Sometimes it takes to convert a voltage from one value to another. The blocking-oscillator circuit, describerd here, converts low voltage of 6 Volts into higher voltage of 40 (or less) Volts.
Let's see how it works. The bias current goes through resistor R1 to the base of transistor VT1, the transistor is half on. A current through the winding "I" grows, it induces an e.m.f. in the feedback winding "II", the positive e.m.f. applies to the base of the transistor through capacitor C1. The transistor opens, the voltage at the collector drops to almost zero, but the current through the winding "I" still grows until the capacitor C1 is charged or the core of T1 is saturated. Then the positive feedback voltage at the transistor base gets down, it reduces the collector current, then the transistor closes, it cuts the collector current off. A positive voltage pulse appears at the collector (see the Fig.2.), the voltage of this pulse may be several times higher than the power supply voltage Uin. This pulse is getting ever higher across the winding "III", it's only takes to rectify it with diode VD1 to get an output DC voltage.
Fig.1. Circuit diagram of the voltage converter
Uin = 6V; Uout ≤ 40V;
R1 = 100k; C1=4.7nF; C2=0.1μF;
VT1 - KT201B (hFE min = 30, VCE max = 20 V);
VD1 - KD522B (a common silicon diode just like 1N4148);
T1 - on a ferrite ring of 10x6x4 mm, μ/μ=1000...2000; "I" - 12t; "II" - 3t; "III" - 60t.
The high voltage of pulses at the collector VT1 requires a high voltage transistor. To protect the transistor VT1 from a high voltage produced when the load is disconnected, use a Zener diode or a neon lamp (60...80V) connected in parallel with the primary winding "I". The pulse width of the blocking oscillator can be adjusted by matching value of capacitor C1, the frequency - by value of resistor R1.
Fig.2. Voltage at the collector of transistor VT1
The transformer T1 is wound on a ferrite toroidal core with dimensions of 10x6x4 mm with the relative permeability μ/μ0=1000...2000. Windings I, II, and III having 12, 3, and 60 turns respectively of any suitable enameled copper wire. The voltage converter consumes 0.5 mA from 6V voltage source, provides output voltage till 40 Volts. The output frequency is 12.5 kHz, the pulse width is about 2 μs.
V. Polyakov, "Radio" 2001, issue 5.