Overall Summary: The LM78xx series of LDO are simple to use, require minimum components and are relatively inexpensive. Although referred to as "Low Drop Out", they are based on old NPN internal "pass transistor" technology and this term is not particularly "correct" by today's standards. Modern LDO regulators van operate down to Input to Output voltage differentials less than 200 mV at full output current load. The LM78xx series is much higher than this and the drop out voltage Vdo can be as high as Vdo = 2.5 V worst case at Io = 1 Amp (2.0 V typical). The LM7808 version is still adequate for Vo = +8 V when regulating from a 12 V lead acid "Gell Cell" battery supply running at its end of discharge terminal voltage of 10.8 V as this allows 2.8 Volts input to output voltage differential (i.e. 300 mV headroom). A low battery voltage indicator would be a worthwhile addition to the design!
The LM78xx series is available in TO-220 and TO263 (SMD) case style versions for 1 Amp output current rating and TO92 case style versions for 100 mA output current rating (SO-8 SMD versions are also available at 100 mA rating).
 | Although the last voltage drop-out graph suggests Vdo < 2.0 V @ Iout = 1.0 Amps over -25C < T < 125 C, these are typical values. Worst case voltage drop-out may be closer to 2.5 V. |
| LM7808 and LM340 series are similar |
| Input decoupling capacitors should be placed reasonably close to the IC for best stability. |
Note: The LDO IC may need a protection diode if input short circuits to ground are expected (unlikely in this application but similar conditions might occur if the LDO is run from a car battery as the car engine is started - a standard 1N4001 to 1N4007 rectifier diode should be suitable).
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