wire gauge confusion

[Chuck H]

I've been trying to determine the right gauge of wire for some inductors. So many charts have different or incomplete info. How, for example, can a chart state the amount of current a specific gauge can carry without indicating the length or voltage?

From the info on one chart I determined that for the amount of wire in a 12mH inductor, to use it in a 100W attenuator circuit would require a gauge of #2. Now, obviously this inductor would be the size of the front wheel on a Chevy. meanwhile, Parts Express is selling 18 gauge inductors rated for 300 watts. And in a previous post when I mentioned I wanted to use 18 gauge wire for my inductors Steve Connor asked 'why do you need to use such a large wire'. I guess I'm not getting it.

I'd love to use smaller wire if I can get away with it. Considering the low ohmage of a speaker attenuator I would have to take the wires DCR into account. Thats not a problem. But how do I know what size wire I can get away with current wise so I can reduce cost and decrease the size of this 12mH unit.

Guidance?

Thanks

Chuck

[Enzo]

Because voltage is irrelevant. And length of wires is too. Current through a series circuit is the same throughout, so if 10 amps flows through an inch of wire, it is still 10 amps if the wire is 10 feet long.

Current rating is the amount of electrons that can flow through a cross sectional area of wire.

Like a fuse, a wire will heat up and even burn open if its current rating is exceeded.


Your attenuator is not a lab instrument. If it is to be used at 8 ohms, does it really matter if the inductor is .4 ohms versus .8ohms?

[Chuck H]

Because voltage is irrelevant.

Ok... I just figured that since every other thing 'electronics' has a "watt" rating that wire might too. It does have resistance.

And length of wires is too. Current through a series circuit is the same throughout, so if 10 amps flows through an inch of wire, it is still 10 amps if the wire is 10 feet long.

Same answer. I know, for example, that those big adjustable resistors are only good for 10% of their rating when set to 10% of their value. I guess I'm just over thinking this.

But in my defense one source of web info did give wire length limits for a given current.

Your attenuator is not a lab instrument. If it is to be used at 8 ohms, does it really matter if the inductor is .4 ohms versus .8ohms?

No, not alot. But I was surprised at how much difference small 'fudgings' can make. I had to source some resistor values to keep things where I wanted. This attenuator on it's 8 ohm setting swings between about 6.5 ohms and 9.5 ohms. Thats pretty damn good when you consider some of the other attenuators swing +/- 100%+. If I try to get away with using a 10R instead of an 8R, for example, the up swing shifts to about 11 ohms. Gotta draw the line somewhere I suppose.

Thanks

Chuck

[tedmich]

for air coil inductors (and the odd hexaphonic pickups I dabble in) I find this online calculator very useful:
Pronine Electronics Design - Multilayer Air Core Inductor Calculator


Wow they have a TON of useful calculators! wadda ya know!
http://www.pronine.ca/links.htm

[Enzo]

But in my defense one source of web info did give wire length limits for a given current.

OK, in what context? For example speaker wires. You don;t want to send power down a long wire to an 8 ohm speaker and have 8 ohms of wire resistance. it has little to do with the wire handling the current, it is about losses.

Sure things have a watt rating, but in wire, the unit resistance/length is pretty standard, and the current will be the thing that varies. hence the current rating on a wire size instead of wattage. If i was using hte wire itself as a heater, then wattage might be my concern. But in a speaker, it is the amount of current I can deliver to the drivers that matters. One could calculate the wire's limits in terms of watts - how much current would have to flow through the wire's resistance to melt it could be expressed in watts.

those big adjustable resistors are only good for 10% of their rating when set to 10% of their value.

Sure, but you no longer have the whole resistor dissipating power, just the little bit you are using. 10 watt or 50 watt or whatever power size, all will be about the same size at any one power rating. If you have a 50 watt wire wound resistor that is 100mm long (10cm) the entire length of it is dissipating the heat of operation. If you tap it at 10% of its value, you also are using only 10% of its winding. In other words you can;t expect a 10mm resistor to dissipate as well as a 100mm one.

Those resistors are rated for power dissipation, you won;t generally see a current rating. If you wanted one, plug the power rating and resistance into Ohm's Law, and voila.

BY all means try to get as close to your design goals as possible. Replacing a 8 ohm with a 10 is a 25% increase, while a .4 ohm difference is 5%.

[dai h.]

if the 12mH is for the low freq. tank cct. thing part of the load, then I don't think it has to handle max. current (at max. output), and if you go bigger with the mH, it hinders current flow so it doesn't need to handle as much. The 12.5mH in the Marshall Power Brake and SE100 also doesn't have wire that thick if you look at it(also, it's a cored inductor so not as many winds are necessary compared to an air core).

also as I understand something that effects how much current the wire can handle is the ambient conditions, specifically, if it's say inside a transformer, and it gets hot, then it's worse for the wire and makes it more likely to melt from heat(and in the open where it can cool off it's better).

might try this page where you can simulate the load and check out the current flow through various parts of the inductive load to get a better idea:

Combinations of Resistors, Inductors and Capacitors

(couple of example screenshots attached)