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Small desk Power supply.
This is an updated version of the small desk supply, the board is bigger than the previous version, but the
trade off is, there have been a few improvements.
An error in the PCB overlay has been corrected which showed C12 the wrong way around.
Layout has been improved, so that a screwdriver can easily reach the head of any screws holding the
voltage regulators in place. I had been assembling my power supplies on a bracket, so inserting
countersunk screws from behind was not a problem for me, but it did occur that other constructors might
want to do things differently.
I have added complementary rail shutdown circuitry as per the original CPS150. This means that if you
lose one rail (due to a short or a fault) the other rail shuts down. I have repaired many consoles where one
rail has failed without causing further damage to the console so I was debating whether to add this feature
or not, but as it is included in the CPS150 I figured Soundcraft might know something I didn't (and
besides if you do not want this feature, you can simply leave the associated components out).
I have also made provision for a trimpot, to allow the supply rails to be adjusted, alternatively if you
prefer a fixed resistor may be used if you do not want the extra trouble and expense.
Finally I have included protection diodes on the Regulators. These are unlikely to be needed in normal
usage as the decoupling capacitors inside the console will usually discharge much more quickly than main
capacitors in the power supply, but as I am not across every application that these are being used for it is
probably a good idea to be more cautious.
The supply uses a voltage multiplier to generate the 48 volt supply so a readily available 15-0-15 Volt
transformer may be used.
While specifications ultimately depend on the Transformer used and heat-sinking, the power supply is
capable of delivering up to:
+12V at 1 Amp (lamp supply)
+17V at 1.5 Amps
-17V at 1.5 Amps
at 0.5 Amps
The only unusual circuitry is the voltage multiplier consisting of D1, D2, C5, C6 and C1+2.
If we presume a 30V centre tapped supply, on the first half cycle (I.e. the upper AC input is positive and the lower Negative) C5 charges via D1, up to 42.3V (30V*1.414).
On the second half cycle (lower AC input going positive) the negative terminal of C5 goes to 21.21V.
Now D1 is off but D2 conducts, discharging C5 into C6 so that the positive terminal of C6 charges up to somewhere approaching 63.5 Volts.
While the multiplier does have a lot of ripple this more than enough to provide our 48 volt rail.
While the TL783 is rated at 750mA I found that in this circuit, ripple and hum reach unacceptable levels
above 600mA or so. However 500mA should be more than enough to provide phantom power for 24
channels (while a shorted channel will draw 17mA from the phantom power supply, most
devices/microphones draw much less than 10mA, typically 2 or 3mA).
Under test the LM317 and LM337 happily provided a clean, rock steady supply, up until they shut down
due to over current at over 1.5 Amps.
To minimise current variations through the Adjust pin (which would decrease regulation) adjustable
regulators are designed to sink most of their operating current via the output pin, this means that in order
to maintain regulation they need a minimum load. While this is mostly included in the voltage divider
calculations (for the adjust pin), it is also provided by the indicator LEDs, so as odd as it may seem, the
indicator LEDs may be required for stable operation.
Choosing a Transformer
Either a dual 15V transformer or a centre tap 30V transformer can be used, however these are wired differently.
The different options are shown here, note your transformer may not use the same colour code as shown here, so you will need to confirm windings before connecting.
Note that the Centre tap diagram shows the transformer windings labelled 0-15-30 and this seems to be the way most transformers are labelled (according to a quick, very unscientific survey I conducted of Transformers I found lying around), but they may also be marked 15-0-15 where 0 is the centre tap and 15 is the end of the winding.
Frame ground terminals are provided for convenience, they are otherwise isolated from the circuitry on the PCB. If you use a toroidal transformer I strongly recommend that the Power supply ground is connected to frame ground at some point (such as via the terminals provided on the PCB) as the construction of a toroidal transformers means that their insulation is inadequate for use in double insulated power supplies.
Your supply Transformer needs to be at least 15V for a 17 Volt supply, the reason for this is that filter capacitors charge up to peak voltage rather than the RMS voltage and the peak voltage in this case is 15 x 1.414 = 21.21 Volts, however then you need to allow for a volt or two of ripple and a couple of volts for the regulator.
Keeping the unregulated DC supply as low as practical, means your voltage regulators will run cooler and require smaller (i.e. cheaper) heat-sinks as power in the output stage is current times the voltage drop across the regulator, so at 1.5 Amps using a 15V transformer, a 17 volt regulated output will result in (21.21-17) x 1.5 = 6 Watts ,whereas a 17 volt transformer the regulator would dissipate (17 x 1.414 - 17) x 1.5 = 10.56 Watts.
The trade off here is, that this assumes your AC supply is reasonably stable. Since the LM317/337 require up to 2.5V across them in order to function reliably at 1.5 Amps (lower current requires less forward voltage) then for 17 Volts output we need to allow for a power supply of at least 19.5 Volts, if we then add another volt to allow for supply ripple we want 20.5 volts on the unregulated side.
This means, that a 15V
transformer will work fine, until the AC supply drops by as little as 5%.
In Australia the AC supply is specified as 230V +10% / -6% so going by
worst case conditions a 15V transformer will be cutting things a bit fine,
whereas a 16V transformer would allow for a comfortable 10% sag in