Hi! Im just wondering why a small Mosfet legs can handle 100A+ but wires needs to be very big to handle the same current?
Current capacity is not a simple number. It considers current, resistance (which depends on width, height, and length of the conductor), and acceptable heat rise (which takes into consideration thermal mass, and thus time.)
So, can that transistor conduct 100A indefinitely? Or only for 1s? Can it do so with 0.2"-long legs, or would it be able to sustain that load if its legs were six feet long (like a power cable)?
Tamulmol:
Hi! Im just wondering why a small Mosfet legs can handle 100A+ but wires needs to be very big to handle the same current?
Wires ard made up a finer twisted copper wire, the legs on a transistor are solid so there's more area for the electrons flow but even so a transistor can get very hot without heatsinking ....
SirNickity:
Current capacity is not a simple number. It considers current, resistance (which depends on width, height, and length of the conductor), and acceptable heat rise (which takes into consideration thermal mass, and thus time.)So, can that transistor conduct 100A indefinitely? Or only for 1s? Can it do so with 0.2"-long legs, or would it be able to sustain that load if its legs were six feet long (like a power cable)?
Thanks for the reply! Even with the same lenght if the wire size is just the same as the legs of transistor the wire wont handle it.
cjdelphi:
Tamulmol:
Hi! Im just wondering why a small Mosfet legs can handle 100A+ but wires needs to be very big to handle the same current?Wires ard made up a finer twisted copper wire, the legs on a transistor are solid so there's more area for the electrons flow but even so a transistor can get very hot without heatsinking ....
Thanks! I think I understand now. It's just so confusing why a very small metal can handle even 200A+ but wires should be very HUGE.
Tamulmol:
Thanks for the reply! Even with the same lenght if the wire size is just the same as the legs of transistor the wire wont handle it.
Where do you get that idea? What transistor are you looking at, and what wire gauge calculator tells you the same diameter and length is insufficient?
SirNickity:
Tamulmol:
Thanks for the reply! Even with the same lenght if the wire size is just the same as the legs of transistor the wire wont handle it.Where do you get that idea? What transistor are you looking at, and what wire gauge calculator tells you the same diameter and length is insufficient?
Im not really sure lol,im newb at this. for Example a 200A mosfet, the legs is just like a 20AWG wire. Can 20AWG wire handle 200A?
1" of 20AWG copper wire conducting 10v @ 100A will drop 0.2v. However 0.2v at 100A is 20W that it will need to dissipate. I doubt it will last long.
I would be skeptical of a transistor that had equivalent to 20AWG leads and claimed 100A capacity.
SirNickity:
1" of 20AWG copper wire conducting 10v @ 100A will drop 0.2v. However 0.2v at 100A is 20W that it will need to dissipate. I doubt it will last long.I would be skeptical of a transistor that had equivalent to 20AWG leads and claimed 100A capacity.
Hmmm sorry for this another noob question. If the wire durability(dont know what to call) is based on the wattage it can dissipate, then the Wire current rating isn't very accurate? It should be rated in watts? I mean ex. a wire may handlel 10v 100A but not 200V 100A even if the wire is rated 100A?? Thanks and sorry for the stupid question LOL
You are confusing the ability of the wire to dissipate heat due to its internal resistance, and the ability of the plastic covering the wire to resist isolation breakdown at high voltages.
The heat generation and dissipation depends on the current. The insulation breakdown depends on the voltage.
A wire "carrying 100 A at 100 V" is generating the same heat internally as a wire "carrying 100 A at 200 V" or "300 V" or "400 V". The heating depends only the current.
With MOSFET ratings its not unusual for the datasheet to give the current rating
for just the silicon die on an infinite heatsink.
Often you see something like
Max current: 170A, (package limit 60A)
This means that 60A will fuse the gold bonding wires between lead-frame and die, even
if the die could theoretically take 170A pulses. The 170A figure is just marketing nonsense.
If you buy a 100A MOSFET with the intention of putting more than about 30A
through it then you'll likely be dissappointed as it gets extremely hot - the max
current figure is not practical without water cooling, its basically the maximum
power dissipation of the package with infinite heatsink and no insulating thermal
pad (not realistic situation).
You should be choosing a MOSFET by its on-resistance and power dissipation
calculations for the heatsinking you are actually going to use. You also need to
check that Vds isn't so large at your load current that the device starts to switch
off (the gate must be higher in voltage than both the source and the drain when on).
So always be sceptical of large current ratings - are they pulse ratings only?
Are they just theoretical? Do they assume liquid nitrogen is sprayed continuously
on the device? Remember power depends on current squared, so
100A is a hundred times more difficult to handle than 10A for the same wire
or device.
Simple: Metal conducts heat as well as electrons.
On MOSFET legs the heat has a place to go (up into the MOSFET and out through the heatsink).
The heat has no place to go at the center of a long piece of wire.
Tamulmol:
If the wire durability(dont know what to call) is based on the wattage it can dissipate, then the Wire current rating isn't very accurate? It should be rated in watts? I mean ex. a wire may handlel 10v 100A but not 200V 100A even if the wire is rated 100A??
No, the 10v figure I gave above isn't particularly relevant. I used that in a calculation because you have to have voltage to have a voltage drop. 
Wire has a resistance, quoted for a particular unit of length. (E.g., ohms per foot, or per meter, etc.) This value depends on the diameter of the wire and directly affects its current capacity. You can use a thinner conductor at lower lengths for a given acceptable voltage drop, and conversely you will need a larger conductor for longer lengths, assuming current is the same in both cases.
As the resistance goes up (smaller diameter and/or longer length), the voltage drop across the conductor's length increases. This drop, along with the current flowing through it, results in a figure of wasted energy as heat, expressed in watts. Wire isn't likely rated in watts or current -- you get the gauge, and maximum voltage the insulation is rated for, and information you need to calculate its capacity based on your application. (Except when the wire is a cable assembly, like a power cord with terminated ends. Then, its maximum current capacity is known because so is its length. Hence the warnings you'll see about using extension cords with heavy loads like power tools or appliances.)
Generally though, it's pretty obvious that a 1" piece of 20AWG wire will not be able to dissipate 20W in a typical use-case. As Mark said, if you want to super-cool it, you might be able to pull it off. 