9 VAC wall wart

[drhulsey]

Is there anything inside a 9 VAC wall wart other than a transformer?
Is that all that is needed if you want to add a 9 VAC outlet to a pedal
power supply/

Thanks!
Tim

[smallbearelec]

Your questions are a little unclear to me. Is the wall wart something that you have, or something you are thinking to buy? What is your ultimate application?

An AC wall wart is sometimes simply a transformer, though some include a thermal shutdown device or fuse as well.

[R.G.]

Steve is correct about the innards. 9V->AC<- wall warts are usually just a small transformer, with some protective something like maybe a fuse or maybe a thermal cutout.

But let's talk about the greater issue for a moment. a 9VAC output is as dangerous as a pet rattlesnake. Too many pedals to count have been destroyed by plugging a 9Vac power supply into them in a moment of confusion. The reason is that the standard reverse-shunt diode polarity protection provides little if any protection against a 9Vac transformer output. It first kills the diode, then the rest of the circuit. And they have the same size/shape plug on the wire as DC adapters so they'll neatly fit in almost every pedal.

The only thing worse than plugging a 9ac source into a pedal is plugging it into a daisy chain to all your pedals. It will find the weak one. Then the next weakest. Then the next. You don't have all that long to stop the meltdown.

Be really, really sure you want to add 9Vac to a pedalboard power supply.

It's like the old quip: be very careful what you want - you might get it.

[drhulsey]

Thanks for the replies from two of my heroes!
R.G., I have read your warning about AC and pedals many times.
I have an 9VAC wall wart that I took apart to see if I could add the innards to a multi-output power supply.
It was a surplus RCA adapter. There was only the transformer inside. I couldn't think of anything else a 9VAC supply would need, but I just wanted some expert opinion. Boy, did I get experts!
I had planned to place the output for it in a different location on the enclosure, well away from the 9VDC outlets and well-labelled.
My 9VDC is supplied in ten isolated outputs based on a Line-6 power supply for pedal displays in a guitar store. I found them on eBay:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=250729163133&ssPageName=STRK:MEWAX:IT#ht_1810wt_836

I am building one into an enclosure for a bench top supply. I have some Godlyke PowerPumps which will give me the option of 12, 18, and 24 VDC, and I thought I would add a 9VAC option for good measure. I plan to modify another for pedal board use and don't need 9VAC for it.

BTW, I am currently reading your PCB book, R.G., which I bought from Steve. Thanks so much to both of you for your contributions!

Tim

[davent]

If you have to have both AC and DC you could use a totally unique connector on your power supply for the AC such as a Molex and then colour code the cable for it, say red so it's obvious when you have the 'danger to pedals' cord in your hand and are averted from the road to destruction...

dave

[R.G.]

Yep, you're right Dave.

I like to buy the bright yellow "tool handle dip" at hardware stores and paint that on the ends of connectors to watch for. It dries to a flexible plastic in a bright color.

[yeeshkul]

How come 9VAC kills the shunt diode and reversed DC doesn't? It sure depends on the current through the shorted diode (wall wart type) .. did i overlooked something else?

[R.G.]

How come 9VAC kills the shunt diode and reversed DC doesn't? It sure depends on the current through the shorted diode (wall wart type) .. did i overlooked something else?

Much as I hate to say this ( ) I don't really know.

It seemed odd to me, but when we started getting back pedals killed this way, I started messing with trying to recreate it. It may be something to do with the fact that regulated 9Vdc adapters nearly always use a three terminal regulator that limits current to an amp or so, and a transformer does not. I would have delved deeper into it, but the answer is clear - don't plug in a 9Vac pedal.

Actually, I vaccinated all the Visual Sound pedals against this. No current manufacture VS pedal will die on 9Vac, but this is most assuredly not true of most pedals.

A series diode will stop the reverse voltage and let a pedal survive; making this Schottky keeps the voltage losses down. An active series reverse voltage preventer, like the MOSFET and transistor versions on geofex *can* be made to stop it. But a shunt diode will just sit there and fry.

[drhulsey]

R.G.,
In a multi-channel power supply like the Spyder, what size fuse would you use on the primary?
The Line-6 power supply I'm working on has 2 MOVs, but didn't have a fuse as it came to me.
I put in a 0.5 A, Slo Blo. It didn't blow with no load.

Thanks!
Tim

[yeeshkul]

You have to do some measuring before you decide on a primary fuse. First, you have to measure the primary current when the power supply is shorted (biggest possible current) and then, do the same when the power supply is just loaded to it's maximum current draw, but not shorted (slightly lower primary current). Then use a primary fuse for a current in between of those two numbers. Well it is annoying and a bit dangerous if you don't know what you are doing (the load resistor must be a high power one, not just a common resistor). I cannot recommend doing this unless you have more experience than confidence.

A much easier and safer way to do this is to use a secondary side fuse.

