Many thanks for the help Ken, it all makes much more sense to me
now :-)
I think I'll try it with 15-0-15 and check voltage across caps with
scope and fit a 18-0-18 transformer if its looking too marginal
under load - I'm trying to keep the heat from the heatsinks as low
as possible as they will be in the same cabinets as VCO's and I
don't want to warm them up too much. I'll use schotty diodes, but
can't find low drop regulators for negative voltages, only positive
ones such as LM2940 - looks like the cap requirements are a bit
different too, LM2940 needs a medium size electro cap on the output
rail, so I think I'll have to stick with 7815 and 7915's, or is it
possible use low drop regulator on + rail only?
BTW Is low dropout the same thing as low drop with regulators? I've
had a look at datasheets but they assume you know what low dropout
is!
cheers,
Allan
now :-)
I think I'll try it with 15-0-15 and check voltage across caps with
scope and fit a 18-0-18 transformer if its looking too marginal
under load - I'm trying to keep the heat from the heatsinks as low
as possible as they will be in the same cabinets as VCO's and I
don't want to warm them up too much. I'll use schotty diodes, but
can't find low drop regulators for negative voltages, only positive
ones such as LM2940 - looks like the cap requirements are a bit
different too, LM2940 needs a medium size electro cap on the output
rail, so I think I'll have to stick with 7815 and 7915's, or is it
possible use low drop regulator on + rail only?
BTW Is low dropout the same thing as low drop with regulators? I've
had a look at datasheets but they assume you know what low dropout
is!
cheers,
Allan
--- In cgs_synth@yahoogroups.com, sasami@h... wrote:
>
>
> >I'm going to be using the +/-15v psu's to supply some of the
digital
> >modules in my system to keep noise away from analog module power
> >rails. Is there any reason why I couldn't attach the regulators
to
> >the pcb with short (50 to 100mm) flying leads so that they could
be
> >remotely mounted on a single large heatsink, with insulation
washer
> >kits to keep them electrically isolated from each other. I'd
guess
> >that they could be good for 600 to 800ma per rail in this
> >configuration.....
>
> You need to keep the 100n capacitors physically close to the
regulator, so
> if you remote mount them, solder the 100ns directly to their pins.
>
> >Also, could you explain more fully why I would need to go up from
> >15v-0-15v to 18v-0-18v transformer for higher current outputs? I
> >thought that with full wave rectification you get a higher DC
> >voltage than AC anyway - somewhere near 1.3 to 1.6 times higher?
>
> square root of 2 is the figure. The problem is you lose a lot of
voltage in
> the process - 1.2 across the diodes and another 3 across the
regulator. Also
> the output from the capacitor is not a straight line. The higher
the load,
> the more it droops between cycles. The BOTTOM of this droop must
be higher
> than 18V for the regulators to work propperly.
>
> Also, the no-load output of the transformer is irrelevant (apart
from
> determining the minimum voltage for the electros), as you won't be
running
> no-load.
>
> If you really wanted to, the use of low-drop voltage regulators
and schottky
> diodes would save you a couple of volts of losses, allowing a
greater load
> to be taken from a 15-0-15 transformer.
>
> Ken
>
_____________________________________________________________________
__
> Ken Stone sasami@h... or sasami@c...
> Modular Synth PCBs for sale
<http://www.blaze.net.au/~sasami/synth/>
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