[sdiy] Best way to get a +3.3V rail from +15V?

blacet at blacet.com blacet at blacet.com
Thu Mar 7 18:00:20 CET 2013


Just as an example, I've used a 33 ohm 1W resistor in line with a LM317T
(with very small heatsink) on a board needing 5V @ 150 mA from 15V. Both
the resistor and regulator get quite warm; too hot to touch for very long
but well under the max operating temp.

The switching regulators are attractive except for they all seem to be
very small surface mount.



> Scott Gravenhorst wrote:
>>
>> I'm wondering about using another regulator between +15 and +3.3.  Maybe
>> a +8v.  I would think
>> that the heat dissapation should be less for the +3.3 regulator.  Some
>> heat might also be expected
>> on the +8v reg.  It's either that or a power resistor as previously
>> mentioned.
>
> How would that help?
>
> Single regulator:
>   Vdrop x Iload = (15 - 3.3) * 0.2 = 11.7 * 0.2 = 2.34W
>
> Dual regulator:
>   Vdrop1 x Iload = (15 - 8) * 0.2 = 7 * 0.2 = 1.4W
>   Vdrop2 x Iload = (8 - 3.3) * 0.2 = 4.7 * 0.2 = 0.94W
>
>   Ptot = 1.4 + 0.94 = 2.34W
>
> The same power is dissipated, this time shared between two heatsinks.
> You'd get the same effect using a single regulator and a larger
> heatsink.
>
> Neil
> --
> Modules and more: http://www.cesyg.com
> Homepage: http://www.njohnson.co.uk
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