MFOS Box Supply

±12V and a separate 9V from one AC wall wart · A 7812, a 7912 and a 78L09 · By Onion Madder · Mess O' Pedals

Two Music From Outer Space boards going into one box, wanting two different supplies: the 10 Step Sequencer runs on 12V bipolar, plus and minus, and the Noise Toaster runs on 9V. This board makes all three rails from one 12 VAC wall wart, with a power switch in the lead. The sequencer gets +12V, -12V and a ground; the Toaster gets its own regulated 9V and a ground, wired where its battery clip used to go. Fifteen parts, four cuts, and three regulators standing in a row.

The MFOS Box Supply layout drawn as a stripboard diagram, titled MFOS Box Supply, 37 by 4 stripboard. Four long orange copper strips lettered a to d run left to right with thirty-seven columns numbered above them. Strips a and b are each cut once near the middle, strip c is cut twice, and the bottom strip d is whole. Three regulators stand in a row across the top three strips: a large TO-220 body labeled U1 near the left, a small TO-92 labeled U3 in the middle and a second TO-220 labeled U2 on the right. Pink circles mark the electrolytics C1, C5, C9, C6 and C2, small outlined boxes mark the ceramic caps C3 and C4, and four small diode bodies D1 to D4 stand between strips, D1 at the left end and D2 at the right end reaching down to strip d. A dark blue link J1 joins strips b and c just left of the cuts. Pads carrying wires off the board are labeled in the margins: +12, GND_IN, GND_SEQ and AC on the left, -12, GND_NT and +9 on the right. Off to the left an SPST toggle and a DC jack labeled 12 VAC wart are wired to the AC and GND_IN pads, and two module boxes stand for the MFOS 10 Step Sequencer and the MFOS Noise Toaster with wires to their pads. A parts list and an off-the-board list run underneath. Below it the same board is drawn a second time from the solder side, mirrored, with the column numbers running right to left, every solder point as a dark dot and the cuts as X marks. — The board as the layout file draws it. Every part, cut and off-board pad in one picture. The file itself is below. The lower drawing is the solder side, mirrored - the side you actually work on.

What you'll need

tap to expand / collapse

1× 7812 (U1), the +12V regulator. Spot it: TO-220, a black body with a metal tab and three legs, 7812 printed on the face. Printed face toward you: IN, GND, OUT. The tab is ground
1× 7912 (U2), the -12V regulator. Same package, and not the same legs: printed face toward you it is GND, IN, OUT, and the tab is its input, not ground. See the heatsink note under the wiring
1× 78L09 (U3), the 9V regulator. Spot it: TO-92, the small half-round body like a transistor. Flat face toward you: OUT, GND, IN - the reverse of the big ones
4× 1N4001 (D1 to D4). Spot it: black body, silver band at the cathode end. D1 and D2 are the rectifiers, D3 and D4 the output clamps. All four stand up. Any 1N4001 to 1N4007 works
2× 1000 µF, 35V electrolytic (C1, C2), the reservoirs. Spot it: the two big cans, about 12.5 mm across. The stripe marks the negative leg. C2 goes in the way that looks backwards: its positive leg is on ground and its stripe on the negative rail. The drawing is right
2× 47 µF, 25V electrolytic (C5 on +12, C6 on -12). C6 is the other one that looks backwards: positive leg to ground. The 7912 needs it - its datasheet will not promise a stable output without a cap there
1× 10 µF electrolytic, 16V or more (C9), on the 9V output
2× 100 nF ceramic (C3, C4), marked 104, one on each regulator input
1× link of solid wire (J1)
1× stripboard, 4 strips × 37 holes or larger

Off the board

1× 12 VAC wall wart, 300 mA or more (a 500 mA one is plenty). It has to be an AC one: the label says AC output or shows a wavy line. A DC wart will not work - it only fills one side
1× DC jack to match the wart's plug, usually 2.1 mm. AC has no polarity, so it does not matter which way round the plug is wired
1× SPST toggle, the power switch, in the jack's tip lead
Five wires to the boards: three to the sequencer (+12, -12, ground), two to the Noise Toaster (+9, ground)
Optional: two clip-on TO-220 heatsinks. Not needed at these currents, but see the note below before sharing one

The layout file is the guide

Like the rest of this shelf, the build document for this one is the file itself: ↓ Download MFOS_Box_Supply.json - import it into Copper Bottom and the editor gives you the board picture, the placement walkthrough, the electrical checks and a printable build sheet, all from the same file. No coordinates are written out on this page on purpose: the editor is the viewer, and a number typed twice is a number that can drift.

