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> 12vac 500ma Adaptor For A 200ma Device?, Am I going to blow anything up?
kierent
Posted: November 28, 2010 11:27 pm
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Hi everyone.

I have an acoustic guitar pedal that is missing a power adaptor. The product specs say 12vAC 200ma min. and the smallest I can find is 12vAC 500ma. In the specs it does say min. but I just am wary about putting more than double the specified amps through the pedal and my knowledge of electronics is limited. Does anyone know if this will be a problem? The last thing I want to do is fry the pedal.
Thanks
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AdamO
Posted: November 28, 2010 11:34 pm
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It will not be a problem, you can use the higher-rated power supply.

-Adam O.
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AwesomeMatt
Posted: November 29, 2010 12:26 am
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Short Don't Care Answer - You're probably fine, it doesn't work how you're thinking it does.

Longer Learn Something Answer -

Amp ratings on power supplies are max ratings, like the max capacity of an elevator or bridge, or the max speed of a car. So, all you have to do is pick one that can supply *at least* as much as you need, maybe some extra for safety, and not too big because bigger tends to cost more. In your case, it's such a small amount of power, it's to the point where smaller might actually cost more because it's already pretty tiny. Either way, those cheap power adapters are only a buck or two, so, it's academic.

Match voltage, make sure it can supply more amps than you need, pick the cheapest.

In electronics, amps are the result. Voltage causes things to happen, resistance is any device you hook up, and amps are the result.

Amps = Volts / Resistance

So, when you have a device (some resistance), and you apply a voltage to it (whatever your power supply is).. the amps are the result of that.

It is nonsensical to say that any amount of current will flow, because the amount that will flow will depend on what you hook up to it.

The only additional consideration, is that the voltage in a very cheap power supply (unregulated) changes depending on resistance. So, if you're hooking up something that only draws a tiny amount of power, it might not load down the voltage to what it's marked as. For example, a power supply that says 12V 500mA, might actually be giving you 20V if you're only drawing 25mA through it or something. And that extra voltage could ruin a device.

But probably not. Electronics is often a ballpark kind of science, with wide, forgivable tolerances.
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AdamO
Posted: November 29, 2010 02:03 am
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Oh yeah doh.gif good idea to check the adapter first with a voltmeter and make sure it's actually putting out 12VDC. If not, stick a 7812 voltage regulator on it like this http://circuits.datasheetdir.com/38/UTC-LM7805-circuits.jpg

You can buy a 7812 (cheap) but they're also pretty easy to find in junked electronics if you keep an eye out.

-Adam O.
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AwesomeMatt
Posted: November 29, 2010 02:12 am
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Ignore Adam's comments above, it doesn't apply to your situation.. if you stated it correctly. You said you needed 12V "AC"? You sure? Not "DC"? (Not saying it couldn't be AC, just, that it's rare enough that it warrants a double-check).

If it's DC, then Adam's advice applies.
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AdamO
Posted: November 29, 2010 03:08 am
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QUOTE (kierent @ November 28, 2010 11:27 pm)
The product specs say 12vAC 200ma min. and the smallest I can find is 12vAC 500ma.

doh.gif doh.gif doh.gif doh.gif doh.gif doh.gif

did I mention "doh!"?

I need to stop trying to give advice. This is becoming habitual. Yes, do ignore everything I said. Apologies for any confusion.

-Adam O.
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AwesomeMatt
Posted: November 29, 2010 03:11 am
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It's Sunday. Brains aren't required to work on Sunday. It's the day off.
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kierent
Posted: November 29, 2010 08:29 am
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Thanks heaps!
Yes it's AC, that's what's making things difficult. It seems that in australia AC adaptors are rarer than DC. Thanks for the lesson though, I didn't quite understand how it all worked but it's much clearer now.
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kierent
Posted: November 29, 2010 08:35 am
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Actually upon reading the listing of the one I thought I'd found more carefully, it doesn't actually say that output is AC. Anyone know where to get an adaptor that has AC output not DC. Or does this not really matter either?
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MacFromOK
Posted: November 29, 2010 08:41 am
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There is a bit more to this...

Unregulated power supplies (as are most "wall wart" adapters) have their voltage rated at a particular load level. When the load is less, voltage goes up.

So... if you put a 12V 200mA load on an unregulated 12V 500mA adapter, it's voltage output will be more than 12V. And while it will probably work ok, you should be aware of the possibility that it won't.
[EDIT] Never mind, I see Matt already mentioned this. doh.gif



[EDIT] Just noticed your last post...

Your guitar pedal works on DC, so it must have an internal rectifier (turning the AC to DC). A DC adapter should work, but will drop a couple of volts (1.4-ish) by being rectified twice.


