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+/- 15V PSU questions

2004-11-18 by allanjhall

Hi Ken and group,

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.....

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?
(Can't remember the exact figure) Here in the uk I seem to get
higher than the specified AC voltage from transformers, under no
load condition most 15-0-15 seem to give about 18v to 20v AC per
side anyway, so after full wave rectification this would be well
over 20 volts.

Any help and enlightenment most gratefully received

Cheers,
Allan

Re: +/- 15V PSU questions

2004-11-18 by sasami@hotkey.net.au

>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@... or sasami@...
Modular Synth PCBs for sale <http://www.blaze.net.au/~sasami/synth/>
Australian Miniature Horses & Ponies <http://www.blaze.net.au/~sasami/>

Re: +/- 15V PSU questions

2004-11-19 by allanjhall

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

Show quoted textHide quoted text

--- 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/>
> Australian Miniature Horses & Ponies
<http://www.blaze.net.au/~sasami/>

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+/- 15V PSU questions

+/- 15V PSU questions

Re: +/- 15V PSU questions

Re: +/- 15V PSU questions

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