Electronics > Beginners
Small gauge (35AWG) aluminum wire capacity
GregDunn:
I'm having trouble finding AWG charts with current ratings for extremely small aluminum wires. Background: My speakers are unconventional (Magneplanar Tympani 1-C), made with straight runs of very small aluminum wire of such a length as to provide 8Ω total resistance, bonded to Mylar sheets. They are sufficiently old that even the manufacturer does not maintain detailed specifications for them any longer; I have been in touch with them of course. I'm trying to determine whether I'm providing sufficient protective fusing for them, but it's been rather difficult to locate a wire capacity table for AWG35, aluminum wire.
The manual I have for them only gives a vague recommendation for amplifier power and fusing. I have been using a 1A FB fuse which seems in line with their recommendations but it would be helpful to know just what the current capacity of the wire is, so I can make the best possible choice for protection. They are somewhat inefficient, and do require a significant amount of power to perform properly.
Does anyone have a wire chart that might have the info I need?
bob91343:
I presume one could calculate this. First, look at the melting temperature of aluminum compared to copper. Then look at the resistance of the wire per unit length. With copper you can find the value from tables, I assume, and thus see how close the melting temperature is approached at maximum current. Do the same for aluminum and allow a comparable temperature gap.
It's a bit tedious but if you allow a substantial safety factor you are probably okay.
Another approach might be to measure the temperature of the wire as you put various currents through, assuming you have an expendable piece of it. You can do that either with a thermometer of sorts or by measuring the resistance change with current.
Most pure metals have a positive temperature coefficient of resistance of about 0.4% per degree C. So if the resistance changes 20% with current flow, you can figure 50 degrees change. If that is well below melting point you are safe. You can find the correct coefficient from a materials handbook.
Just put calibrated current through the wire and compare voltage drop at various currents.
I also suspect there are tables of safe current for aluminum wire, although probably not for something of such light gauge. Yet you can see the comparison to copper tables and make an educated guess.
floobydust:
For copper wire it's 4A 1 sec, or 27A 32msec, from 1928 Onderdonk's equations. You have to know the fuse's clearing time.
You can put in aluminium's properties for melting point, density, conductance into Onderdonk's equation to get the fusing current. It's a bit of work.
GregDunn:
I just realized, it turns out not to be the wire which is the limiting case, but the Mylar it's bonded to which melts at a lower temperature. So I suppose I can assume linearity for copper at that temperature and find out what the limiting current is without the complex calculations.
Now, to track down the temperature coefficient for aluminum...
floobydust:
The model 1-D fusing is weird, 2-1/2A for the tweeter? Nothing for the mid/bass section?
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