Home | About Us | Contact Us | DIY Projects | Tutorials |Pictures |Retro Computing | The Luddite
| PDF Instructions |
Medium Desk Power supply.

While the 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 rails).

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 used.

While specifications ultimately depend on the Transformer and heat-sinking used, the power supply is capable of delivering:

+17V at 5 Amps

- 17V at 5 Amps

+48V at 1 Amp

Circuit description
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 load).

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.
At the moment I am selling these through Ebay 
which for this project includes:
PCB and
Assembly instructions 

Email if you require any more info....
Copyright © 2011 Australian Technical Production Services