# Can a resistor start a fire?

I read that if you create a short-circuit, this produces heat and can cause a fire or damage an electronic component. The solution is to use a resistor.

The heat generated by electricity is proportional with the amount of electricity passing through a conductor in a period of time

[Source: Make: Electronics Second Edition]

And I read in this question(What does a resistor do?), that

If I switch the power source from 5V to 9V, will I still measure the same numbers after a resistance ?

And got this answer :

The increased electromotive force will allow more charges to flow per unit time, i.e. the current will increase.

If I connect a resistor to a high voltage source, a lot of current will pass through it and the heat will increase.

What will happen ? Does the resistor break and allow more and more current to pass? Could it start a fire ?

• There is not such thing as "amount of electricity". – Olin Lathrop Dec 27 '15 at 14:35
• Here also take a look at this video youtube.com/watch?v=3G7tQVGXtBs – Orkun Dec 27 '15 at 14:52

## 4 Answers

It's somewhat unclear what you are really asking, but resistors dissipate power and heat up accordingly.

Electrical power is EMF times current. In MKS units these are measured in volts and amps, with the product of the two being watts.

Resistors are described by Ohm's law:

EMF = current x resistance

In MKS units:

volts = amps x ohms
V = A x Ω

Since power being dumped onto a resistor is volts x amps, and we can relate one to the other by Ohm's law, power is also:

W = V2
W = A2Ω

That is all to determine how much power is heating a resistor. How hot it actually gets depends on how well it can get rid of the heat. This is defined by the thermal resistance to ambient, and is dependent on the mechanical properties of the resistor.

For example, a physically small resistor might be rated at 500°C/W. If ambient is 20°C and you tried to dump 1 W onto it, it would stabalize at 520°C. That's way too hot for most resistors, so it would catch fire, vanish into a puff of greasy black smoke or the like before it got to that temperature.

On the other hand, a larger power resistor may be rated for 50°C/W. Dumping 1 W onto it would only raise its temperature to 70°C, which is likely well within its normal operating range.

This is why resistors not only come in different resistances, but also different powers. Resistors that can dissipate more power are physically bigger. Typical 0805 SMD resistors are usually good for 125 mW or so. It takes something 15-20 mm long and 5 mm in diameter to be able to dissipate a few Watts without damage.

• Yeah -- one of our first labs in Intro to Electronics where I went to college at involved lighting a 1/4W resistor on fire -- it caught fire briefly (enough to light a candle or cigarette I suspect) and then self-extinguished, as all good components should. – ThreePhaseEel Dec 27 '15 at 17:17

A resistor has a certain rating of how many Watts (i.e. power) that it can safely radiate into its environment. Beyond that the resistor may burn open circuit, could burst into flames or go to a lower resistance causing other components to fail.

A resistor can start a fire if you force enough current through it. I have in the past used them to light matches (about 1W power through a 1/4W metal film resistor)

in the fire prevention role the resistor must be chosen so as not to overheat when used with the voltages that will be present.

some resistors are designed to fail open circuit (like a fuse) when they overheat. this type will not start fires, but your circuit will stop working.

Some resistors go on fire when grossly overloaded. Some are self extinglishing and some are not. If you have a source that has a very high prospective fault current you must be very careful.

A good example would be a 100V 200AH lead acid battery system. Say if you have a 1 ohm 2watt resistor in your circuit. What happens to the resistor when you have a short circuit fault? What's wanted is for the resistor to safely go open circuit but as mentioned before that does not always happen. Fusable resistors are specified in this respect and are recommended.

If you are looking at existing designs and want to change the type of resistors you should retest to make sure before putting your "better" resistors into production. Remember that these things can also happen on a 12V lead acid battery.

We did some tests and found that the Indian resistors flamed up and would not go out. There is a safety message here and that's why people back up the resistor with a fuse. When doing auto electrical stuff I would test on my 100AH 12V battery to be sure. You can't rely on people to use the correct fuse.