Power Supply and Enclosure update

The original plan for the Lunch Box was to have an external supply. One of the reasons for this was the relatively high dissipation in the 12V heater regulator and the corresponding size of heatsink required which meant it would not fit inside the rack. After a lot of discussion with Sven on groupdiy.com, this situation has changed. Sven was very keen to have a built in power supply option and wanted to squeeze it into a 28HP wide, 3U high module. I had already tried and failed to squeeze the power supply PCB layout into a 3U high Eurocard format mainly because of the size of the heat sinks. However, with a built in power supply you can only fit a further four modules into the lunch box. Even if all four are mic pre-amps, the 48V phantom power requirement is much reduced. This means the size of its heat sink can be reduced considerably. It also became clear that for good screening, fitting the PSU into a Fischer Electronik cassette would be a good idea. One advantage of using such a cassette is that it can also be used as a heat sink provided the regulator is electrically insulated from the cassette. This is easy to do with the TL783 regulator used for the 48V phantom supply so it was decided to delete its heat sink entirely and rely on the cassette.

Only having four modules to power also reduces the 12V heater supply requirements to just under 2 amps easing the burden its heatsink. To reduce this even further it was decided to replace the TO3 package LM338 regulator of the original 12V supply design with a TO220 package low drop out regulator the LD1084. This has a guaranteed maximum drop out voltage of 1.5V at 5 amps. Not only can this reduce the overall dissipation but it means we can reduce the transformer secondary ac voltage from 14V RMS to 12V RMS which is a much more convenient standard value. I had already built  a small bench supply to power the 6U sub-rack based test rig for the MK3 mixer using the original lunch box PSU board. So I removed the LM338 regulator from this board and replaced it with an LD1084 and changed the transformer secondary voltage to 12V. The temperature rise in the 12V heatsink was much reduced - with a four module load it was less than 20 degrees C which is quite satisfactory.

I later tried the same regulator on an external power supply using the same PCB and a much larger off board heat sink. With a 6 module load, the temperature rise was less than 15 degrees C.

By removing the 48V heatsink, enough room was made available to allow a Eurocard sized PSU PCB to be designed and still retain the existing 12V regulator heatsink as shown below:

As you can see, the TL783 regulator for the 48V supply on the left has no heat sink.

The next question was could it all be made to fit inside the 28HP cassette. First we needed to know how big the mains transformer would be. To allow it to also be used in an external power supply it was decided to rate the 12V heater supply for 6 modules and also to add a 12V utility supply winding. The specification we came up with was:

Primary 0-115, 0-115 @ 50/60Hz for series/parallel operation

Electrostatic Screen

Sec 1 240V @ 140mA

Sec 2 50V @ 70mA

Sec 3 12V @ 4.3A S

Sec 4 12V @ 1.6A GOSS band

Diameter not more than 95mm

Thickness not more than 50mm

Quotations from two manufacturers determined that this specification could be met and Sven and I each ordered a transformer. 

Extensive calculations and measurements then followed to establish if the toroid transformer and the 3U PSU board could all be fitted into a 28HP Fischer cassette along with an IEC mains input connector, a fuse and an on/off switch. I was also concerned about the heat dissipation in the 12V rectifier at higher current and you can see in the picture of the external power supply that I have replaced the on board DIL rectifier with an off board 25 amp rated type that is bolted to the chassis to act as a heat sink. Knowing the Fischer cassette could also be used as a heatsink, we looked at bolting a higher current rating bridge rectifier to it. Below is a sketch showing a possible layout of the major components:

In the last few days, prototype PCBs, the custom transformer and the 28HP cassette have arrived. Here they are fitted together:

So far it looks as though it will all fit in reasonably comfortably.Next step is to complete the build and test the power supply. In the meantime, a 3U rack has been obtained from SRS and filled with four 14HP modules and the 28HP power supply module to give an overall idea of what the lunch box will look like: