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Short:

Are carbon film resistors and metal film resistors equally safe to burn?

Long:

I want to explain to children that they need to carefully calculate and check resistor values. I don't only want to explain it, but also let them feel and see it.

In one step, I use three resistors in parallel, operating at their defined 0.25 W. They will get hot and the children can feel it. However, this may not show the danger enough, so I also want to let one of them catch fire.

A lot of things are obvious when performing this experiment

  • keep the children in a safe distance
  • use a fire-proof surface
  • do the experiment outside, so there's good ventilation
  • have a fire extinguisher near, just in case
  • in any of our experiments, we use low voltage DC power source, < 24 V

I was also looking up online sources like Blowing up a resistor and I'm glad that resistors burn but don't explode.

However, there's one open question to me: we have 0.25 W carbon film resistors and 0.25 W and 0.6 W metal film resistors available. Are they equally safe to use?

My assumption would be that the carbon film resistors just burns to ash, but the metal film resistors might melt into liquid metal. Is it maybe even educational to do both experiments, because the result is so different?

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    \$\begingroup\$ Most important is safety goggles for eye protection. Carbon resistors cannot burn to ash, it is already burnt. My guess is the carbon resistors will break in the middle, but it can also be that the electrodes will disconnect due to melting. Why don't you try it out first? Also, other than concentrating on burning resistors, also explain safety margins, heat dissipation, airflow etc. \$\endgroup\$
    – Indraneel
    Commented Feb 10, 2019 at 11:51
  • \$\begingroup\$ @Indraneel: certainly I will try this myself first. Before I do that, I want to make sure that I'm safe as well. \$\endgroup\$
    – Thomas Weller
    Commented Feb 10, 2019 at 11:53
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    \$\begingroup\$ @ThomasWeller You are safe, nothing will explode. Just wear eye protection. Hopefully you're using a low voltage (<30V). 60V may be too much for small children with wet fingers.... although I seem to have survived some nasty 220V shocks as a kid (dry fingers though). \$\endgroup\$
    – Indraneel
    Commented Feb 10, 2019 at 11:57
  • \$\begingroup\$ Use low value carbon or film resistors, for example 10 ohms. Then, even 5 or 10V will set them alight quickly. If you want a resistor that will get hot for the kids to feel the heat, I suggest use a bigger wirewound resistor on a ceramic core, which is "guaranteed" NOT to catch fire and won't be damaged running at temperatures too hot to touch. They wll be able to feel the heat from a safe distance - you could even make a wire mesh shield if your "elf and safety" people don't like this sort of thing.. Experiment yourself to find the supply voltage to give the amount of heating you want. \$\endgroup\$
    – alephzero
    Commented Feb 10, 2019 at 22:31
  • \$\begingroup\$ Prehaps get a current controlled power supply and wind the current slowly up and it will slowly starting burn, smoke and glow. May be an easier to do. \$\endgroup\$
    – Curious Diode
    Commented Feb 11, 2019 at 17:28

2 Answers 2

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Carbon film leaded resistors can "track" and end up a glowing red ceramic rod, depending on the value and applied voltage.

Metal film resistors usually just open up in undramatic fashion, sometimes with a tell-tale spot of black through the lacquer for through-hole types.

Cement wirewound resistors (the rectangular ceramic type) can explode.

There are bound to be nasty chemicals liberated from the burning lacquer in the first case.

If you're using relatively high (eg. mains voltage) to fry your resistor, you may wish to ensure you're using an appropriate (eg. Class C) fire extinguisher so you don't accidentally demonstrate electrocution.

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    \$\begingroup\$ Decades ago some newly hired new-graduate technicians wondered about resistors burning up. I personally used batteries in my experiments--- no 60Hz noise for my 80dB amplifiers, and no shock risks --- and always did P= V^2 /R crudely, to ensure was safe. You can provide a nomograph of that for the kids, to pick non-hot conditions. Anyhow: with the new techs, we found 5 volts and 4.7 ohms at 5 watts power, would "instantly" cause1/4watt AllenBradley resistors to flame up. These were carbon comp. \$\endgroup\$
    – analogsystemsrf
    Commented Feb 10, 2019 at 12:14
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The whole thing sounds overly dramatic to me. Instead, you should be teaching your students that every component has at least two important parameters that need to be accounted for in component selection. For example:

  • Resistors - resistance and power rating
  • Capacitors - capacitance and voltage rating
  • Inductors - inductance and current rating
  • Transistors - VCE (or VDS) and power rating

... and so forth. Obviously this is a simplification and you can quibble over the details, but if you learn to pay attention to these basic parameters, you generally won't be experiencing dramatic component failures in your prototypes.

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    \$\begingroup\$ Teachers know that students love drama however, and I don't think that goes away with age :) \$\endgroup\$
    – Ibraheem
    Commented Feb 10, 2019 at 14:25
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    \$\begingroup\$ I find your statement useful and I'll definitely use the chance to address your point. It does not answer the actual question, however. \$\endgroup\$
    – Thomas Weller
    Commented Feb 10, 2019 at 14:29
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    \$\begingroup\$ You also want to demonstrate what can go wrong, both so they're less afraid to make mistakes (they know it won't explode) and so they aren't tempted to try it themselves to see what happens. \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Feb 10, 2019 at 23:02

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