[sdiy] L7915 - Confusion

[Stewart Pye]

Hi Aaron,

The down side of running them all from the unregulated supply is that 
the 5V reg for example will dissipate quite a bit of power (heat) 
because of the voltage drop. At very low currents this won't be much of 
an issue, but I'd certainly put a heatsink on at least the 5V reg.

A low dropout 3.3V reg could be run from the 5V reg.

You may want to do something like run the 5V supply from the 9V, but 
keep the 15V rails separate. This way if you only want 15V rail and a 5 
or 3.3V rail, the 9V reg will lower the voltage to the 5V reg to reduce 
dissipation in the 7805, while no extra current is drawn from the 15V 
rails. As you probably won't need much current from the 9V supply this 
could be a good compromise. You'll need to allow enough heatsink on each 
reg to dissipate the power caused by the voltage drop and current drawn 
from that supply (including any cascaded supplies).

Regards,
Stewart.

lanterma at ece.gatech.edu wrote:
> On Jan 21, 2011, at 5:30 PM, Matthew Smith wrote:
>
>   
>> Just for the record, this is a little +15/-15V 0.6A bench supply I'm putting together with ammeters on both rails for development and debugging purposes.
>>     
>
> Speaking of which... I'm working on something similar...
>
> If one wants multiple voltages - say 5V, 9V, 12V, and 15V, in some combination - is it better to "cascade" voltage regulators? (I.e. get 12 V from 15, 9 V from 12 V, 12 V, 5 V from 9, etc.), or "parallel" them (have all the regulators get their inputs from the same original >15V supply)?
>
> - Aaron
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