I read about a center-tapped joule thief and decided I might be able to use
this as a better voltage booster circuit. Unfortunately, it did not seem to
work as well as I had hoped.
The basic idea is that, with a center tap there are two outputs of opposite
polarity and these can be used to "double" the available power into a capacitor
to be used by a low-current circuit needing a higher voltage.
In playing with this circuit, I didn't feel it really had any advantages over
the basic joule thief circuit in supplying a steady voltage to even an LED
unless the LED was a simple 20 mA LED.
I wanted to use this circuit to drive a flickering amber LED for more realistic
Christmas Window Candles. The voltage dropped to around 2.0 - 2.2 Volts and,
while the flickering LED worked, it seemed dimmer than I wanted.
The schematic shows the initial center tap. Initially I used 20 loops of
wire, so ten on each side.
Thanks to Dean who caught the fact that I previously had the transistor
collector shorted to ground which is NOT the way this circuit was designed. I
had simply removed the LED that was there for the standard Joule Thief
The below trace shows that the positive and negative pulses happen at the
same time and are about the same value, as expected. This leads to a problem:
there is no second pulse coming during the off-time of the first. This gives
a short duration pulse and then a longer down time. In other words there is no
negative going pulse to keep the voltage even.
Trace 1: center tap trace
In thinking about this I was wondering what would happen if the center tap was
ignored. I measured the output from the extra loops of wire end to end. The
trace below is for the 22 loop version.
Trace 2: End-to-end coil of extra wire, 22 loops
In this trace it can be seen that there are two pulses: a large positive pulse
and a smaller, lower-voltage, negative pulse during the off-time of the
positive pulse. I liked this better, even though the negative pulse is only
about 2+ Volts. That is better than no Volts with the center-tapped version.
Using a full wave bridge rectifier now gives around 2.4 - 2.5 Volts to drive
the flickering LED. This worked much better and the LED was noticeably brighter.
I did notice a problem when using a capacitor with this circuit: If the LED is
not attache and the circuit is powered up, the voltage on the cap will be as
high as the voltage-boosting circuit. I believe this can be in the 30 volt
range, the peak to peak was closer to 60 Volts. See Trace 3.
Trace 3: Free wheeling voltage boosting circuit, Left side
If the LED is then connected to the circuit, it dies almost instantly, whether
the circuit is energized or not, unless one shorts out the capacitor, thus
lowering the voltage stored there. This being the case, I decided it might be
prudent to use a 330 Ohm resistor to help reduce the current going to the
flickering amber LED.
When this was done and the voltage across the capacitor was measured, the
voltage was found to be 4.0 Volts. If the voltage is 4.0 Volts, then a resistor
of ~ 200 Ohms should be sufficient to supply the LED with ~ 20 mA of current. I
used a 330 Ohm resistor. Since it worked fine, I left it in.
In the schematic I have just a resistor LED combination, but this was replaced
with the flickering amber LED with its circuit, either internal to the LED or
an external circuit with an amber LED attached. Whatever I found in the batch
of dead flickering candles we had. (They were "dead" because of dead 2032
batteries, and I wanted to run them from this circuit using dead AA or AAA
Phil Karras, KE3FL
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