LED filaments are thin, stick like LEDs. They are used in many Edison bulb look-a-like LED bulbs.Each of those thin sticks contain many - 20 to 30 LEDs connected in series. So they are quite bright and energy efficient, however typically require over 70V to turn on.I wanted to make a small, battery operated lamp using those LED filaments. After trying out many circuits, I discovered a very simple circuit to light a filament LED with only one, 1.5V battery.
Step 1: Circuit Designs
I knew I should be able to use boost converter to boost the low voltage from a battery, however didn't think I can easily get the 70V. I have tried using specialized boost LED driver ICs with success, but those ICs need 3V or higher to operate.Next I experimented with a simple, two transistor Joule Thief (blocking oscillator) circuit. I did not have a transistor that can withstand 70V, so I used charge pump circuit to double the output of the Joule Thief. This way the transistor is exposed to only half the final output, or 35V.
This circuit worked, and I was happy with the performance for a while, but still wanted to reduce the component count. So I obtained a few transistors that can handle over 70V of voltage, and tried to see if I can light filament LED with just a Joule Thief. After some tweaking of the component values I found the circuit to work just as well as the charge pump assisted Joule Thief circuit!
Step 2: Final Circuit
So here is the final circuit. It is deceivingly simple, but works as well as the previous version with many more components.
The key is using transistors that can handle high enough voltage. I used KSP06, which has Vceo of 80V, just high enough for this project. Other specs such as hfe and Vbe is still good enough to operate at low supply voltage.
I tuned the components to not draw too much current, since the power source is AAA battery which is small. You can adjust R1, R2, as well as C1 to draw more current and light LED with more power if you want. For example R1: 470 ohm, R2: 47k ohm, and C1: 22pF would produce higher output, but the battery will drain much quicker.
Step 3: Final Touch
I designed a PCB to fit into a glass test tube.
It uses a single AAA battery (alkaline or NiMH) and draws about 50 mA.
I also added a tilt switch to turn on the LED when the unit is standing upright, and turn off when resting. I inserted the unit into the tube to make it look a bit like a vintage tube.
I put together the PCB and components as an easy to assemble kit - available at my website: https://www.theledart.com/products/jt-filament - if you are interested.