Bipolar power w/ wallwart? Multiple effects?

[D Wagner]

Hey Guys,

What is the easiest way to get bipolar power from a wallwart that supplies only V+ and ground?  Someone mentioned that I would have to use an AC, (not DC) wallwart, full wave rectify that, use two (or four) large filter caps, and the intersection of those capacitors is my ground.  Two regulators (+12 & -12), and I am in business for +/-12V DC.  Does this sound right?

Anyone know of a good source for 15v or 18vAC wallwarts?  (I'll give Mouser a shot in a little while.)

Now the tricky question about powering multiple effects that are in the same "unit".  If I have a bipolar opamp effect, and a transistor fuzz that requires only V+ and ground, can I connect it to V+ and ground of the effect (assuming that my grounding procedures are good, ie star ground everything to the junction of the filter caps)?  Is there anything that I need to look out for, or avoid?

Thanks in advance!

Derek

[Rodgre]

I made a bipolar 9v supply once by using two single supply 9v adapters and wiring them together. I used it on a Craig Anderton Octafuzz.

Somewhere out there, there is an explanation on how to convert a single supply 18v supply into bipolar 9v. I think it involves a network of resistors and caps to establish a new ground between the two lines. Maybe I read it in Craig Anderton's book....

Anyway, my question has always been that if you have an 18v supply and you create a voltage splitter to make a bipolar supply out of it, can you then just pick the +9v half of the supply to feed a bunch of standard 9v effects, and maybe tap off the -9v supply side to feed a germanium fuzz face? Do you have to draw equally from both halves of the power supply, or does it matter?

Roger

[R.G.]

What is the easiest way to get bipolar power from a wallwart that supplies only V+ and ground? Someone mentioned that I would have to use an AC, (not DC) wallwart, full wave rectify that, use two (or four) large filter caps, and the intersection of those capacitors is my ground. Two regulators (+12 & -12), and I am in business for +/-12V DC. Does this sound right?

The simplest way to get a bipolar supply is to use an LT1044 charge pump IC to make a minus voltage from the + voltage. See "+9V and -9V from one Battery!" at GEO http://geofex.com/circuits/+9_to_-9.htm.

That works for +/-9V. In the same section of GEO you'll find ways to make +17 from +9, and you can extend the technique to negative voltage doublers; see the app notes on line for the LT1044 and other charge pumps.

If power IC's don't appeal, get an AC wall wart, and look at "Power Supplies Basics" at GEO http://geofex.com/circuits/+9_to_-9.htm. Down at the bottom is a schematic for getting +/- raw DC power supplies from an AC source without a center tap. You'll need bigger caps, but it's far easier to get wall warts without CT's. Modem wall warts have CTs sometimes.

Failing that, you can make a power CT by using an opamp or power amplifier chip to make a fake ground. No secret here - set a resistor reference at the middle of the raw voltage you have, then use the opamp or power amp chip to creat a low impedance ground by buffering the midpoint bias voltage.

Now the tricky question about powering multiple effects that are in the same "unit". If I have a bipolar opamp effect, and a transistor fuzz that requires only V+ and ground, can I connect it to V+ and ground of the effect (assuming that my grounding procedures are good, ie star ground everything to the junction of the filter caps)? Is there anything that I need to look out for, or avoid?

The problem you find is imbalanced DC currents from the ground of the power supply. If you have independently regulated + and - supplies, you can freely do this. If you split one DC power supply with a resistor and caps, the "ground" will move as the DC current imbalance causes the DC point in the middle to move.  This is the problem that buffering a resistor/cap power supply splitter solves. The buffer equalizes the DC draw on each side and keeps the "ground" in the middle.

Now the tricky question about powering multiple effects that are in the same "unit". If I have a bipolar opamp effect, and a transistor fuzz that requires only V+ and ground, can I connect it to V+ and ground of the effect (assuming that my grounding procedures are good, ie star ground everything to the junction of the filter caps)? Is there anything that I need to look out for, or avoid?

This is another version of the same question. You can't successfully do this with a passive power splitter. You CAN do it with an active power supply splitter. There are several ways to split one DC power supply. There's the R-C method, which will imbalance the DC levels. There's the opamp buffered RC, which works fine up to the current limit of the opamp. There's the power amp chip method, which works fine up to the current limit of the power amp chip. Some of the national semi power amp  chips can work from up to 28Vdc, and self bias in the middle. I think the LM2877 does this. One of these chips with both inputs grounded and no other components makes a quiet power ground in the middle of a single power supply.[/url]

[yano]

Keep this thread posted on your findings if you would, Derek.

