I received a Google Alert with a link for this Youtube video of a JT powering a LED night light. 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. But when run from the 120VAC, there is a huge difference in the voltage of the supply and the LEDs. What do we do to allow the LEDs to be connected to 120VAC? Well, the difference has to be removed, by dropping the difference across a resistance, reactance, or combination of the two, impedance, or by conversion.
Let’s look at conversion first. 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. 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.
Instead, the other method is used to drop the excess voltage because it is cheaper and simpler. 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.
If a resistance is used, the circuit is very inefficient. A LED typically needs 20 milliamps or 0.02 amp of current. The voltage drop is 3.3 volts for a single LED. So the remaining voltage, about 115 volts, has to drop across the resistance or reactance, or both.
Let’s do the math for a resistance. 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. Essentially, the 2.3 watts is being wasted as heat. This is very poor efficiency and wasteful of power.
How do we get around this waste? 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. We can have our cake and eat it, so to speak.
Now for the Joule Thief Instead of using a wasteful resistor, the LED night light uses a capacitor to drop the excess voltage. 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. 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.
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. Therefore since the frequency is a thousand times higher, the reactance is a thousand times lower. At 60 kilohertz, it should take much less voltage from the Joule Thief to light the LED brightly.
Skip the following paragraph if you don’t understand the math. The LED night light needs a capacitor to drop the voltage at 60 Hz. This cap must have a reactance of 115 volts divided by 0.02 amps, or 5750 ohms. 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. This turns out to be 0.46 microfarad, or rounded to the nearest common value, 0.47 microfarad or 470 nanofarads. If we look inside of the nightlight we should find a 0.47 uF capacitor in there. It might be marked U47J250 or something similar.
At 60 kHz, the 0.47 uF capacitor has a reactance of 1/1000 of 60 Hz, or 5.75 ohms. This is practically nothing when it’s in series with the LED. 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.
But how will this higher frequency affect the rest of the LED night light circuit? 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, The 60kHz from the JT will be rectified very inefficiently, which could waste a lot of the power.