D3 may not really be needed unless reverse polarity hookup is a possibility. It does help drop the voltage a bit.
As Bertus has noted, you may want to consider running the 3.3V regulator directly off the supply so that its current is not added to the 5V regulator. The decision depends in part on the current demand from each.
Yes, whatever the current demand is for the 3.3V regulator, driving it from 12V will increase its heat dissipation compared to driving it from 5V. However moving it to the 12V source will cause the same amount of extra heat to be removed from the amount that must be dissipated by the 5V regulator. So the question is, which chip do you want to take the heat load? If the 5V regulator is already warm because of the current demanded at 5V, adding the 3.3V heat might overdo it.
hi Diego,
Looking on the web using your part no: AML1117 there a many different ready built AML regulator PCB's, some have heat sinks, others appear to have minimal AML1117 cooling methods.
Which version of these ready built regulators are you buying.?
For a light load I will in series all the regulators, as this.
In your case, if you want to connecting all the inputs from 12V, for the heat dissipation that I will using power resistor to bear it.
Vmin_U2 = Vo+Vdrop = 3.3 + 3V = 6.6V
V_R-limit2 = 12V - 6.6V = 5.4V
R-limit2 = 5.4V/250mA = 21.6 Ω, you could choose 20 Ω to get a little more current.