| amt |
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
amt |
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| Bill Fitzpatrick |
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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| amt |
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.
amt |
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| Bill Fitzpatrick |
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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| JTT |
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.
JohnT |
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| amt |
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/...eaks/index.html
amt |
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| runebivrin |
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.
Rune |
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| Bill Fitzpatrick |
| 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.
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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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| runebivrin |
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...
Rune |
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| amt |
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.
amt |
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| Bill Fitzpatrick |
I can't find my stash of warts so I must have taken them to the dump.
If you get me the resistance of the wire and buy the available wall wart I can come up with a resistor value and wattage to put in series with it. |
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| amt |
Bill,
Sounds good to me. I just ordered the tweeter kit yesterday and they dont ship until they get temps high enough not to jell the waterbase adhesive (MN). Hopefully I will see them next week. I will probably buy a regulated supply with a 3amp output.
I'll hook up when I got all the parts. Thanks.
amt |
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| amt |
Well the tweeter kit arrived so Im really to start. I was rooting through the garage and spotted my battery charger and had an idea. Will it supply the current I need. Its a standard automotive 12/6 volt version with a meter that reads to 4 amps. Can a resistor be used in line with this to produce a stable 1.75 amps or is this not the proper device?
amt |
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| macboy |
| quote: | | Its a standard automotive 12/6 volt version with a meter that reads to 4 amps. Can a resistor be used in line with this to produce a stable 1.75 amps or is this not the proper device? |
I'd say that's an excellent idea. If you consider the resistance of the wires to be low enough to ignore, then the resistor that you need is 12/1.75 = 6.9 ohms. However, the rating of 12 V is nominal and it is more likely to be around 14 to 15 volts under a load of 1.75 A. So just to be on the safe side, 15/1.75 = 8.6 ohms. Try a 10 ohm in parallel to a 47 ohm. Use a multimeter to measure the current. Also make sure that the resisitors are of the proper wattage: V^2/R. So for the 10 ohm, at least 20 watts, and for the 47 ohm, at least 5 watts. |
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| amt |
Macboy,
Thanks for the info. Im headed out to get some resistors.
amt |
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| sam9 |
"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."
The theory is right but Magnepans are typically rated 4-ohms. I've seen a impedance plots posted by MUG and over most of the range it was actually 5-ohms.
However, since individual Magnepans drivers are almost resistive rather than reactive in nature (per info gleened from the MUG), and since this is an older model, it seems to me that simply measuring resistance with a DMM would get the a sufficiently accurate value. If you have a DMM, you can also read the current to be sure it stays below the max. In fact if the wires stay in place with a current lower than the max, that might be a safer proceedure. If all else fails, one can call Magnepan. I've found them very helpful and friendly -- once they answer the phone, which is another matter. |
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| Bill Fitzpatrick |
amt:
Grrrr. Haven't been able to get online till just now.
Turns out the supply I have won't put out the current you need.
22 feet of 31 ga copper wire is about 2.8 ohms. Alum, depending on the method of mfg and alloy will be about 5.3 ohms. Assuming
the battery charger puts out 14v with a 1.75 amp draw, the resistor
you need in series with your wire will be 2.7 ohms with a rating
of 10 watts. Personally, I'd go for 2 5.6 ohm, 10w in parallel.
These are standard values and should be available at
Norvac or The Shack. Be sure to use your multimeter to check the
current before placing the wire on the membrane.
Bill |
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