Electronics > Beginners

Resistance of resistor leads ?

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lordvader88:
I use the trimmed leads of 1/8W and 1/4W resistors, as breadboard wire. So how much current could I really pass before it would matter over a few centimeters ?

I'm just wondering, since I just soldered on some big heavy wire wire, to handle 1A under 30V, just to go a few centimeters, I surely could just use resistor leads still, on the perf.board

mariush:
Diameter is probably ~0.45mm .... equivalent to ~ AWG 25.
A copper wire with AWG 25 gauge has ~ 106 mOhm per meter of wire, but resistors use tin plated steel or other materials with higher resistance... so let's go with 150 mOhm per meter or 1.5 mOhm per cm.

Let's go with 4cm of wire because it makes math easier ... that's 6 mOhm of resistance ... or 0.006 ohm

V = I x R , P = I^2xR

At 20A , you're dropping 20 x 0.006 =  0.12v so you shouldn't use such a lead to power something with 3.3v at up to 20A (for example a bunch of lithium batteries in parallel)

P = 20x20x0.006 = 2.4w dissipated in 4 cm of wire ... congrats you got yourself a small lightbulb

MosherIV:
I once asked a similar question once about current rating for wire.
The answer is........

Wire will pass any current you like BUT how hot it gets is dependant on the wire parameters.

To answer your question, yes at 0.5A upwards you are right to question whether a resistor lead can handle the current safely.
Compare the resistor lead to similar cable/wire ratings and you will have an approximate answer.

I think 1A is roughly the current limit/capability of perf/strip board.
You can increase it by soldering over the track or by adding parallel lead. Dave did a video about pcb track current capability.

tooki:
Current carrying capacity (ampacity) depends on the thickness of the leads*, which varies wildly. Cheap chinese 1/8W resistors have super thin leads (which might not even be copper), while a name-brand 2W resistor will have thick copper leads.

If you really want to know, first use a magnet to make sure they're not steel, and then use calipers to measure the diameter. You can then use any of the many online wire gauge tables (e.g. on wikipedia) to determine what gauge it is. (Or use your middle school geometry to calculate the cross section in mm2, if you prefer metric wire sizes.) Then once you've determined this, you can refer to an ampacity table to determine its current carrying ability.

But for 1A, you only need around 29ga wire, which is hair thin. Any name-brand component lead is going to be thick enough to carry several amps, but even the cheap Chinese ones should be thick enough for an amp or two. (The voltage is irrelevant to wire gauge, but defines how good the insulation must be. But 30V is nothing at all. It matters once you get to mains voltages and beyond. High voltage stuff, like in the kV, is where insulation really starts to become critical.)


* and on cooling. Bare wire hanging in free air can cool better than insulated wire in a bundle of cables, for example. The better the cooling, the more current a given wire can carry. A PCB can act as a heat sink to an extent, so for example if you solder a wire to a bunch of unused perfboard pads, they'll both increase heat sinking, as well as increasing the effective thickness of the wire, reducing resistance. But for 1A that's not even needed.

HighVoltage:
It also largely depends if you send DC or pulsed current through the wire?

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