[sdiy] DIY PSU

[John Luciani]

On 8/14/06, Dave Kendall <davekendall at ntlworld.com> wrote:

> I recently got given a nice looking transformer (N.O.S from RS) It has
> 120V/240V inputs, and AC output taps at 0V, 4.5, 9, 12, 15, 18, 20 and 24V.
> It says it can deliver up to 10A from any one tap.
> A friend suggested that if these taps are RMS measurements, then it should
> be possible to take 0V, 12V and 24V, use the 12V as ground, and rectify and
> regulate the other outputs down to +15V and -15V, as the peak voltages would
> be more like around + or -20V or so.

The taps are most likely RMS. You should measure the outputs to verify.
You could use the 12V as a "center-tap" and rectify the output. 12V RMS is
apx 17V peak. You could probably get a diode (or bridge rectifier) with a drop
of around 0.5V which leaves you with 16.5V.

You need to determine (1) how much current you are going to require and (2)
the dropout voltage (Vdo) of your linear regulator to calculate the
value of your holdup
capacitor.

    I = C dv/dt

dv = Vpeak - Vdo - Vout
        Vpeak is the peak output voltage out of the bridge
        Vdo is the dropout voltage from the regulator datasheet
        Vout is the regulator output voltage
dt  = 8.3ms (fullwave rectifier)

> The other suggestion was to run rectified DC at ±20 ish (whatever it works
> out to after diode drops) and use local voltage regulation at the modular
> distro boards and inside the rack synth(s) that are planned - that way lower
> current regulators could be used. I guess they'd have to be pretty accurate
> for driving VCOs though.

How much current are going to require? For large currents I would distribute
the regulators for small currents I would not. For either solution you
will want local
filtering.

For a VCO you may want a voltage reference rather than relying on the power
supply regulation.

> Is any of this at all feasible? I haven't yet found any schemos or circuits
> that are relevant, except possibly the LM723 datasheet, so if anyone knows
> any sites?.....

On-Semi or National Semicondutor should have a variety of app-notes on
linear regulators.

(* jcl *)

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