Following my previous not so bright 1.5V LED flashlight I wanted to see how much
that circuit could be improved to get higher currents through the LED. This is because I got curious not because I wanted to change my flashlight. I have left
this new high power 1.5V LED flashlight at the breadboard stage since there are plenty of commercial LED flashlights available in stores. It is just interesting to see how to do it. I found that the previous one transistor only oscillator can not easily go beyond 10mA LED current with 1.4V power input. I needed to change the circuit. Just improving the transformer/coil did not do it.
The new circuit with 2 transistors can produce much sharper current pulses in the coil and therefore get higher output
currents. Using the same ferrite toroid with 50 turns of AWG25 enamel insulated copper wire I was able to get
easily 15mA LED current at 1.4V power input. A bigger ferrite toroid with AWG 22 wire produced immediately 32mA LED current. It did not seem that the new circuit was already at its limit with the bigger ferrite toroid. I just used that toroid because I found it "as is" in the basement.
high power 1.5V LED booster circuit
high power 1.5V LED breadboard setup
Oscilloscope probe at the collector pin of the NPN transistor, shows nice sharp current spikes with 24kHz
The circuit has more components but it is still simple enough and the coil is actually easier to build because it is just one coil and not a high frequency transformer setup. This allows you to use off the shelf commercial inductors (a 50uH to 100uH inductor will work well). Using a bigger inductor will result directly in higher output power. Note also that
a higher quality inductor with a lower internal resistance (bigger wire diameter) may produce better results because it will allow for stonger magnetic fields with fewer losses in the coil.
Large toroid with awg 22 wire, this produces 32mA DC with a white LED
Small toroid with 50 turns of awg 25 wire, this produces 15mA DC with a white LED
The circuit starts to work at 0.8V input voltage. That is a bit higher than the previous one but it is still very low and
good enough to re-use old batteries that you would throw out otherwise. Any already used battery with 1V to 1.5V will produce good light output. This is better than most commerically available LED flashlights.
update: 2021-01-02
I have been searching for circuits designed by others and I found this blog entry:
It is just one capacitor more than my previous one transistor only oscillator and
it is not as good as the above circuit with 2 transistors but that one extra capacitor improves the switching characteristics of the transistor resulting in higher output currents. I was able to get 12mA LED DC current at 1.4V input using
the same coil/transformer that had previously only produced 8mA. You have to play a bit with the value of the capacitor
attached to the base of the transistor. Try not only 2.2nF (222) but also 1nF (102) and 3.3nF (332). The
optimal value of capacitor depends a bit on the transistor. Here is the circuit diagram:
Improved Joul Thief, 1.5V LED flashlight, circuit diagram
