# Why is the voltage across my nichrome wire so low?

I have a 37 AWG nichrome wire (0.6 mm diameter) with 5 ohm (about 3 m wire) resistance, 12 V DC supply with max 1 A output.

I only measured 1-3 V across the nichrome wire, the wire does warm up but the V across it is too low, the multimeter didn't read any current through the wire for some reason. I tried it with a short nichrome wire and the voltage went 0 V probably because the resistance was too low.

I want the wire to reach 100°C. The datasheet (RD 100 / 0,6) says that needs 2.21 A, according to Ohm's law that's 24 W and with 3 V across it that needs 8A (I = P/V).

How I can increase the V across wire so that less current would be required? Why is the voltage so much less than the supply? I obviously need a supply with higher current rating but with an actual 12 V across wire I would only need 2 A and supplies with lower ampere limit tend to be cheaper.

• You need 2+ amps and your supply can only give 1 amp max. Do you think this could have anything to do with the problem? Feb 1, 2021 at 23:26
• @Sredni Vashtar It sure does, but if 1A was going through the wire then the Voltage across wire should be 5V, I don't understand why it's mostly hanging around 1.5V instead. Feb 1, 2021 at 23:33
• That depends on how the current limiting is set in your supply. If it's a lab power supply ou can usually set the current limit beforehand and you might have forgotten about it. If it's a fixed supply it might try to off itself to prevent burning up. Feb 1, 2021 at 23:37
• The basic problem, you need "24W". Your power supply can only deliver 12* 1 = 12W. Also don't confuse "temperature" with "power" or "heat". You didn't say what you're trying to do. But if your plan is to use that "100 degree wire" to heat something else to 100 degrees, it isn't going to work. 24W gets the wire up to temp when it's hanging free and not touching anything. You'll need ALOT MORE WATTS if you wanna actually use it to heat something else. Feb 2, 2021 at 0:55

I want the wire to reach 100°C and datasheet (RD 100 / 0,6) says that needs 2.21 A, ...

Correct. And since the resistance is 1.73 Ω/m you need 3.8 V/m of wire.

... according to ohm law that's 24 W ...

That's Joule's law, P = VI.

... and with 3 V across it that needs 8A (I = P/V).

No, R is fixed so you can't just set the voltage to any value like that. At 3 V you get 1/4 the voltage you get at 12 V and that gives 1/4 the current resulting in 1/16 the power.

Now my question is how I can increase the V across wire so that less current would be required?

The way to do that is to draw out the wire so that it has 1/4 the cross-sectional area (1/2 the diameter). The resistance will then be 6.9 Ω/m.

Why the V is so much less than the supply?

I obviously need a supply with higher amp but with an actual 12 V across wire I would only need 2 A and supplies with lower amp limit tend to be cheaper.

There cheaper because they are not as powerful, of course.

• Thank You, I wonder how I could have avoided buying the wrong nichrome wire, because I thought I would just cut out enough wire to get 5ohm, but practically it all went wrong. If I were to replace the wire with a 5ohm resistor, would it still overload it ? Feb 2, 2021 at 0:38
• ^^ yes. 12V / 5ohm = 2.4A Your supply can not deliver that, so it will respond by cutting voltage. Doesn't matter if the 5ohm is 'wire' or a 'resistor'. 5 ohms is 5 ohms Feb 2, 2021 at 0:53
• "I thought I would just cut out enough wire to get 5 ohm, but practically it all went wrong." You can, but you should have calculated the required current first. "If I were to replace the wire with a 5ohm resistor, would it still overload it?" If the resistance is the same how could it be otherwise? Feb 2, 2021 at 9:12

Your power supply has ovecurrent protection. Where is two basic types of overcurrent protection. 1st is output current restricted. 2nd - output of or very low voltage. You probably have the 2nd.

• Overdue rent protection? I think that autocorrect has been at work there. Feb 1, 2021 at 23:49
• Eye h8 wind dat happen Feb 2, 2021 at 0:53