Wall Wart Power Supply (+/-9V to +/-15V)

Added 2.4K Load Resistors (Jun 2010).

Article by Ray Wilson

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This is an advanced project and I do not recommend it as a first project if you are just getting started in synths or electronics. Only the circuit and some explanation are shown here. A lot of project building experience and electronics knowledge and equipment ownership (scope, meters, etc.) is taken for granted. If you are interested in building this project please read the entire page before ordering PC boards to ensure that the information provided is thorough enough for you to complete the project successfully.

Features

Excellent Sound Lab power supply.
Can use LM78XX/LM79XX or LM78LXX/LM79LXX regulators
Supports plenty of capacitance for a clean supply.
Convenient source of bipolar (+/-V) power from a wall wart.
Low parts count.

Introduction
Tired of buying batteries for your synth-diy projects. Build this wall wart supply and kiss batteries goodbye. Simple and versatile this board will power your projects in style.
!!! IMPORTANT SAFETY CONSIDERATIONS !!!

Use only a CE or UL approved wall wart for this project.
Use only an AC output wall wart DO NOT USE A DC OUTPUT WALL WART.
Do not exceed a 15VAC wall wart.
Make sure the wall wart outputs between 300mA and 1000mA.
All capacitors must have working voltage of 35V or greater.
Use electrolytic caps that fit the board Radial Lead Spacing = 7.5 mm (.3 in.)
If you heat sink DON'T SHORT HEAT SINKS TO ONE ANOTHER OR ANYTHING ELSE
Do not exceed the current capability of the wall wart or regulators used.

Some AC Wall Wart Sources

PLEASE READ THIS... A DIYer recently wrote to ask me the following:

"I have a question about the Wall Wart PSU. I don't see any information here about how much current this circuit can safely draw, nor anything about recommended heatsinking on the project page for this. DO you have any further information regarding these important points?"

I thought my answer might be useful to anyone interested in this project.

The current that can be safely drawn from a power supply that uses one of the wall-wart PCBs is based on several factors:

The current capacity of the wall wart. They come in many current capacities. I recommend that you do not use a wall wart that supplies greater than 1000 mA. It is for eliminating batteries and so should be used to provide similar currents however it can provide the capacity to run several synth modules as long as the wall wart in combination with the selected regulator can provide clean power. The page explains that the output voltage of the supply will depend on the selected regulator pair (e.g. LM7805/7905, LM7809/LM7909, LM7812/LM7912, LM7815/LM7915)

The output voltage of the wall wart. Wall warts come in a variety of output voltages IT IS VERY IMPORTANT TO USE AN AC OUTPUT WALL WART. So lets say the wall wart outputs 15VAC. Typically the VAC value of a transformer is it's RMS value. I'm thinking that the transformer manufacturers thought Volts AC was a little more marketable than Root Mean Square (RMS). So a 15VAC transformer puts out a peak voltage of 15 * 1.414V or 21.21V. If you were viewing the output on an oscilloscope you would see a sine wave (50 or 60 hz. depending on your locale) that had positive and negative excursions of 21.21V peak. This value when rectified and filtered by the diodes and capacitors in the wall wart will be capable of supplying voltage to any of the regulators mentioned above. The spec for the LM78XX/LM79XX series of regulators says that the input voltage should not exceed 35/-35 volts respectively so keep that in mind when buying your wall wart.

The current capacity of the voltage regulators used. Voltage regulators vary in their current ratings. The LM78XX and their corresponding negative partner regulators (LM79XX) claim to provide up to 1A of current. If you use them with a 500mA wall wart you should only expect to draw up to about 400 mA from the supply. If you try to draw more current you will begin to get ripple and degraded regulator performance and the wall wart could become warm and even fail. The LM78LXX (and LM79LXX) regulators can provide up to 100 mA. If you use a higher capacity wall wart and a regulator that can provide more current you can draw more current from the supply without degraded performance.

Heat sinks can be applied but they should not be shorted to one another or anything else. If you can find a heat sink that fits on the board without causing a short between the two regulators great! Otherwise you would need to fashion your own heat sinks from Aluminum L-channel usually found at a hardware store. It is usually sold in 3 to 8 foot lengths. Since you will only use 3/4" inches at a time you will have a lifetime supply for about 4 to 10 bucks. The metal tab on the 78XX series is at ground potential but the metal tab on the 79XX series is at -V potential thus shorting the heatsinks to each other (or ground in the case of the LM79XX will cause sparks and burned out regulators).

