|Medium Desk Power supply.|
original purpose for the supply was a lowish cost replacement for larger
analogue mixing consoles it can also be used for many other uses where +/-
17 volts (other voltages can be set) and +48 volts is required such as 500
and 51x style racks (although 2 boards will be needed, one for the +/-16V
and 48V supplies and the second to deliver the +/-24V
The supply uses switching regulators, with buck regulators for
the +/- supplies and a boost regulator to generate the 48 volt
supply. This keeps the regulator efficent and means a readily
available Dual 25 Volt transformer may be
specifications ultimately depend on the Transformer and heat-sinking used,
the power supply is capable of
+17V at 5
17V at 5 Amps
at 1 Amp
This circuit is based
around Texas Instruments LM2585 and LM2679 simple switchers regulators, I
did consider using linear regulators, but that approach would have ended
up generating up to 200W of heat, (or almost as much power as the supply
delivers). Using switching regulators dropped power dissipation to a much
more manageable 16 Watts.
There are two basic Switchers in use here, IC1 and IC21 act as buck
regulators, while IC11 is a boost regulator generating the 48V supply.
They are configured to run off a 21- 40 Volt input, so a readily available
dual 25 Volt 300VA transformer may be used (any Transformer with dual
windings between 18 and 27 Volts with power rating of over
225VA should do the job).
Following feedback from the small
desk supply project, I elected to make this supply trimmable with a range
covering 13-18 Volts which should cover the common supply voltages,
typically 15V, 16V and 17V.
Based on experience with a bench supply
that had anoisy Pot the wiper arm for the trim-pot is between the feedback
line and the shunt to ground, this means that should the contact go open
circuit, the regulator will decrease the output voltage to its
internal reference, of around 2 Volts.
As the Switchmode integrated
circuits used in this supply have a fast rise time, this can result in
some ringing on the output with a fundamental above 30-35Mhz (I had
recorded 35Mhz for my first prototype, but measured 65Mhz on my final
build, so I am not sure if I measured incorrectly the first time, and as
the Prototype is now in studio in Melbourne, I am not in a posituion to
easily remeasure this).
I did consider and test an alternative circuit
with a slower rise time, however this increased power dissipation
(although in the scheme of things this was not significant), circuit
complexity and the size of the PCB.
As any ringing was well above
Audio frequencies, I decided it was not worth the extra effort and
added secondary filters of L2, C101, C102 etc. which largely deal
with this noise.
Having said this however, I strongly recommend the
power supply is mounted in an earthed metal box and
ferrites placed on any input and output leads.
As this is intended as
an external supply I would expect this to be mounted in a metal case
anyway, but it is worth mentioning here.
I chose the
LM2679 over the LM2678 as it has soft start capability, so it should be
able to start with a highly capacitive load. Having said this though,
tests with the LM2678 found that it started happily even with
15,000uF across it.
I would not expect that any mixing consoles would
have any issues with the soft-start, however I have not tested every desk
out there, so it may be safer to assemble the power supply with soft-start
disabled (this is done by leaving off C4 and C24) and only fitting them if
soft-start is required (i.e. if the power supply struggles to start under
Unfortunately the cost of
Electronic components and shipping from Australia is not cheap, so I am
selling these as short form kits (PCB, Instructions and any programmed
parts) rather than complete kits.
the moment I am selling these through Ebay which for this project includes:
PCB and Assembly
if you require any more info....
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