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Old 16th April 2004, 04:32 PM   #1
amt is online now amt  United States
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Default Reducing amperage of a DC powersupply

I need to use a wallwort that has 1.75 amps at 12DCV but can only locate a 2.5amp version. I can use the higher voltage later but for now I need 1.75amps. This is a temporary setup to be used to heat up wiring. Do I put a power resistor in line and if so, how do I calculate it and measure it? Thanks

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Old 16th April 2004, 04:55 PM   #2
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What kind of wire are you trying to heat up and why?

Sorry I haven't been back to you about that beer but I've been rather constrained.
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Old 16th April 2004, 05:21 PM   #3
amt is online now amt  United States
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Dont worry about it- my lifes not my own either.

Im installing new tweeter wires on my old Magnepans. The old method was to use 3M 77 to attach the wire and then apply the waterbased adhesive to the wires/mylar. The new and improved methom is to apply voltage to the wires, making them magnetic and thus attracting them to the magnets and keeping them firmly against the mylar. No 3M = less mass = mo betta sound. I need 1.75 amps for the tweeter wires. Any more and the heat generated may melt the mylar.

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Old 16th April 2004, 06:08 PM   #4
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Is this method part of a set of specific instruction supplied to you by someone or have you made some assumptions and decisions yourself?

That aside, I'll check my wart stash for a 1.75a unit.
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Old 16th April 2004, 06:14 PM   #5
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If you're feeling suicidal (which does seem to be the case), why not give your Magnaplaners to a friend, and seek out a high building?

Seriously though, a wallwart PS is unlikely to give you 12v at 1.75A (or 2.5A, for that matter) no matter what it says on the box. They are just not that precise.

If you must try something like this, then it will be essential to use a supply with variable current limiting, applying power to the wires, slowly increasing the current level, and relying on observation of the mylar/wire junction to decide when to stop.

My honest opinion is that this is a procedure you just shouldn't attempt, unless the speakers are already u/s and this is a last ditch attempt to save them.

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Old 16th April 2004, 06:26 PM   #6
amt is online now amt  United States
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This method is apparantly being used by Magnepan. Ive gotten all the information on repair from the Magnepan Users Group (MUG). Check the repairs threads under Misc at the MUG site.

So whats a semi - disposable (read cheap) PS that can be used ?



http://www.integracoustics.com/MUG/M...aks/index.html


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Old 16th April 2004, 07:17 PM   #7
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Not knowing much about Magnepan speakers, I offer you Ohm's law:

If the tweeter has 8 ohms of resistance, 12 volts will give you 1.5 amps of current, regardless of the maximum amp rating of the wall wart.


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Old 16th April 2004, 07:26 PM   #8
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Quote:
Originally posted by runebivrin
If the tweeter has 8 ohms of resistance, 12 volts will give you 1.5 amps of current, regardless of the maximum amp rating of the wall wart.
That's true but at this point we don't know the resistance of the length of wire. Now if he were to measure it . . .
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Old 16th April 2004, 07:32 PM   #9
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That's true. Either way, it's not a particularly good idea to just connect a regular wall wart and let that run into current limiting. They usually don't contain much in the way of protection against overload...

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Old 16th April 2004, 09:05 PM   #10
amt is online now amt  United States
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The wires are about 25ft long. I havent measured them since I dont have the speakers disassembled yet. The speakers are rated @ 5ohms if that helps.

So what are the options for a power source? The author of the article Im following purchased a supply from RShack but the model number isnt given.

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