 to 0.6 volts, so if the supply is below this voltage it will not start.\u00a0 But once it <\/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":[6,4,12],"tags":[14,15,16],"class_list":["post-171","post","type-post","status-publish","format-standard","hentry","category-electronics","category-joule-thief-smps-dc-dc","category-led","tag-low-voltage-joule-thief","tag-mosfet-joule-thief","tag-simplest-joule-thief"],"_links":{"self":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/171","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=171"}],"version-history":[{"count":56,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/171\/revisions"}],"predecessor-version":[{"id":173,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/171\/revisions\/173"}],"wp:attachment":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=171"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=171"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=171"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}