AC-DC ALTERNATE POLARITY INPUT FOR YOUR BOX

[hawkeye2u]

I DON'T KNOW IF ANYONE HAS TALKED ABOUT THIS REALLY SIMPLE BUT EFFECTIVE POWER INPUT DEVICE FOR THE STOMP BOXES
IMAGINE IF YOU COULD PLUG IN ANY POLARITY DC OR AN AC POWER PACK TO YOUR BOX & ALWAYS HAVE THE RIGHT POLARITY
THE ANSWER IS SO SIMPLE!!
ON THE POWER INPUT JACK OF YOUR BOX CONNECT A BRIDGE RECTIFIERS AC INPUTS TO THE PLUG
THE OUTPUT OF THE BRIDGE RECTIFIER HAS TO BE POLARIZED THE RIGHT WAY AROUND IE + TO POSITIVE & - TO THE NEG TERMINAL OF THE BOX'S PCB
WHATEVER THE POLARITY OF THE INCOMING POWER IS YOU WILL ALWAYS HAVE THE RIGHT POLARITY TO THE BOX!!
AC OR NEG TIP OR POS TIP ALWAYS RIGHT (BE CAREFUL WITH THE AC SO THE DC VOLTAGE DOESNT GET TO HIGH
DC VOLTAGE=AC VOLT X 1.732 (FOR A FULL WAVE RECTIFIER, BRIDGE)

[puretube]

wasn`t that 1.414 times?

[R.G.]

Like most so simple answers, there are a couple of gotchas.

1) Bridge rectifiers do a good job of making any polarity into the right polarity. Unfortunately, they subtract two diode drops of voltage to do this trick. That doesn't matter much when you have plenty of voltage to spare, but when you are using a 9Vdc wall wart, you only get to use 7.6V of the 9V. This is a problem with some (but not all) pedals.

The trick is to use low voltage drop diodes. Schottky diodes get down to about 0.4V, and germanium can be as low as 0.2V. But you can get down to a few millivolts by using what the power industry calls a synchronous rectifier. This is a power device with a saturation voltage that is quite low, and which has a signal input for when to turn on. The "Improved polarity protection" circuit at GEO using a MOSFET, and the cheaper version with more parts using bipolar transistors are both designed this way. As the MOSFET protection article notes, there is a four-MOSFET circuit which can be set up as a bridge to do as you suggest. The only problem is that you have to buy four MOSFETs, at about US$0.50 each to do it. Not a huge problem, but there is always someone going to complain about it, or the complexity of having so many parts.

2) The conversion of sine-wave AC to DC by a rectifier does indeed produce  a voltage equal to the peak of the AC wave. This will be 1.414 times the RMS value of the AC wave in, minus diode losses.

If you have 9Vac in, you get 9*1.414 - (2*0.7) = 11.36Vdc out. Some pedals work OK with this, some pedals will pop - 10Vdc is a common electrolytic value and some poorly designed commercial pedals use this value.

Also, small AC power supplies are notoriously poorly regulated. When the transformer says "9Vac/300ma" what that means is "at least 9Vac when a full 300ma is being used". So the voltage is significantly higher when fewer ma are being used.

You might want to read "Power Supplies Basics" at GEO for some more background, as well as the articles on power protection at GEO.

[niftydog]

Regulators get hot... not ideal in a tiny, sealed box.

Also, this is tempting fate.  You may become complacent and rely on it to handle ANY input voltage.  Then you hook up 30VAC and your un-heatsinked regulator has thirty seconds to live!

My method is to keep all the 9-10VDC plug packs in one spot in my music room and have a few stashed in my gig bag.  All other plug packs are far away in another part of my house.

[hawkeye2u]

Well point taken everybody
I forgot to mention to add a 2200uF electrolytic and a 7809 regulator (heatsinked) and a 1uF, across the output
then U R pretty much covered for all occations (still have the option of a 9v batt connected, after the output of the 7809) :oops:

[RedHouse]

If you only need 100mV using an LM78L09 will do you right, it's the size of a TO-92 transistor and takes up very little space.

I use them locally inside my boxes bringing in 18v from my main power supply box on the pedal board. Heat from the regulator has never been an issue, I can't even feel 'em getting warm.

Few effects use more than 100mA unless say it's a Univibe or you're using multiple LED's, and if so, running LED's at 16mA/each (using a 560 ohm resistor on your 9v regulated circuit) you can run like 3 and still have 50mA for the effect).

Checking the device Data Sheet from National Semiconductor, it indicates the capacitors can be substantially smaller, the typical app note shows .33uF on the input side and .01uF output side.
(of course Wall-Warts usually already have 220uF up to 1000uF filter caps built-in)

[smoguzbenjamin]

By RG's description, Boss effects are poorly designed effects My BF-2 has 10v caps in it, so does my DD-2. Maybe they ain't stock though, I got them both second hand...

[Samuel]

No I think Boss uses 10V electros in their pedals. Not alot of breathing room! The guitarist in my band definitely lost a DD-5 (I think) when our singer slapped a 12V supply into it.

PLUG

What's that smell?

[puretube]

...that`s the price to pay for small footprint enclosures...

[R.G.]

By RG's description, Boss effects are poorly designed effects  My BF-2 has 10v caps in it, so does my DD-2.

Yep.  IMHO, anyone who deliberately puts a 10V cap in a circuit with a 9V nominal battery supply that ranges up to 9.6V on some batteries and also lets you plug in an external power supply is either a true beginner, or has been driven penny-mad by MBAs or the equivalent. That's begging for problems. And you can tell Boss I said it.
8-)

[TheBigMan]

By RG's description, Boss effects are poorly designed effects  My BF-2 has 10v caps in it, so does my DD-2.

Yep.  IMHO, anyone who deliberately puts a 10V cap in a circuit with a 9V nominal battery supply that ranges up to 9.6V on some batteries and also lets you plug in an external power supply is either a true beginner, or has been driven penny-mad by MBAs or the equivalent. That's begging for problems. And you can tell Boss I said it.
8-)

And in those days most Boss pedals (although not the DD-2) used the 12V ACA adapter.  One resistor protects the entire pedal against a power spike or anything else, plug in a 15V or so adapter and you have an ex pedal very quickly.