The Ultimate Power Supply
Over the course of years I made and bought a bunch of different power supply for my effects or amplifiers, but at any given moment I needed at least to of them to power everything I needed. So I set about designing and build a power supply that can power all mu present (and future) gadgets. It needs to have as many isolated outputs as possible, i.e. outputs that do not share common ground, to avoid potential noise caused by ground loops. It also needs to be able to provide various voltage outputs, ranging from 6V (for small germanium amps) to 18V (for EQ and overdrive pedals). Finally, it needs to provide high current outputs for current-thirsty digital pedals, such as digital delays, processors, etc.
At the heart of it is a custom toroidal transformer. It has a 230V/30VA primary and 5 independent secondaries: 18V@0.2A, 18V@0.2A, 12V@1A, 12V@0.5A, 9V@0.5A. The same could be achieved with two or three center-tapped generic 12V and 18V transformers, but custom toroids can be ordered cheaply here (around $20 for a 30VA transformer), so it made more sense to go that way. 30VA transformer weighs around 0.45kg (~1lbs) and is around 70mm in diameter, so it’s also more compact than several EI-core transformers.
The circuit contains an array of five simple regulated and filtered AC/DC rectifiers, each corresponding to secondary. Each bridge rectifier is followed by two cascaded 78xx voltage regulators to provide two (non-isolated) outputs with different voltages. That way we can achieve having 9V output from each secondary, as it’s the most commonly used, together with higher or lower voltages that other circuits may need. Circuits for each secondary are independent from each other, so DC outputs coming from different branches will be isolated.
It’s easy to adapt this circuit for any voltage you may need but keep in mind that each 78xx regulator needs at least 2V extra voltage at the input to get the desired output voltage, e.g. 7809 needs at least 11VDC at the input pin. Bridge rectifier drops around 1.4V due to silicon diode forward voltage drop. Taking all that into account, here’s how the math would look for one of the secondaries. 9VAC secondary should give around 1.41x higher DC voltage after rectification, or ~12.7VDC under full load. If we subtract bridge rectifier voltage drop of 1.4V, we get 11.3VDC. I’ll probably never load it to the maximum, so the actual voltage will be slightly higher. That means that we are in the safe zone and won’t go below the minimum input voltage required for the 7809 to run properly. To be safe, it’s maybe better to have 10VAC secondary instead of 9VAC, but this should be fine as is. In other instances with 12VAC and 18VAC secondaries, we have more voltage to spare after rectification when we apply the 1.41x rule, so they are absolutely fine.
It is also possible to make a version with higher current capabilities. 78xx regulators are rated up to 1A, but there’s also a 2A-rated version of the regulator with “S” in their name, e.g. 78S09.
There’s a lot of regulators in the circuit and some of them are dropping considerable voltage, so heat management should not be taken lightly. 78xx regulators have thermal resistance of 65C/W, meaning that for every watt they need to dissipate, the temperature of the regulator will rise for another 65°C. That’s a lot! As a rule of thumb, TO-220 body can dissipate around 1W without having to install a heatsink or around 3W with a generic small heatsink. Dissipated power can be calculated as P = Vdrop x Iload. Taking that into account, the cases that worry me the most are regulators that drop the most voltage and regulators that supply the most current.
If we take our 7809 that drops from 18V to 9V into consideration, it needs to dissipate the excess 9 volts. Under the maximum load of 120mA, that will translate to around 1.1W of power dissipation. Without a heatsink, that means ~72°C increase in temperature. Another example is the 7809 that drops from 12V to 9V, but under potentially higher loads. 3V drop under 600mA load will dissipate around 1.8W and that’s significant when we take thermal resistance into account, as it would increase the temperature for 117°C – well above the boiling point! All regulators should be equipped with heatsinks, ideally with some thermal paste in between to maximize the effect.
Since I didn’t think about heat management before making the layout, none of the standard heatsinks could fit between the components, so I had to figure out an alternate way to make heatsinks. We found some aluminum L-shape profile used as floor trimming that seemed perfect and cut it to size that could fit the layout. I needed five of them in total – one to cover each pair of regulators. They have thermal resistance of roughly 30-40C/W which isn’t great, but it’s better than nothing and it should help bring the temperature of regulators below the boiling point (we don’t want to boil the capacitors).
Note that the back of each regulator is grounded to the central pin, so we want to keep heatsinks from touching anything else, or they could cause a short.
Couple of years ago I designed a compact eyelet-board layout but did not take heat management into consideration, so none of the heatsinks that can be bought could fit between the regulators and capacitors. If I were to build it again, I’d definitely make a larger board (or separate identical boards for each branch) and have a nice large double TO-220 heatsink for each pair of regulators. I will post the layout used in my build, but I would recommend modifying it to allow for heatsinks to be mounted between each branch of the circuit.
Note that the layout doesn’t show 0.1uF filter caps. I added them later to improve filtering and mounted them above the bridge rectifiers.
For the circuit board I used a thick fiberglass eyelet board and the enclosure is a custom aluminum 14.5x11x5cm box. For cable connectors between the power supply and pedals I decided to use 3.5mm stereo jacks, inspired by Dunlop DC Brick which uses mono 3.5mm jacks. Since our outputs are isolated, we need stereo jacks to avoid connecting output’s grounds to the enclosure. Everything just fits inside with not much room to spare. Capacitors are nice golden strip Panasonic FM and regulators and rectifiers are whatever I found in a local store, any should be good enough for this application. Try to get decent filter caps rated 105°C as it could get warm inside.