[sdiy] power supply pain

[Scott Bernardi]

The problem is using resistor dividers to drop the voltage down for your
lower voltage regulators. The current your load draws passes through the
regulators, and so it will also pass through the resistor dividers. When
you "tested" your outputs, you probably used a multimeter or DVM which
has a high equivalent resistance, and so does not load down your
resistor divider. When you plug in the actual circuit as the load, it is
probably pulling a few milliamps, which drops the voltage on your
resistor dividers. 
Measure the voltage at the inputs of your +9v and +5v regulators with
the ADSR hooked in - you'll probably see that it is too low. In general,
the input to each of your regulators should come directly from a filter
cap.
So to fix it, you need to get rid of the resistor dividers. Your +/-12v
and +/-9v regulators can probably run off the same filter caps just fine
(the voltage at the filter caps should be at least +/-15v for the 12v
regulators to work right). You may want to have a separate transformer,
diodes, and filter caps for your +/-5v regulators (a 12.6VCT would do
well here) to avoid excessive power dissipation (a 15v input for a 5v
regulator is kind of high).


Gavin Russom wrote:
> 
> Hi there
> So I built my own power supply for my modular (which I know everyone
> reccomends AGAINST) but the reason I did it was because I needed +12, +9,
> +5, -5, -9, -12 outputs but couldn't find a suitable one for sale.  I built
> it and it tests perfectly at all the outputs.  But when I hook it up to my
> ADSR that runs off +5 the voltage immedeately drops to about +2.2 volts,
> even if i test it at the output of the power supply.  If I run the ADSR off
> a 9volt battery it works perfectly, but if I connect it to the +9 of the
> power supply again the voltage drops to about +4.3 volts even at the supply
> output.  If I hook up the ADSR to the +12v out put the voltage doesn't drop
> but its too much voltage for the ADSR.  The power supply uses a 7812 to get
> the +12V, then taps off a resistor network to get the +9, and goes through a
> resistor network and a 7805 to get the +5.  The variable voltage source
> that's tapped off the same resistor network works great no matter what its
> hooked up to.   Any suggestions?   I'd even buy one at this point if it had
> the right outputs, but I'd rather fix my own!
> thanks
> MYSTIC

-- 
Scott Bernardi
sbernardi at home.net