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kierent
Posted: November 29, 2010 10:18 am
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QUOTE (MacFromOK @ November 29, 2010 06:41 pm)

Your guitar pedal works on DC, so it must have an internal rectifier (turning the AC to DC). A DC adapter should work, but will drop a couple of volts (1.4-ish) by being rectified twice.

Just wondering how you know my guitar pedal works on DC? Is that from something I wrote or do all things work on DC?

I confess I'm a total novice when it comes to this stuff. I have a multi-voltage adaptor at home and it's 12v setting runs at 700ma but it's DC and when I connect it up to the pedal it either does nothing or if I switch the polarity it powers up but there is a 'clip' light that also glows red which isn't normal. Others on an acoustic guitar forum have had this problem and it is a result of using the wrong transformer and it causes all sorts of hums and buzzes when you plug it in.
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CWB
Posted: November 29, 2010 01:21 pm
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pretty much all electronic devices of the type you are using actually "run" on DC . the active circuitry requires DC to function .

the power supply has the job of converting the voltage from an AC source to DC .
there are a couple of ways this is approached in the design of equipment :
the "power supply" is external to the unit .it is a self contained unit and provides the correct DC voltage(s) to run the equipment .
part of the power supply is located externally (ie ; transformer) while the rectifiers and filter capacitors are located internally . in this case the job of the transformer is to supply AC to the unit for conversion .

why is part of the power supply located externally ?
think of how much bigger that effects pedal would have to be if the transformer were located internally .
it is also a matter of design choice ... i have seen pedals with the complete power supply built into the unit (just like a lot of other equipment) .

the reason for making certain that the input to the effects pedal is AC or DC is that interchanging the two can result in a dead unit .


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AwesomeMatt
Posted: November 29, 2010 04:12 pm
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QUOTE
the reason for making certain that the input to the effects pedal is AC or DC is that interchanging the two can result in a dead unit .


As can hooking it up with reverse polarity DC, (like he already did), no? Or, hrm, I guess if it has an internal rectifier it would take care of that.

We're going a bit over his head though. Should at least link what we're talking about with rectifiers, transformers, caps, etc...

http://www.kpsec.freeuk.com/powersup.htm <-- Very simple tutorial. You don't even need to read it, just watch the pictures. It progresses from the AC that comes out of your wall, adding one component at a time so you can see how you get down to the DC that you use.

What people here are saying, is that the 4-piece power supply for your pedal is likely broken up into two sections. One section is the thing that plugs into your wall, the other section is inside the pedal. So, the power supply being 12V AC, likely means it's just a transformer and nothing else, and the pedal has the rectifier, capacitor, and regulator (if any).

There's not really any reason the plug couldn't have all 4 components in it.. and that's why there's so few AC outputting power adaptors. Everything needs DC anyway, might as well mount it all in the same place.

There is a small chance that part of the foot pedal circuit actually requires some AC.. I don't know what the pedal does or what's inside it. So maybe there's a reason for that.

In any case.. if what you require is a 12V AC power plug, and all you can find are 12V DC power plugs... if you feel like being experimental.. chop one of the DC ones apart. They're usually plastic welded, so use a wood chisel or something around the corners, a few hard whacks and suddenly it'll pop open.

Inside you'll find 4 components. The transformer is easy to identify, it's the big heavy boxy thing made out of steel. Power comes in from the wall prongs into it.. then out to a small circuit board, then out to the output cord. If you have a soldering iron, simply desolder the output cord from where it's attached.. and re-solder it to the 2 points that come out of the transformer (at the circuit board you'll have lots of room, don't try to touch the actual wires coming out of the transformer, they're fragile).

Ta da, then you have an AC adapter that has bypassed the extra crap that a DC adapter adds on.

It's really very simple. 1 minute of work, moving 2 wires. If all of this is information overload, we've been over-explaining. Just crack open the 12V power supply and take pictures of each side and post them here. We'll tell you what to move where.

It'll take you longer to take pictures than to change the wiring.
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tekwiz
Posted: November 29, 2010 09:26 pm
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QUOTE
As can hooking it up with reverse polarity DC, (like he already did), no? Or, hrm, I guess if it has an internal rectifier it would take care of that.


This depends on the type of rectifier used in the pedal. Results can range from nothing at all to fried parts inside the pedal. The same goes for an AC/DC mixup.

Adapters must match in voltage, polarity, & power type...AC or DC. Current rating of the adapter must be the same or greater than what the pedal requires.
Polarity of the pedal's requirements should be marked on it somewhere.

Kierent, something to remember is that amps are drawn by the load, not pushed by the power source. Therefore a load will only draw what it needs, as long as the voltage is correct.


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