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If I connect a USB power supply, rated for 2.4 A max. at 5 V, to a 1 ft 21 gauge nichrome hot wire (resistance 0.79 ohms/ft), what would happen?

Would the wire still heat up, but not much, or will my power supply fail?

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2 Answers 2

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Current is voltage divided by resistance, so if 1 foot of wire has a resistance of 0.79\$\Omega\$ then the current at 5 V will be $$\frac{5 V}{0.79\Omega} = 6.33 A $$ Your supply will not handle this so either the fuse will blow, current limiting will kick in (if the supply has this feature), or the supply will overheat and probably fail.

To keep the current within the limits of the supply the resistance would need to be more than $$\frac{5V}{2.4A} = 2.08\Omega $$

Which would be $$\frac{2.08\Omega}{0.79\Omega/ft} = 2.64 ft $$

Mind you, this is the maximum current, so you'd want to leave some breathing room, maybe run it at 80 % which would be around 3.3 ft.

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  • \$\begingroup\$ This is really helpful! Thanks so if I understand it correctly. I can add more resistance without damaging the supply so I should aim to get a higher resistance. My question is so then if the resistance increases won't the heat output also be higher? Or is my power supply the limiting factor in how much it heats up? \$\endgroup\$ Commented Dec 6, 2023 at 14:10
  • \$\begingroup\$ @VisuaLHippocracY Increasing resistance will only increase power (=heat output) if you're using a constant-current supply. You are using a constant-voltage supply, so increasing resistance will decrease power. \$\endgroup\$
    – Hearth
    Commented Dec 6, 2023 at 15:09
  • \$\begingroup\$ @VisuaLHippocracY The heat output will be power which is the voltage squared divided by resistance, so for 5V it's 25V/R, so for 2.08 ohms it's 12 watts. Higher resistance will be lower wattage. All of the relationships between voltage, current, resistance and power come from Ohm's Law, look that up and learn the formulas, it comes in handy. \$\endgroup\$
    – GodJihyo
    Commented Dec 6, 2023 at 20:19
  • \$\begingroup\$ Ahhh, so increasing resistance will reduce power which will lead to lower heat generation. Got it. I knew about ohms law but wasn't understanding how it would work. In my head I was supplying a constant 12 watt power so increasing resistance would just cause all the power to convert to heat. So in my head if I increased resistance the heat generated would keep increasing, but that's not how it works \$\endgroup\$ Commented Dec 7, 2023 at 7:43
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The wire would want to draw about 6.3 Amp at 5 volts, which is much more than your power supply can supply.

Edit due to miscalculation

  1. A simple supply will probably be damaged when asked to provide almost three times its rated current.

  2. The supply might reduce its voltage to keep the current under 2.4 A. (unlikely for that much overload)

  3. If the supply has overcurrent protection, it might latch its output to near zero volts and need to be turned off to reset.

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  • \$\begingroup\$ thanks! so If I want to use a nichrome wire for any heating purpose it would have to be of a higher resistance then? \$\endgroup\$ Commented Dec 5, 2023 at 19:52

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