Volodymyr Smotesko

Use 5V power, Luke

Wed, 24 Jul 2019 00:00:00 +0300

Many Eurorack synthesizer power supplies provide not only ±12V, but also +5V power line. This is useful for digital modules, where most power is consumed by 5V or 3.3V devices. But instead of taking power directly from the +5V line, module manufacturers tend to regulate it down from +12V, overloading the +12V line unnecessarily. There are two major reasons for this:

Backward compatibility with older and/or simpler power supplies, which do not provide +5V
Insurance from poorly regulated 5V power supplies, which can sometimes output much larger voltage and fry the module.

The second reason was well explained by Émilie Gillet, she has also suggested to use a 5V TVS diode for protection of modules powered from a system 5V line. I’ve built on that idea and made a little adapter board that converts any +12V powered digital module into a +5V powered one. Here the TVS diode SA5.0A protects the circuit when voltage on the +5V line rises above 5V. In that case diode starts to reverse conduct and clamp the voltage down. The clamping current flows through the fuse F1, diode D1, and into the ground. When the clamping current raises above 200mA, the fuse “blows” and line becomes disconnected. F1 is a PTC “self-resetting” fuse, so it will recover when the voltage returns to the normal 5V level. Ferrite bead FB1 helps to filter down noise coming both from the system into the module, and the other way round. This adapter connects 2x8 system bus connector to the 2x5 module connector. On the bottom of the board there is a 5Vout point that should be attached to the point on a module. On the target module there would be a voltage regulator, usually “78L05” or “1117-5”, that has to be removed, or not ever installed when assembling a DIY module. I find the output pin of the voltage regulator and wire 5Vout to that point. Here is an o_C board with adapter soldered on. During assembly I skipped the 1117-5 voltage regulator and one capacitor at the input of the regulator. I did install the output capacitor. Adapter is soldered directly onto the 2x5 header and it’s a very sturdy connection without any flex.

BOM

F1	PTC fuse 200mA 1206	Littlefuse 1210L020WR
D1	TVS diode 5V	Vishay SA5.0A
FB1	Ferrite bead	Murata BL01RN1A1F1J

🎁 I’ve shared the board at OSHPark. You can download the kicad-pcb file or order PCBs from them.

How to find 5V injection point

This is the section of o_C schematic where I found the point to which I should connect the 5V supply. The leftmost 10uF capacitor was not installed because it would now do nothing (almost). Let’s look at another example: find the +5V point on Mutable Instruments Grids schematic. Here C19 and IC7 would be removed and wire attached to the pin 2 of IC7.

Eurorack power kit: designing a power supply

Sun, 03 Sep 2017 00:00:00 +0300

For a single row Eurorack case, I want to have a power supply that’s not too big, and can be fed from different power sources: AC-DC adapter, li-ion battery pack, car battery, USB. Unfortunately, I wasn’t able to find an easy solution for delivering 500mA on ±12V from a 5V source, so no USB this time, but for input voltages from 9 to 36 VDC this power supply should work.

The core is two Murata NCS12 series DC-DC converter modules. They cost $30.46 each today. One delivers ±12V, the other +5V. They are isolated devices, meaning there’s no galvanic connection between input and output, even on the GND terminal. This gives protection when working with poor quality AC-DC adapters that may have insufficient isolation from mains potential. Isolation was not my initial goal, but the DC-DC converters I chose happen to be isolated, so I’ve embraced this and designed the PCB to have sufficient gap between input and output traces.

According to datasheet, max output current should be 500mA for ±12V lines and 2400mA for 5V line. This should be enough for a single row system, if most of the digital modules used would be powered from 5V line.

Input connector is 5.5/2.1mm barrel jack, center positive. This seems to be the most popular one. I’ve also added a screw terminal for connecting an external power input connector. If using an external connector in a metal case, the connector must be mounted on a non-conductive mounting plate, to maintain isolation between input negative voltage and system GND.

P-MOSFET transistor Q1 does reverse polarity protection on input.

Common-mode choke L1 and capacitors C8, C9 form an input and common-mode noise filter. L1 is the Murata part recommended in the NCS12 datasheet, C8 and C9 are Y-rated 250V 10nF capacitors. These components are optional. You can replace L1 with two wire jumpers and just skip C8 and C9.

C3 should be at least 50V 47uF low ESR electrolytic capacitor. All other capacitors are surface mounted on bottom side: 1uF 50V ceramics and 10uF 25V tantalums on output.

The three LED indicators connected directly to output lines are not going to melt because they are special LEDs with built-in current limiting resistors.

There are two options for mounting the PSU board. First, it can be affixed to the case back panel on four 5mm high threaded spacers. Second, it can be mounted on a panel using L-brackets. Distance from the front edge to centers of mounting holes is 6.35mm to fit Keystone 612 threaded brackets.

Full schematic PDF and KiCAD source files are published in git repo.

Eurorack power kit: power distribution board design

Fri, 01 Sep 2017 00:00:00 +0300

This power bus (part of my “eurorack power kit”) was designed to be used in a single row case. I’m planning to build a small Eurorack synth system, and decided to make my own power supply and distribution.

Most Chinese PCB fabrication services are very inexpensive if your board’s longest dimension fits within 100mm.¹ This is the reason the power kit bus only fits 5 module connectors: the PCB fab will produce 10 boards anyway, so I can just install 2 or 3 of them in a single row case. With parallel connection to the power supply, it will even have lower electrical resistance (and hence, lower voltage drop).

Bus board is completely flat on the back side, so it can be installed in the case without drilling: just stick it with a 3M VHB adhesive tape or any appropriate glue. To achieve this, I’ve selected surface mounted connectors: the obvious Molex C-Grid 0.1” headers for connecting modules; and Phoenix PTSM terminal block for power input.

Meng Qi Flat Back Busboard inspired this project.

Four 3mm mounting holes are also provided, so it can be mounted with screws. It’s easy to use the board itself as a drilling template when making holes in the case back panel. The bolt holes are copper plated and electrically connected to GND wire. It is safe to install the bus board into metal Eurorack case because the chassis is supposed to be connected to GND.

Wires connecting this bus to the power supply should be solid copper with cross-section of 0.5mm² max (⌀ 0.8mm, 20 AWG), as required by PTSM terminal spec. It may be possible to use tinned stranded wire, but I wasn’t able to test it yet. Phoenix PTSM connector is push-in spring connection, so wire must be stiff enough to be pushed in without bending itself.

The C-Grid header is non-shrouded, in accordance to recommendation of mr. Doepfer to not use polarized headers on bus boards and modules. Silkscreen line across the bottom row of contacts marks the -12V power line. Remember: Red Stripe Down!

All images in this post were created in CAD, I wasn’t yet able to actually build this board and test it. Look for my future posts to see it built.

KiCAD source files are published in git repo.

It can be just $5 for 10 boards! ↩