[sdiy] 5V regulators

[harry]

If you need 500mA, you will be dissipating 3.5W.... (7 volt drop)
this is a LOT. Thermal resistance of the TO-220 package is 4 degrees
C per watt... Case to Ambient is 50 degrees C per watt... you are looking at
a temperature (without heaksink) of about 175 degrees C, without a heatsink!

OUCH

Also... you will draw the same 500mA from the 12V supply... so it will be
heavily loaded as well.

I'd go with a separate 5V supply... or IF you can give away the 500mA of
12V supply... then I'd use 1 regulator chip PER oscillator... that would only
give about a 60 degree rise on each... much smaller heatsinks would do !!!

A string of forward biased diodes could drop .7V each... that would dissipate
.35W per resistor... Six resistors in series could be used to get the input
voltage down to about 8V... making life MUCH easier on the regulator.

In general... its a BAD idea to generate +5v from +12v with a linear regulator...
unless the current draw is very small, or you literally have "Power to Burn" !!!

Sorry...  ;^(

H^) harry

Keith Winstanley wrote:

> Hi All,
>
> I have put together some VCDOs in a MOTM style layout with
> Schaeffer panels, there are going to be three of these in a cabinet
> and I think the only modules requiring +5V. Now instead of putting in a +5V
> rail I was thinking of putting a voltage regulator on an empty bit of board
> (7805? I need 500mA)with an appropriate capacitor to reg the +12V down to +5V.
> This way I can rig up an MTA power connector to the board and not worry about
> special cables and mixing up with MOTM/Oakley/Synth Systems/Blacet cables.
>
> I dont see any problems with this....any comments/suggestions welcome
>
> TIA
>
> Regards...
>
> Keith
>
> http://home.freeuk.net/lowpass