If I have a coil with the following characteristics:

Winding: 3750 Turns +/-10% OF 40 AWG double build polyurethane wire with min. of 130 ºC temperature rating.
Resistance: 290 +/-10% ohms at 25 C.

How do I calculate the max current it can carry without breaking the wire? And how long can it carry that current?

 

Being that it's an inductor, the answer would depend on the frequency it is operated at.

 

From the information you provided, you cannot. You need yo know how much heat it can dissipate to the environment, which depends many unstated factors.

Bob

 

Being that it's an inductor, the answer would depend on the frequency it is operated at.

Let's say it's operated at DC with a ripple of 360 Hz.

 

You're asking how much current will flow through a resistor of - say - 100 ohms. You need to give a whole lot more information in order to get an answer. In my example of the 100 ohm resistor, 10 milli amps can flow through it IF you use one volt. However, if you use 100 volts, then you're going to get 1 amp. One million volts will give you 10,000 amps. And we've not even covered wattages yet.

Your question can only be answered by looking at the data sheet for that device. Other critical things that can apply to your coil is the material it's made of and the diameter. Temperature comes into play as well.

 

What are the physical characteristics (size, shape) of he coil?

 

If the outside of the coil is open to the air then the outer turns will be cooler than the inner ones. So the geometry of the coil is important and makes determination of the maximum current capability something of a guessing game.

 

Here is a picture of the coil

 

Fusing current for #40 AWG copper wire is 1.77 amps. I would try to not even get close to that current.

 

@ 24 v about 2 W so stay under 1 hour , @ 12 V -continuous.

 

Kind of looks like an old doorbell striker, the kind with the solenoid, two musical note bars with two resonating chambers. Ones I've seen operate on 8, 16 or 24 VAC. And at that - momentarily.

Let's assume it has 290Ω impedance. For the sake of making me NOT be wrong, let's assume it's a resistive load measured in ohms. At 8 volts, 290Ω, the current draw would be (8 ÷ 290 =) 28 mA. At 16 volts; 55 mA. At 24 volts; 83 mA.

Now, exactly what that solenoid is used for and at what voltage, all that makes a difference. Without knowing how that was used, or how you want to use it - the answer will vary.

 

My doorbell runs on 240 VAC -there is a lot of wire in that thing. Note that doorbells are not intended to be energized continuously.

 

@DickCappels Interesting. Every doorbell I've ever messed with in the US has been 120 VAC through a transformer with the afore mentioned secondary voltages. I have such a doorbell transformer down stairs still in the package. Bought it from a hardware store going out of business. I figured "Never know when I might need one of those voltages." 8 VAC RMS rectified and filtered would be 11.3 VDC. 16 VAC RMS becomes 22.6 VDC, and 24 VAC RMS becomes 17 VDC (16.97 VDC). So for around $3.00 I grabbed it. It was their last one too. Packaging specifically said it was a doorbell transformer universal replacement.

Wondering what the TS wants to do with this coil and how he knows it's 3750 turns or any of the other characteristics he's listed. That many turns - that small a gauge wire - GOT to be low voltage.

 

I made a 4,000 turn coil just for the heck of it by chucking the core in a drill press and knowing the RPM of the chuck I just would for a certain number of minutes. Never found a use for the coil. 30 Years later I still have it.

 

Also, keep in mind the temperature of the wire will be compounded by the turns and the surrounding wires.

The temperature is directly related to the maximum power dissipated on the wire, which is in turn directly related to the maximum current passing on the wire.

As @DickCappels mentioned, the 1.77A for a #40 wire will take it to an extreme temperature where the metal is fused. Imagine now many turns of wire reaching such temperature.

 

I made a 4,000 turn coil just for the heck of it by chucking the core in a drill press and knowing the RPM of the chuck I just would for a certain number of minutes. Never found a use for the coil. 30 Years later I still have it.

I re-wound many transformers when I was a kid, and I didn't have mechanical assistance... Only gobs of spare time counting turns one-by-one. Transformers used to be quite expensive and finding one with the exact characteristics was quite difficult.

 

In high school electronics class we made crystal radios. I don't remember the turns ratio but I think one was 100 turns. I measured the thickness of the #26 magnet wire and insulation and calculated how far winding 100 turns would transit on a cardboard tube (exhausted toilet paper roll). Marked my starting point and measured to my ending point. Then set the cardboard tube on a dowel and taped it on. Then put a screw into the dowel, and cut the head off. Then chucked it up and spun it until I transited the distance. Had to be close to 100 turns; probably give or take a single turn. Never had the opportunity (or need) to wrap a multi-layer coil. Knowing physical dimensions of the wire in use, number of turns could be calculated to width and then how many layers. The final layer might be 1/2 the distance across the bobbin. But like I said, never had reason to do so.

 

I did this as well; 100 turns of AWG 26 on top of ferrite for MW and most SW bands.
FM was much less turns but harder to get it right: 2~3 turns on open air (about 5mm diameter) but you needed to manually adjust the gap between turns and then lit a candle and wax the heck out of it, so it wouldn't move.

 

X + 7 = Q

Without knowing what X is or what Q is - the equation can not be answered. Except in that X can be anything; meaning Q, likewise can be anything (minus 7).

If we don't have enough information then it's not possible to answer your question. and from the photo, you're talking about a coil that I certainly did not imagine we were talking about. I was thinking of something on a toroid or in a transformer.

[edit] come to think of it - 3700 windings on a toroid would be pretty hard to make.

 

The question, I may have missed in all this is WHY does he need to know the answer anyway?

With no stated purpose, you can give answers all day long, but knowing the reason behind the question is certainly part of the "equation".

And as somebody stated, more or less, what the part can withstand vs what you want to do with it may be miles apart.