Cgs synth

So how did it work out ? Last time I built a + - 15 psu I used a 36v
center tap xfrmr and an LM 723. I'd be interested in a cheaper
solution. Everything else I tried was anemic, broadcasted, spikey, got
hot fast... basically crap.

G wong

> Date: Fri, 19 Nov 2004 09:51:49 -0000
> From: "allanjhall" <allanjhall@...>
> Subject: Re: +/- 15V PSU questions
>
>
> 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
>
> --- 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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