I hope this is helpful information but I recommend that if you have not built power supplies before that you buy a pre-made unit rather than try to build one. Most of us DIYers have burned out our share of regulators, cooked a couple of wall warts and seen the results of connecting electrolytic caps backwards. It's a jungle on the workbench and not for the faint of heart. I believe the page provides more than enough information for someone who has built power supplies before and is the reason I put the following information at the top of the page in bold letters:

"This is an advanced project and I do not recommend it as a first project if you are just getting started in synths or electronics. Only the circuit and some explanation are shown here. A lot of project building experience and electronics knowledge and equipment ownership (scope, meters, etc.) is taken for granted. If you are interested in building this project please read the entire page before ordering PC boards to ensure that the information provided is thorough enough for you to complete the project successfully."

If you are just getting started in electronics I wish you success and hope you keep going and gain experience and knowledge and come to enjoy the hobby as much as we nutty DIYers do.

Cheers

Ray

Does it matter if I use a EURO style wall wart?

As long as the secondary voltage of the Wall Wart is correct (9 to 12 VAC... NOT DC) you can use a EURO or US type wall wart. The MFOS Wall Wart board does not care if the primary input is EURO or US it will work fine with either.

Wiring Multiple Modules to One Wall Wart Supply

Depending on which size capacitors and what regulators you use (78XX/79XX or 78LXX/79LXX) you can wire more or less modules to the outputs of a MFOS Wall Wart supply. Read the entire page to get an idea of what I'm talking about. When you wire multiple modules to one power supply (and this goes for any power supply) wire them in parallel. That means that: the +V inputs of the modules all get tied to the +V output of the supply, the -V inputs of the modules all get tied to the -V output of the supply, and the ground inputs all get tied to the ground connection of the supply. Each module draws the current it needs from the supply. If you connect too many modules to a supply they will not be able to get the current they need from the supply and performance will suffer. That is why each module on MFOS has info about the current consumption for that module. Remember that 78LXX/79LXX regulators supply about 100 mA max and that 78XX/79XX regulators supply about ten times that (or 1000 mA). When multiple modules are connected their current consumptions add up. For example: three modules that consume 30mA each connected to the supply will consume a total of 90mA.

Wall Wart Power Supply Schematic Page 1 (June 2010 Version Added Load R's) PDF

The load resistors were added as a convenience for testing without a load and are not necessary. The negative regulator needs a slight load to begin regulating which is normally provided by the circuit being powered. These resistors provide a small load so the output voltage can be observed prior to connecting the supply to your circuit.

Wall Wart Power Supply Schematic Page 1 (Original Version for Reference) PDF

This supply uses half way rectification to produce a positive and negative DC voltage with respect to the WW2 (wall wart 2) connection. Just so you know there is no 1 or 2 as far as wall wart secondaries are concerned but be aware that one side of the output is going to be used for ground (as shown in the schematic) and you do not want to short that to the other output. How could you do that? Well if you use a wall wart jack and plug when you install your wall wart supply in a case be aware that if the plug exposes either wire of the wall wart secondary to any metal involved in mounting the plug then be sure you use that side of the wall wart secondary for your ground (WW2 connection). So in case the wall wart secondary plug mounting contacts your circuit's ground you don't short the wall wart secondary. You might just want to mount the wall wart plug (if you use one) to non-conductive material like plastic or wood. If you don't have any idea what I'm talking about here don't build this so you don't shock yourself, burn up the wall wart, or experience any other untoward event.
Diodes CR3, CR4 and CR5, CR6 are protection diodes recommended in National Semiconductor Application Note AN-182 for protecting three terminal regulators from unwanted current paths that the output capacitors can inadvertently cause under certain conditions. Although the ones for the negative regulator are not considered as critical, better safe than sorry I always say. The 1uF tantalums close to the regulator pins are also recommended by National Semiconductor for operational stability.

