# Three-terminal linear regulator expansion flow circuit diagram

Posted on Mar 28, 2007

Circuit is a very common three-terminal linear regulator expansion flow circuit, when we actually use, because there is no encounter some well-considered or is lower false failures.

1. First, it says disadvantage of this power 1.1 This power is a linear regulator circuit, all have their unique internal power loss, all the pressure drop is converted to heat loss, low efficiency, so pay special attention to heat issues. 1.2 Since the operation speed of the core member 7805 is not too high, so that the input voltage or the load current changes abruptly slow response. 1.3 This circuit does not increase the power supply protection circuit 7805 itself overcurrent and temperature protection but no expansion flow transistor TIP32C unprotected, there is a big drawback, if after 7805 to protect the state, the output circuit will be Vin-Vce, circuit output than expected, pay special attention to this point. 2. Power advantage 2.1 circuit is simple, stable and easy commissioning (almost no debugging). 2.2 cheap, suitable for cost demanding products. 2.3 circuit almost no high frequency or low frequency components of the signal radiation, low frequency, EMI, etc. easy to control. 3. The circuit works Io = Ioxx + Ic. Ioxx = IREG - IQ (IQ 7805 for the quiescent current, typically 4-8mA) IREG = IR + Ib = IR + Ic / ( is TIP32C current magnification) IR = VBE / R1 (VBE TIP32 base for the turn-on voltage) So Ioxx = IREG - IQ = IR + Ib - IQ = VBE / R1 + IC / - IQ Because IQ is very small, it may be omitted, then: Ioxx = VBE / R1 + IC / Check TIP32C Manual, VBE = 1.2V, it is preferable 10 Ioxx = 1.2 / R + Ic / = 1.2 / 22 + Ic / 10 = 0.0545 + Ic / 10 (here take the main map in 22 OHM) Ic = 10 * (Ioxx - 0.0545) Assuming Ioxx = 100mA, Ic = 10 * (100 - 0.0545 * 1000) = 455 (mA) Then Io = Ioxx + Ic = 100 + 455 = 555 mA. Suppose Ioxx = 200A, Ic = 10 * (200 - 0.0545 * 1000) = 1955mA Io = Ioxx + Ic = 200 + 1955 = 2155mA Examples can be seen from the above two, the output current is greatly improved. 3.2 size of resistance R R to adjust the size of a great relationship through 7805 current, different values into the formula to see the bigger .R, the situation is the same output current flowing through the current 7805 to be smaller, and vice versa . However, generally such circuits, for expansion flow transistor TIP32 additional heat sink, and no need for 7805, but the value of R can not be too large, with the proviso that: R <VBE / (IREG - IB). 3.3 7805 circuit input capacitance values is an error, a friend already analyzed, mainly cause a surge in the instantaneous output power is far greater than 5V, the subsequent damage to the circuit. When in actual use, in order to suppress the self-excited oscillation of 7805, this capacitor is usually taken 0.33uF (most common spec. are recommended for this parameter) This circuit is used in a commercial device, the real argument is not a 100uF capacitor circuit in addition to that, and the main paste the parameters, there are thousands of sets of products into the market, proved that it can be used. The reason is because open discussion posted colleagues used a new model, we change this parameter capacitance, surge caused the problem, and burned a lot of peripherals, and therefore re-analyzed.

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