You don't want to do that there not going to do what you think. One will try to put out more the it should and basically cutoff the second one. See there not going to supply the same voltage.I would do this use a PNP to get you more current
Well the following are 12vdc 3 amp linear regulators in TO-3 package:78H12NTE933and lambda sometime ago use to have a 12vdc 5 amp regulator in a TO-3 package that you can sometimes find sold as new old stock:http://www.ebay.com/itm/LAMBDA-LAS1912-12-VOLT-5-AMP-TO-3-REGULATOR-NOS-78H12-NTE933-EQUIVALENT-/380547135339?pt=LH_DefaultDomain_0&hash=item589a60776bLefty
Get 2 tip42
my bad, i forgot the voltage drop of zener diode.
If you really need 3amp, you can look into the datasheet of your voltage regulator to see how to expand their current capabilities.
it didn't work
so i cannot use npn transistor?
Let's go back a few stepsQuestion - Why do you think you need a 3A regulated supply ? What is your actual load ? Just because the power supply can provide 3A+ doesn't mean your regulator has to be rated at that figure. If 6 volts meets you back-up needs than "losing" 12.5 across a linear regulator is a shear waste of power (12.5 x 3 = 37.5 watts). If you really need 6 or 12 volts (you seem to be a bit mixed on voltages) then I'd suggest a switched mode regulator that offers over 90% efficiency with very little heat.
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