Wall Wart Power Supply (+/9V to +/-15V)
Article by Ray Wilson
Back to "Music From Outer Space" Analog Synth Pages
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.
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Some AC Wall Wart Sources
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.
Buy Wall Wart Power Supply PC Boards via PayPal
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Tired of etchant eating your hands and the tedious work of drilling hundreds of
tiny holes. Tired of that tingly guilty feeling you get as you flush ferric
chloride into the environment. Then buy a ready-to-go, super high quality PC
Board!
Purchase Printed Circuit Boards online! Click the Add To Cart button to
make a purchase or go to PayPal
to sign up for a PayPal account. You can also set the number of boards you
would like to buy on the PayPal shopping cart page.
Please note that you are buying an un-populated PC board
only. The glass epoxy, double sided, plated through-hole PC board is
professionally manufactured, pre-drilled and silk-screened with a parts layout
legend. You must purchase all of the parts for the project and build it
yourself. But since that is the whole idea behind DIY (Do It Yourself) that's a
good thing.
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Wall Wart Power Supply PC Board Size (4.3" x 2.2")
These little suckers come in really handy... buy more and save $$
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(1) One Wall Wart Power Supply PC Board $15.00
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(2) Two Wall Wart Power Supply PC Boards $26.00 (Save $4.00)
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(3) Three Wall Wart Power Supply PC Boards $35.00 (Save $10.00)
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Wall Wart Power Supply Schematic Page 1 PDF
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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 inadvertantly 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.
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Wall Wart Power Supply Theory PDF
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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
recitifer CR2 steers the negative excursions on the filter caps (C6,C7 and C8). The rectifed and filtered
positive voltage on C1,C2 and C3
is fed into voltage regulator U1 (LM7812 +12V Voltage Regulator) and the rectifed 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.
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| 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 (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 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.
Notice The 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.
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 |
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| 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 |
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