[sdiy] Multiple LM317s - serial or parallel?

[Seb Francis]

Depends on the current requirements of the various rails.  If the 
current requirements of the lower voltage rails are relatively high then 
might be worth 'chaining' the regulators.

As an example: if you are supplying 100mA on the 5V rail from 15V then 
you need to dissipate (15-5) * 0.1 = 1W as heat.  You can spread this 
over 2 regulators or just over 1.  If you run the 5V from the 12V then 
you need to dissipate 0.7W as heat, but then the other 0.3W will have to 
be dissipated by the 12V regulator, which will also have to dissipate 
the heat for the requirements of the 12V rail.

Add it all up and arrange it so the power dissipation is spread evenly.  
Or alternatively stick a heatsink on 1 of the regulators and arrange it 
so more power is dissipated by this regulator.

Seb


Justin Owen wrote:
> Hello,
>
> I have an LM317 in a TO-220 package outputting 12V DC from a 15V DC supply.
>
> I have a second LM317 in a TO-92 package outputting 5V DC and a third outputting 3V.
>
> My plan was to run the 5V off the 12V and the 3V off the 5V - which is working just fine. There's beefy caps on the 12V input and all three have diode protection and Tantalums on the outputs.
>
> Is there any reason why I should consider running all three LM317s off the main 15V DC supply?
>
> I'd all but made up my mind on this - but now I'm second guessing myself.
>
> Just checking...
>
> Thanks.
>
> Justin
>
>
>
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