Research website of Vyacheslav Gorchilin
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Kacher voltage converter (joule-thief)
In the previous part of this work, a pulse generator based on the kacher effect was introduced. This solution allows you to get away from the mandatory inductive coupling between the windings of the generating coil and use separate chokes. This allows you to generate pulses more efficiently in terms of energy costs, because. there are no losses on such a connection. A slight modification of this circuit allows you to make miniature high-efficiency voltage converters for LEDs, which can be powered from low voltages - from 0.5 V, and having another rather strange name: joule-thief.
A diagram of such a device is shown in Figure 5. Generating circuit: L1, L2 and the junction of the transistor VT1, using the bias formed by R1 and C1, generates pulses on the collector, exceeding the supply voltage U by several times in amplitude. This pulsed voltage allows the LEDs D1-D2 to light up. Recall that a direct connection to the power source will not allow them to ignite, because. their minimum operating voltage starts at 1.6V each. And some brands of LEDs start to glow only from 2.5 V.
Fig.5. Schematic diagram of a quality voltage converter (joule-thief)
Fig.6. Oscillogram of the signal on the VT1 collector with two LEDs and a supply voltage of 0.5 V
One of the advantages of such circuitry is the miniaturization of the device when using SMD components. Moreover, there is no adjustment or adjustment of the turns of the coils, and their relative position. The device uses standard parts, including chokes L1-L2, the relative position of which does not play any role.
The ideal solution for transistor VT1 is the following brand: 2N2219. Among other things, this transistor allows you to start the device from 0.45 V. In this case, the value of R1 can be increased to 33 kΩ. But if the economy of the circuit is not a priority, then, in principle, any silicon n-p-n transistors will do, e.g. S8050.
If an even lower Joule Thief trigger voltage is needed, then the A733 transistor can be used. Just keep in mind that this is a p-n-p transitor and the power supply of the circuit must be reversed. If the device will be started from a voltage of 0.3 volts, then the resistor R1 should be 470 Ohm, and instead of two LEDs, there should be only one.
Capacitors C1,C2 can be propylene or ceramic. Their nominal value can be in the range of 10-33 nF without a significant change in the generation effect.
Chokes L1 and L2 can be either axial or radial type, for example: 470uH and 100uH.
It is desirable to use LEDs D1-D2 with a diameter of 3 mm, green or blue glow.
Schematic Power Notes
To perfectly match this circuit with LEDs, it is desirable that their number would approximately correspond to the expected supply voltage. The calculation can be made based on two LEDs - for every half a volt of power, starting from 1 V. For example, for the expected supply voltage of 1.5 V, it is necessary to connect 4 LEDs connected in series to the VT1 collector. With a supply voltage of up to 1 V, two LEDs are sufficient, as shown in Figure 5.
Another version of the scheme
The circuitry of the generators presented in this work does not work in radio circuit simulators, because their software does not contain the kacher effect of a transistor p-n junction. Another confirmation of this is the following circuit, in which the inductor L1 is moved from the base of the transistor VT1 to the supply input, and LED D1 is connected in parallel to it (Fig. 7). Such a generator, with minor modifications, has already been described by the author here.
Figure 7. Schematic diagram of a joule-thief with LED protection and 0.6-12V supply
Such an unusual mode of operation of the generator allows you to protect the D1 LED from burning out in the supply voltage range from 0.6 to 12 volts. In addition, at a supply voltage of about 1.5-2 V, the device switches to an over-economical LED glow mode. The usual scheme of a joule-thief and a blocking generator does not have such properties.
By the way, D1 can be turned on and vice versa, then its mode of operation will change slightly, but, at about 5 volts and above, it stops glowing, which ensures its complete protection against burnout. True, in this case, a slightly higher current consumption of the circuit is observed with the same other parameters.
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