Leave room for the big cans. Copper Bottom draws every electrolytic the same small size, so it cannot see that C1 and C2 are 12.5 mm across. The layout keeps the two columns either side of each one empty for them. Do not fill those holes with anything else.

The netlist

Quoted verbatim from the layout file. It was written from the board, so checking the board against it can only pass - what it is for is carrying the circuit somewhere else, and telling you what each strip is supposed to be doing while you stare at it.

# MFOS Box Supply - written from the board # 4 strips of 37 holes, 15 parts, 4 cuts # # One AC wall wart, 12 VAC, half-wave split: D1 fills C1 on the positive # half-cycles, D2 fills C2 on the negative ones. A 7812 and a 7912 make # +12 and -12 for the sequencer; a 78L09 off the regulated +12 makes +9 # for the Noise Toaster. D3 and D4 keep each output from being dragged # past ground at switch-on. The 7912 tab is its INPUT, not ground. AC: @AC D1.A D2.B +RAW: D1.B C1.A C3.A U1.I -RAW: D2.A C4.B C2.B U2.I +12: U1.O C5.A D3.B U3.I @+12 -12: U2.O C6.B D4.A @-12 +9: U3.O C9.A @+9 GND: @GND_IN C1.B C3.B C5.B C9.B C6.A C4.A C2.A D3.A D4.B U1.G U3.G U2.G @GND_SEQ @GND_NT

The off-board wiring

Seven pads leave the board: two for the wall wart, three for the sequencer, two for the Noise Toaster.

Pad	Goes to
AC	The power switch's middle lug. The switch's other lug goes to the DC jack's tip
GND_IN	The DC jack's sleeve. This lead of the wall wart becomes the ground for everything in the box
+12 / -12 / GND_SEQ	The 10 Step Sequencer's +12V, -12V and ground. Twist the three together on the way
+9 / GND_NT	The Noise Toaster, where its battery clip's red and black leads went. Take the clip off; nothing else on that board changes

The 7912's tab is its input. The 7812's tab is ground and the 7912's is the raw negative rail, so bolting the two to one bare heatsink, or to a metal box, shorts the negative side to ground. Give each its own clip-on heatsink, or use a mica pad and an insulating washer on the 7912. At the currents two MFOS boards draw, neither needs a heatsink at all.

Before the boards go on

Power the supply on its own first, with nothing connected to the output pads, and meter each rail to GND:

+12 should read about 11.5 to 12.5V, and -12 about -11.5 to -12.5V
+9 should read about 8.6 to 9.4V
If a rail reads near zero, check that regulator's legs against the list above first. The 7812, the 7912 and the 78L09 all have different leg orders, and one in backwards is the likeliest fault on this board
If the negative rail is missing entirely and the wart is running warm, check the wart is AC. A DC one fills only one side

Then connect both boards, switch on, and check +12 again with everything running. If it holds, the supply has the headroom it needs.

How it works, in one breath

One lead of the wall wart is ground and the other swings about 17V either side of it, sixty times a second. D1 lets the positive half through into C1 and D2 lets the negative half through into C2, so the two big caps charge to about +16V and -16V and hold there between peaks. The 7812 and 7912 turn those into steady +12V and -12V; each wants about 2V more than its output at the bottom of the ripple, and with 1000 µF and a hundred milliamps or so of load it gets that. The 9V comes off the regulated +12V rather than the raw one, so the 78L09 only burns off 3V and the Toaster never sees any ripple at all. D3 and D4 do nothing in normal running. They are there for the moment of switching on, when one regulator can come up faster than the other and a load across both rails drags the slow one past ground; the diode catches it.

Things to try

If +12 sags or hums with both boards running, the sequencer is pulling more than this was drawn for. A 2200 µF, 35V for C1 halves the ripple, but it is a bigger can and needs more room than the layout leaves
A power LED. An LED and a 4K7 in series from +12 to ground, off the board, lights whenever the box is on
A third rail. Another 78L-series regulator off +12 the same way the 78L09 is - a 78L05 for 5V logic - with its own 10 µF on the output

No circuit to credit: the half-wave split supply is the one every regulator datasheet draws. The layout is Onion Madder's, drawn in Copper Bottom. The two boards it feeds are Ray Wilson's, from Music From Outer Space - see the MFOS rebuilds.