Simple power supply


Martin McBride, 2020-08-07
Tags power supply diy modular synth
Categories computer music sound synthesis modular synth

If you are building a modular synthesiser, one thing you will definitely need is a power supply. Most modular synthesisers require a dual rail system (0V, +12V and -12V being typical), and it can also be handy to have a 5V rail too if you are planning to use logic chips or processors such as Arduino boards.

There are several options:

  • Purchase a ready modular synth power supply - these can be quite expensive.
  • Purchase a bench power supply - again this can be quite expensive, and most of the cheaper products only have one rail (+12V).
  • Build your own - cheaper, but you will be working directly with mains voltages, so you need to be confident you know what you are doing.
  • Build you own using off-the-shelf mains adaptors - the cheapest and easiest method which we will use here.

Here is the sort of mains adaptor you might use. You will need two of them to give a dual rail supply:

Theory

You probably know that if you place two batteries in series you get twice the voltage. Here are two 1.5V batteries that give a total of 3V:

Notice, of course, that you must connect the positive of the first battery to the negative of the second so their voltages add together. If you measured the voltage at point B, you would find that it is half of the total, ie 1.5 volts.

In fact, as the right hand side of the diagram shows, we are free to decide what we define as the 0V point, so we can treat the two batteries as a +/- 1.5V supply if we want. Voltage is relative so we can choose whatever point we like as zero.

If we have two 12V DC mains adapters, similar to the one below, the exact same applies:

Although both adapters are plugged into the same power socket, the output of each adaptor is isolated (because there is a transformer between the main power and the 12V output). This means that we can treat each output a bit like a 12V battery. If we connect the +ve of one to the -ve of the other, and call it 0V, we get a +/-12V supply:

Our power supply

The two mains adapters do all the difficult work of converting the AC mains voltage into 12V DC. But we still need to provide connectors so that we can connect multiple devices to the power supply - for example, a couple of small racks and a breadboard we might be working on. Remember the power supply will only supply 1A (or maybe 2A depending on the adaptors you choose) - if your setup gets too big you will need more than one power supply!

Our power supply is basically an empty box where we mount the power input sockets, a number of output sockets, and some LEDs to show that the power is on. Here is the finished item:

I decided to use banana plugs for the power, for several reasons:

  • They are nice and chunky, so they can easily handle a couple of amps at 12V.
  • They come in a variety of colours.
  • Each power rail has its own lead, so if a device doesn't need -12V you can just not plug it in
  • You aren't going to confuse them with the patch cables and accidentally plug your best headphones into the 12V supply!

That said, banana plugs are sometimes used for amplifier inputs, so if you have such an amplifier do take care to avoid connecting the power supply to its input!

There are two types of socket. The smaller one is just a socket, the larger one is a socket and screw terminal combined, so you can also attach bare wires to it (for example to power a development breadboard). I went with the larger ones, there is very little difference in price and they are more versatile. You also get gold plated banana connectors - they are for high quality audio, and it honestly isn't worth paying extra for them if you are just using them as power leads.

The power supply shown has extra sockets for a 5V supply, but I haven't actually added it yet, so the 5V (green) terminals are currently unused. You would simply need to add a 5V regulator on the +12V supply.

Components

Here are the components you will need:

The list is:

  • A plastic project box (about 200 by 120 by 60mm).
  • Two 12V 1A DC adapters.
  • Two 6mm barrel sockets (or whatever your DC adaptors require).
  • 16 4mm banana sockets (4 each of 4 different colours).
  • 3 LEDs - I used red, yellow and green to match the colours I used for the banana sockets for the 12V, -12V and 5V lines.
  • 3 1K resistors for the LEDs.
  • Half a metre of wire, use something thicker than normal electronics wire - 16 AWG stranded is ideal.

Schematic

Here is the schematic:

It is very simple. The power comes in on the two barrel sockets. We connect the positive of one to the negative of the other, and that becomes our 0V. The other two sides of the barrel sockets become out 12V and -12V.

The three power rails are wired directly to the corresponding output banana sockets.

There is an LED and 1K resistor in series from the 12V rail to 0V, and another LED and resistor from -12V to 0V. Notice that the second LED is reversed (its +ve pin is connected to 0V).

That is all there is to it!

Variations

You can vary this project in several ways:

  • Different number of connectors - the project has 4 sockets for each power rail. You could have fewer if you think you won't need that many. Think carefully, though, the sockets are quite cheap and it would be a shame to only fit 2 or 3 sets and discover later that you needed an extra one. Equally you could add more than 4 connectors per rail, but take care then to not exceed the power limit of the power adaptors. 4 seemed like a happy medium to me.
  • The LEDs aren't strictly necessary and could be left off. I like them because I can see that all the rails are powered (ie that I haven't forgotten to plug one of the adaptors in), but it is up to you.
  • Use a different case. The size of the case isn't critical, if you have a spare case available use that. I wouldn't make it too much smaller, because if you have a very small case with a lot of things plugged into it, it will tend to hand off its wires rather than sitting on the bench. This can lead to a lot of cable strain, and eventually connectors might fail.
  • Leave off the 5V rail if you think you will never need it.

Construction

Drilling

The first step in construction is to drill the holes. Most of these are on the top of the case:

The 16 holes for the banana sockets are on a 4 by 4 grid. Try to keep them at least 2cm apart so you can plug the cables in easily. I found 4.5mm holes to be ideal, but it might vary depending on the type of sockets you have.

I placed the 3 holes for the LEDs at the top of the 12V, 5V and -12V rows of sockets. I found that 6mm holes were perfect - the LEDs pushed into place snugly.

The 2 barrel sockets for the power in from the adaptors are best placed at the back of the box, out of the way. The holes aren't round, the are rectangular with a semi-circle at one end. You need to drill then file the holes into shape.

Placing the components

Here is a view of the top of the power supply:

The sockets are held in place by nuts underneath the panel:

Important if you are using the larger screw terminals, it is essential that you use both nuts provided. Make the first nut fairly tight, then tighten the second nut against it so it can't come loose. If the nuts aren't tight, when you turn the screw terminals to attach wires you will find that the socket will eventually loosen and start turning.

The LEDs push into the 6mm holes, they should be quite a good fit but it us worth using a bit of glue to secure them.

The barrel sockets can either be secured with small bolts, or glue in place.

Wiring up

Solder each of the black (0V) sockets together with 16 AWG stranded wire, and connect them to the common terminal of the barrel sockets using insulated wire (16 AWG again - white in the image because I didn't have any black).

Repeat for the red (12V) sockets. The red wire goes to the +ve terminal of the barrel sockets.

Repeat again for the yellow (-12V) sockets. The yellow wire goes to the -ve terminal of the barrel sockets.

The 1K resistors can be soldered directly to the legs of the LEDs, which can then be connected with wire to the power rail (+12V for the red LED, -12V for the yellow LED) and ground.

Here is how it should look:

Testing

To test the unit, plug the two power adaptors into the barrel sockets, and then into main power sockets.

Both LEDs should light.

If you have a voltmeter, use it to check that there is around 12V between each pair of red and black terminals, and -12V between each pair of yellow and black terminals.

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