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Mike I understand what you a saying but I'm making an effort here. Im new to all of this so im pretty sure im going to make some mistakes along the way. Ive done the working out (even if it simple maths) but it would put my mind at rest if you can confirm ive got this right before I buy a power supply unit.
An LM317 at its dropout input voltage is not regulating anymore.
The datasheet shows a graph of the "typical" dropout voltage but do not show the max dropout voltage of weak but passing ones.
Is it a case then that I should go for a higher voltage (but still within the input voltage range) like 18V just to be sure that there is sufficient power going into the regulator. I really have no idea how this kind of thing works.
12.00V + 3V for the worst case drop out means the input should be >=15.0V. I wouldn't go to 18V, because that will produce a lot more heat which you will have to get rid of in the heatsink. Chances are, the 15V supply you are buying is regulated, so as long as it doesn't sag at ~1.5A, you are good to go.
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