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Wednesday, August 16, 2006

Extend current lm78xx regulator circuit

LM340/LM78XX Series
3-Terminal Positive Regulators
The LM140/LM340A/LM340/LM78XXC monolithic

3-terminal positive voltage regulators employ internal
current-limiting, thermal shutdown and safe-area compensation,
making them essentially indestructible. If adequate heat
sinking is provided, they can deliver over 1.0A output current.
They are intended as fixed voltage regulators in a wide
range of applications including local (on-card) regulation for
elimination of noise and distribution problems associated
with single-point regulation. In addition to use as fixed voltage
regulators, these devices can be used with external
components to obtain adjustable output voltages and currents.

Applications circuit of LM340/LM78XX Series

Io = Iq + Ireg
Iq = B *Ib
Ib = Ireg – Vbe/R1

Iq = B(Ireg – Vbe/R1)
Io = B*Ireg – B*Vbe/R1 + Ireg
Io = (B+1)*Ireg – B*Vbe/R1
R1*Io = R1 (B+1)*Ireg – B*Vbe

B*Vbe = R1[(B+1)*Ireg - Io]

R1 = B*Vbe / [(B+1)*Ireg - Io]

We want to make regolater circuit 3 A
We choose current of regulater 0.6 A for safety
2N6133 for Q1
Io = 3A
Ireg = 0.6 A
Vbe = 1.4 V
B = 20

R1 = 20*1.4/[(20+1)*0.5 – 3]
R1 = 2.91

We choose R1 = 3 ohm