Bench power supply

[mmaatt25]

Hi,

I'm trying to put together a bench power supply using a 12v sealed lead acid battery (alarm battery). I want three taps 12v, 9v and 5v. I'm using a 7805 and 7809 rated a 1amp and 1.5 respectively. Both inputs have .33uf caps to ground and .1uf on the outputs.
I've seen some schematics with diodes on the inputs of the 7809/7805 for protection. Can I put a 1N4001(1amp/50v) in series between the battery and the inputs of the 7809/7805s for polarity protection. Or is this not needed?

Thanks

Matt

[amptramp]

You can add a polarity protection diode for the 5 volt supply, but you have to watch the dropout voltage for the 9 volt supply.  If the battery is allowed to discharge from the rated 13.8 for a fully charged battery to the nominal 12 volts, your 9 volt line may sag under high current loads of over a few huindred milliamps and the current at which this effect occurs will change with temperature.  You have to decide whether there is any possibility of the battery being reversed.

A separate issue is the use of reverse-biased diodes from output to input of each regulator that protect against the possibility that the load could retain a higher voltage on its output filter caps than the input voltage if the battery is disconnected, reversing the voltage across the series pass transistor in the regulator.  This would kill the regulator and protection is always advisable.