Wall Wart Power Supply Theory PDF

As the diagram shows if you look at the output of a wall wart with a scope you see that the output voltage goes positive and negative with respect to the side you connected to the scope ground. For our 12VAC wall wart the voltage goes about 17V positive and -17V negative (in practice the voltage was actually a bit higher). Rectifier CR1 steers the positive excursions onto the filter caps (caps C1, C2, and C3 in parallel actually just make one bigger cap) and rectifier CR2 steers the negative excursions on the filter caps (C6,C7 and C8). The rectified and filtered positive voltage on C1,C2 and C3 is fed into voltage regulator U1 (LM7812 +12V Voltage Regulator) and the rectified and filtered negative voltage on C6,C7 and C8 is fed into voltage regulator U2 (LM7912 -12V Voltage Regulator). The outputs of the voltage regulators provide... drum roll... regulated voltages. If you know what you're doing you can see that you could use different wall wart voltages (9VAC, 12VAC or 15VAC) and different regulators (see the table) to get different regulated output voltages. I use +/-12 and thus the schematic shows that setup.

Desired Voltages Higher Currents U1/U2 Lower Current U1/U2 Wall Wart Output

+/-9V LM7809/LM7909 LM78L09/LM79L09 9 VAC Output

+/-12V LM7812/LM7912 LM78L12/LM79L12 12 VAC Output

+/-15V LM7815/LM7915 LM78L15/LM79L15 15 VAC Output

Wall Wart Power Supply PCB Parts Layout (June 2010 Version Added Load R's) (Parts Side Shown) PDF

Notice You do not have to add all of the large electrolytic filter caps if your application draws low current. One set of caps worked fine for my Sound Lab.

Wall Wart Power Supply PCB Parts Layout (Parts Side Shown) (Original Version for Reference) PDF

Notice You do not have to add all of the large electrolytic filter caps if your application draws low current. One set of caps worked fine for my Sound Lab.

Wall Wart Power Supply PCB Bottom Copper (Parts Side Shown)

Wall Wart Power Supply PCB Top Copper(Parts Side Shown)

Wall Wart Power Supply PCB Top Silk Screen

Wall Wart Power Supply PCB Populated

Aren't you lucky in that you get to learn from my mistakes :-) Buy the right size capacitors for the board. I got these "on sale" but the lead spacing was too big. As you can see you can make 10 mm (.4 in) radially spaced leads work (the caps do not seat all the way) but for best results here are some Mouser Electronic's part numbers to use to get caps with the correct 7.5mm (.3 in.) lead spacing:

140-XRL35V3300-RC 3300 uF (18 x 36) mm with 7.5 mm lead spacing.
140-XRL35V4700-RC 4700 uF (18 x 36) mm with 7.5 mm lead spacing.

PLEASE NOTE! Currently shipping PC boards do have the '+' symbols screened on. Older PC boards do not have the '+' symbols silk-screened on them but the plus side of the electrolytic caps go on the side with the line (as if they did have the plus symbols shown in the parts layout).

Also note that you do not have to add all of the large electrolytic filter caps if your application draws low current. One set of caps worked fine for my Sound Lab.

PLEASE NOTE! Nederlander Arthur Bennis noticed that the negative regulators need a 3mA load before they begin to regulate. He noticed this while using the 7915 regulator. He solved his problem by adding a 4.7K resistor from the regulator's output to ground. You will not notice this unless you test the output of the board without having it connected to a circuit. Once it is connected to a circuit in which it supplies more than 3mA the regulator will output the expected regulated voltage.

Wall Wart Power Supply PCB Using Low Power Regulators

You can use LM78LXX and LM79LXX regulators with the boards too.

Wall Wart Power Supply Project Parts List

Qty. Description Value Designators

6   1N4004 General Purpose Rectifier   1N4004   CR1, CR2, CR3, CR6, CR4, CR5

6   Capacitor Electrolytic
with 7.5 mm (.3 in.) radial lead spacing   3300uF @ 35V   C1, C2, C3, C6, C7, C8

1   LM7812 +12V Voltage Regulator   LM7812   U1

1   LM7912 -12V Voltage Regulator   LM7912   U2

4   Tantalum Electrolytic (35V)   1uF @ 35V   C4, C9, C5, C10