I'm in the market for a bipolar wallwart (9 or 12V) or maybe just a regular one that I can put through a circuit similar to the one posted earlier here...

[D Wagner]

Thanks for the replies guys.

R.G., after looking at the MAX1044 datasheet, I did some searching and found exactly what I was looking for.  It is the MAX628.  That application notes show a circuit that generates multiple isolated supply voltages from a DC supply of up to 18V/1A.   There are minimal parts involved;

http://www.maxim-ic.com/appnotes.cfm/appnote_number/1932/ln/en

This will allow me to use a wall wart and have my bipolar supply.  Nice.

Thanks for pointing me in the right direction, and for the great articles on the charge pumps (among othr things)!

Derek

[R.G.]

MAN! That's GREAT! I was working on exactly that output circuit - capacitor isolated voltages from a high speed complementary oscillator, full wave bridge and all, only I hadn't found an IC that had the capabilities of the MAX628. The closest I had come was running two 555's or one SG3524 to do the job.

Hot spit! There's a new Spyder in the offing!

[R.G.]

OK, got it. The MAX628 is not easily available, but Digikey stocks a TI replacement that will work.  The circuit looks like:
- input rectifier/filter cap
- TO-220 7809 regulator sitting on top of two schottky diodes to ground to get the right output voltage.
- TI UCC37325 MOSFET driver driven by CMOS hex inverter oscillator.
- for each output: two 0.1uF low ESR caps, four 1A DO-41 schottky diodes, and a filter cap.

You iterate the caps/bridge/filter cap for as many outputs as you need, probably up to about a half dozen.

I'll see if I can get this tinkered up.

[D Wagner]

OK, got it. The MAX628 is not easily available, but Digikey stocks a TI replacement that will work.  The circuit looks like:
- input rectifier/filter cap
- TO-220 7809 regulator sitting on top of two schottky diodes to ground to get the right output voltage.
- TI UCC37325 MOSFET driver driven by CMOS hex inverter oscillator.
- for each output: two 0.1uF low ESR caps, four 1A DO-41 schottky diodes, and a filter cap.

You iterate the caps/bridge/filter cap for as many outputs as you need, probably up to about a half dozen.

I'll see if I can get this tinkered up.

R.G.,

I'd like to see what you have in mind.  I'd like to stay as close to the circuit in the application notes as possible.  I am trying to get a 12V bipolar supply to power a synth type circuit from a 15v - 18v DC wallwart, since they are so easy to find.  I'm trying to make this compatible for US and Europe, since I'm building it for someone in Norway.  (That is my issue with avoiding a transformer.)  I'll use my 120VAC/16VDC wallwart, and he can use the European equivalent.   Both compatible via a Boss type 2.1mm power adapter.

The entire power supply circuit will have a really small footprint.  I would imagine that you'd have more space on the PCB for V out pads than the actual components.

Do you have any information on what to do with the unused pins for the hex inverter?  I downloaded the datasheet for the one in the application notes, but haven't spent much time looking at it.  Is there another IC with better characteristics that I should look into?  

Thanks,  Derek

[R.G.]

I'd like to stay as close to the circuit in the application notes as possible.

The operational part is the same circuit - the TI part is pin and specification compatible with the MAX628. The MAX628 suffers from being made of pure unobtainium. The additional gorp is to create the environment that makes the circuit work, things that are left out in app notes to avoid obscuring the point of the note.

The entire power supply circuit will have a really small footprint. I would imagine that you'd have more space on the PCB for V out pads than the actual components.

That is your real challenge. That pretty much pushes you to surface mount components.

At a minimum for your 12V thing, you'll need:
- Max628 equivalent
- local power supply smoothing caps
- series output caps
- four schottky diodes
- output smoothing cap
- CMOS IC containing at least two inverters
- two resistors
- two timing caps
- power supply for the CMOS inverters; standard CMOS is likely to be kind of slow in transitions, although it can take the 12 to 18Vdc supply if you get the B version. I suspect you may want to use a resistor and 5V zener to make 5V for HC or LVC CMOS. The 74LVC204 (I think) is a dual surface mount inverter that's probably the smallest footprint part. The 74HCT04 is probably the easiest to find, although bigger.
- probably an input smoothing capacitor

Do you have any information on what to do with the unused pins for the hex inverter?

Yeah - ground the inputs, leave the outputs open.

Is there another IC with better characteristics that I should look into?

As I mentioned, the TIxxxxx part is pin, function, and specification compatiable, and is available from Digikey for a couple of bucks. TI has some more capable parts in its MOSFET driver family.

Layout will be critical to keep this thing from emitting RFI.