[blooze_man]

I don't really know.

I think I just saw a flying pig.

[drhulsey]

Thanks, yeeshkul!

[R.G.]

You have to do some measuring before you decide on a primary fuse. First, you have to measure the primary current when the power supply is shorted (biggest possible current) and then, do the same when the power supply is just loaded to it's maximum current draw, but not shorted (slightly lower primary current). Then use a primary fuse for a current in between of those two numbers. Well it is annoying and a bit dangerous if you don't know what you are doing (the load resistor must be a high power one, not just a common resistor). I cannot recommend doing this unless you have more experience than confidence.

A much easier and safer way to do this is to use a secondary side fuse.

I need to note here that primary and secondary side fuses are for different things. Secondary fuses protect the power transformer against overloads. Primary side fuses are there to prevent fires and electrocution if the transformer has a fault. They do different jobs.

It is possible to compute an estimate of the primary fuse size by calculating the maximum load on each secondary, adding them up, including any additional RMS currents cause by full wave bridge rectifiers causing heavy peak currents into caps, and then computing an estimate of the primary RMS current, adding a startup surge amount and a safety factor. For beginners, it's simpler to get an assortment of fuses from smaller than you think you'll need to larger, then starting with the small ones and seeing if it will start and run without blowing a fuse. The right size is the first one that doesn't blow at startup and under maximum load.

As Yeeskul notes, it is tiring.

But don't count on a secondary fuse for safety issues, and don't ignore safety issues.

[yeeshkul]

I have truly no experience with secondary fuses, i knew that they work like an overload prevention, but i thought that overload of the secondary side causes extreme current on the primary side and that can cause big balls of fire . But as i say - i went through the annoying procedure of picking the right primary fuse to make my home-made wall wart safe.

[R.G.]

I have truly no experience with secondary fuses, i knew that they work like an overload prevention, but i thought that overload of the secondary side causes extreme current on the primary side and that can cause big balls of fire . But as i say - i went through the annoying procedure of picking the right primary fuse to make my home-made wall wart safe.

You are correct in that they do secondary protection on the secondary side. But secondary fuses can't protect against a fault in the primary side of the transformer. You need a primary fuse for that.

My personal preference is for a primary fuse, and also a secondary fuse if there is no other protection on the secondary. A regulator which limits load current, like all three terminals these days qualifies for secondary protection.

For things like tube amps with both a high voltage secondary and a heater winding, you often need a secondary fuse per secondary. The heater winding won't always blow the primary fuse, so the transformer melts down.

[drhulsey]

I have started with a 0.5A slo blo. With no load, it starts fine. I will begin adding load and see how it goes. No one can find any information on the transformer even by contacting the company (MCI Transformer Corporation West)! It doesn't seem to be a part of their normal inventory, and I'm sure it was designed for Line-6's project. It seems like a well-made board and, though it is not small (6x6x2"), it can be fitted under a pedal board easily after moving or removing the mains selection switch. I have the mains grounded, and finally found the primary grounding pad at the junction of the MOVs.
I have tremendous respect for 60 Hz 115V electricity. 60 Hz will stop your heart in a New York second! I've treated a lot of electrical burns, and I don't like that part, either.
Thanks for your help!

Tim

[Paul Marossy]

I have tremendous respect for 60 Hz 115V electricity. 60 Hz will stop your heart in a New York second! I've treated a lot of electrical burns, and I don't like that part, either.

I've been shocked by 115VAC several times. Maybe I'm just lucky?

[Hides-His-Eyes]

I have tremendous respect for 60 Hz 115V electricity. 60 Hz will stop your heart in a New York second! I've treated a lot of electrical burns, and I don't like that part, either.

I've been shocked by 115VAC several times. Maybe I'm just lucky?

Across the hand or across the chest?

[R.G.]

I've been shocked by 115VAC several times. Maybe I'm just lucky?

Across the hand or across the chest?

Yes, you are lucky. And HHE is correct - current through the chest is what sends your heart into ventricular fibrillation. The amount of current through the heart muscle is what determines the deadly effect. As little as 20ma through the chest can kill you.

50-60Hz is particularly bad as far as starting fibrillation. The heart is sensitive to that. The reason DE-fibrillators restart the heart is that they are a DC discharge that clamps everything down simultaneously and lets it all go at once. Re-synchronizes the various parts of the heart so that if it can re-start, it does so in sync.

[drhulsey]

60 Hz can also lead to cardiac standstill– no electrical activity. Always do CPR on electrical shock victims (when they are no longer in contact with the source). There is a high success rate. There is seldom direct damage to the heart, even from high voltage, unless the entry point is adjacent to it, i.e., on the left chest. R.G. is right. The heart being in the path of the electricity from entry to exit is important.
Paul, I knew we were lucky to have you. I just didn't know how lucky!