{"id":1794,"date":"2012-03-24T17:18:35","date_gmt":"2012-03-25T00:18:35","guid":{"rendered":"http:\/\/rustybolt.info\/wordpress\/?p=1794"},"modified":"2012-05-05T16:20:24","modified_gmt":"2012-05-05T23:20:24","slug":"2012-03-24","status":"publish","type":"post","link":"https:\/\/rustybolt.info\/wordpress\/?p=1794","title":{"rendered":"2012-03-24 Joule Thief Powers 120VAC LED Night Light"},"content":{"rendered":"

I received a Google Alert with a link for this Youtube video<\/a> of a JT powering a LED night light.\u00a0 I have some thoughts that I’d like to share.<\/p>\n

A LED or three in a night light may take only a quarter watt or less, if run from a battery.\u00a0 But when run from the 120VAC, there is a huge difference in the voltage of the supply and the LEDs.\u00a0 What do we do to allow the LEDs to be connected to 120VAC?\u00a0 Well, the difference has to be removed, by dropping the difference across a resistance, reactance, or combination of the two, impedance, or by conversion.<\/p>\n

Let’s look at conversion first.\u00a0 If you plug your laptop or PC into the wall, the power supply or adapter converts the 120VAC to a dozen volts or so, or multiple voltages at 12 volts, 5 volts, etc.\u00a0 This switching power supply does several funcyions, is too complicated and too expensive to be used to power a LED night light from the wall.<\/p>\n

Instead, the other method is used to drop the excess voltage because it is cheaper and simpler.\u00a0 A resistance or reactance, or both is\/are connected in series with the LED or LEDs, and the voltage difference appears across this or these components. Picture this: a ‘voltage dropper’ and a LED connected in series and then connected across the power plug pins.<\/p>\n

If a resistance is used, the circuit is very inefficient.\u00a0 A LED typically needs 20 milliamps or 0.02 amp of current.\u00a0 The voltage drop is 3.3 volts for a single LED.\u00a0 So the remaining voltage, about 115 volts, has to drop across the resistance or reactance, or both.<\/p>\n

Let’s do the math for a resistance.\u00a0 The 115 volts at 0.02 amps is 2.3 watts, and for the LED, 3.3 volts at 0.02 amps is 0.066 of a watt. This comes out to 2.8 percent of the power in the LED, and the remaining 97.2 percent of the power in the resistance.\u00a0 Essentially, the 2.3 watts is being wasted as heat.\u00a0 This is very poor efficiency and wasteful of power.<\/p>\n

How do we get around this waste?\u00a0 If we replace the resistance with a capacitance, the excess voltage is dropped across the reactance of the capacitor and the capacitor dissipates almost no heat.\u00a0 We can have our cake and eat it, so to speak.<\/p>\n

Now for the Joule Thief<\/strong>\u00a0 Instead of using a wasteful resistor, the LED night light uses a capacitor to drop the excess voltage.\u00a0 When we connect the Joule thief circuit with its 700 or more turns coil to the LED night light, this capacitor is dropping most of the JT’s voltage and the remaining is used to light the LED.\u00a0 The big difference between the power at the wall socket and the Joule Thief’s output is that the frequency of the wall socket is 60 Hz, but the Joule Thief is a thousand or more times higher, or 60kHz or more.<\/p>\n

Since the capacitor’s reactance depends on frequency, and the frequency is much higher, the reactance is much lower, since a capacitor’s reactance varies as the inverse of the frequency.\u00a0 Therefore since the frequency is a thousand times higher, the reactance is a thousand times lower.\u00a0 At 60 kilohertz, it should take much less voltage from the Joule Thief to light the LED brightly.<\/p>\n

Skip the following paragraph if you don’t understand the math.\u00a0 The LED night light needs a capacitor to drop the voltage at 60 Hz.\u00a0 This cap must have a reactance of 115 volts divided by 0.02 amps, or 5750 ohms.\u00a0 The value of this cap at 60 Hz. is 1 divided by 2, divided by Pi, divided by 60 Hz, divided by 5750 ohms reactance.\u00a0 This turns out to be 0.46 microfarad, or rounded to the nearest common value, 0.47 microfarad or 470 nanofarads.\u00a0 If we look inside of the nightlight we should find a 0.47 uF capacitor in there.\u00a0 It might be marked U47J250 or something similar.<\/p>\n

At 60 kHz, the 0.47 uF capacitor has a reactance of 1\/1000 of 60 Hz, or 5.75 ohms.\u00a0 This is practically nothing when it’s in series with the LED.\u00a0 The full 120VAC will not be needed to light up the LED night light; it should light up brightly at a much, much lower voltage with the 60kHz JT output connected to it.<\/p>\n

But how will this higher frequency affect the rest of the LED night light circuit?\u00a0 If the circuit uses a 1N4005 or similar rectifier, it will have a very slow reverse recovery time because it was made to operat at 69 Hz,\u00a0 The 60kHz from the JT will be rectified very inefficiently, which could waste a lot of the power.<\/p>\n","protected":false},"excerpt":{"rendered":"

I received a Google Alert with a link for this Youtube video of a JT powering a LED night light.\u00a0 I have some thoughts that I’d like to share. A LED or three in a night light may take only a quarter watt or less, if run from a battery.\u00a0 But when run from the <\/p>\n

(Read More…)<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4,12,13],"tags":[],"class_list":["post-1794","post","type-post","status-publish","format-standard","hentry","category-joule-thief-smps-dc-dc","category-led","category-lighting"],"_links":{"self":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/1794","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=1794"}],"version-history":[{"count":11,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/1794\/revisions"}],"predecessor-version":[{"id":2618,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/1794\/revisions\/2618"}],"wp:attachment":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1794"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1794"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1794"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}