Stepping down 16VAC to 12VAC

[gearjunkie]

Hi

I have a pedal power supply that has almost all my needs covered

However I have one output left on that PS which gives me 9 or 16VAC at up to 1.3A and I need 12VAC 1.0A into that pedal. While the pedal can take > 12VAC it is much quieter at 12VAC. PS...I don't want to use another wallwart

Have searched the forum - most solutions tend to be DC based or larger step downs.

I was initially hoping to use a voltage regulator ie IC7812 with a heatsink but I understand they only deal with DC voltage

Just wondering what the simplest and most reliable way to do this step down is...

Any solutions would be gratefully received

Andrew

[Rob Strand]

However I have one output left on that PS which gives me 9 or 16VAC at up to 1.3A and I need 12VAC 1.0A into that pedal. While the pedal can take > 12VAC it is much quieter at 12VAC. PS...I don't want to use another wallwart

There's no nice way to do it that is practical.

There's two approaches.   Lossy and lossless.

Lossless would be to use a 16V (or 9V) to 12V auto-transformer.   It's ridiculous to consider this idea as the transformer would be almost the size of the plug-pack and cost somewhat more.

Lossy is more of a sledge-hammer approach.   If your load is relatively constant you can use a resistor to drop the voltage.  It will get darn hot.  If the load isn't constant then you can use a number of back to back diodes to drop the voltage down.  Here the diodes will get hot.  Very ugly solutions but if you want to use that transformer then they can made to do the job.

Anything in between these two solutions is going to be too complicated and more trouble than it's worth.  Using transistors or MOSFETs to drop the voltage are probably less reliable than diodes and resistors.

(Edit: there is one other voltage drop method.  A series inductor.   It won't get as hot as the resistor but it will be larger and costly.)

[R.G.]

Back tp back pairs of power diodes...   

[GibsonGM]

Any use for a capacitive voltage divider >> rectifier + caps >> 7812?

[Rob Strand]

Back tp back pairs of power diodes...   

No claims for neatness.

I suppose you could reduce the number of diodes by putting one strip inside a bridge rectifier.

[R.G.]

Yep, no claims for neatness. That lies in the hands of the constructor.   

[PRR]

Bunch-of-diodes makes a very ugly "sine" AC wave.

Since he already has a problem (unexplained), I hate to go there.

The "clean cheap" fix is a transformer. Even an autotransformer which could be 4VA. However a 16V:12V is a very odd part; likely comes with a 120V winding we can ignore, except by taking half the space it doubles the stray resistance.

[Rob Strand]

While the pedal can take > 12VAC it is much quieter at 12VAC.

I'm a little confused how you know that is true?  If you did a test with a different wall-wart then maybe the 16VAC unit has an issue.

In other words changing the voltage is solving the wrong problem.

Since he already has a problem (unexplained)

Give that,

While the pedal can take > 12VAC

Maybe all he needs to do is increase the resistor until the "problem" goes way.

The "clean cheap" fix is a transformer. Even an autotransformer which could be 4VA. However a 16V:12V is a very odd part; likely comes with a 120V winding we can ignore, except by taking half the space it doubles the stray resistance.

IMHO, replacing the wall wart *is* the cleanest solution.  If you are going to use a transformer you might as well replace the wall-wart.

[Sooner Boomer]

Do you know what's on the power input side of the pedal?

My suggestion is to treat it just as you would a DC input - rectify, filter, and stick on a regulator (if you go with a 3 pin fixed regulator, you might stick a diode between the regulator and ground.  this will kick the output up by the amount of the voltage drop across the diode, say a half volt).  I bet the AC input will "eat" DC just fine.  Or it could start a fire and burn the house down.  Either way, problem solved.

BTW, would not use a simple dropping resistor if trying to reduce AC, I would use a pair in a voltage divider configuration.

[gearjunkie]

Thanks all
I thought this would be quirky/tricky/inefficient or all of the above
Might just have to settle for another wallwart of find a DC driven pedal to replace...
Andrew