Small desk Power
supply.
While the original
purpose for the supply was a lowish cost replacement for smaller analogue
mixing consoles it can also be used for many other uses where +/- 17 volts
(other voltages can be set) and +48 volts and 12V is
required.
The supply uses a voltage multiplier to generate the 48 volt supply so a readily available 16-0-16 Volt transformer may be used.
While the specifications ultimately do depend on the Transformer used and heat-sinking, the power supply is capable of delivering:
+12V at 1 Amp (lamp supply)
+16.88V at 1.5 Amps
-16.88V at 1.5 Amps
+48.66V at 0.5 Amps
Circuit
description
The circuit uses standard run of the mill voltage
regulators, with a TL783 for the 48 volt supply, and a LM317 and LM337 for
the main positive and negative supply rails.
IC1 and IC2 are the
negative and positive voltage regulators the output voltage is determined
by R11 and R12 for the negative rail and R21 and R22 for the positive rail
using the formula Vout = 1.25 x (1+ Rx2/Rx1)
I went for a fixed rather
than adjustable design, for a couple of reasons:
Firstly minor
differences between positive and negative rail do not really matter for
op-amp based circuitry as it uses the 0v rail as a reference and well
designed circuitry in general will assume DC offsets between stages anyway
and will compensate for that. Secondly adding a trim-pot does increase
setup complexity and one noisy trim-pot can cause major problems or damage
to equipment connected to the supply (and yes I have seen damage caused by
a dodgy pot in a power supply).
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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 650mA 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) If you do require more current from the 48V supply a 10uF decoupling capacitor between the junction of R31 and R32 and Ground will improve ripple rejection of the TL783.
The LM317 and LM337 showed that they could happily provide a rock steady supply, up until they shut down due to over current at over 1.5 Amps.
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
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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
power.
Kit costs $13:50
(Australian) and includes:
PCB
Assembly
instructions
Email
for more details or Paypal instructions (to order) - note at this stage I
will have to work out mail costs on a case by case
basis...
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