Two transistor constitute a constant current source can drive power

Two transistor constitute a constant current source can drive power
http://www.siliconray.com/instrument.html
In lighting lamps and lanterns and the battery charger is inseparable from the constant current source circuit. The appended drawings is with two PNP transistor and four resistors constant current source circuit, through choosing high power transistor and heat sink can make constant current reached more than 1 A.
The transistor Tr1 and resistance R1, R4, R2 composition main circuit. R2 and R4 series of the transistor Tr1 base after the bias resistors. When the power is connected later, because of current flow in the base, there will be in Tr1 collector current flowing emitter current flows through R1 in R1 ends after will produce voltage drop. When R1 on both ends of the voltage drop to 0.6 V, Tr2 will then conductivity, Tr2 collector current flows through the R2. On the base of current form Tr1 shunt. Tr1's base will also reduce the current, collector current also will decrease. So after Tr2 conduction