I've researched for a few hours but I'm still concerned.

I'm building a very low budget quadcopter (think toothpicks) and as I've seen on this site, a D10N05 mosfet would do the job. I've visited 3 of the largest electronics shops in my area but unfortunately, they don't have this specific part, or any mosfet as a matter of fact.

Then, I saw this post here on se and lucky enough I have some 2222's collecting dust in a container. I tried it, and it works like a charm. But, one thing I noticed was the transistors would run so hot to the point that I couldn't touch them. Is this to be expected? I have some small heatsinks and some thermal adhesive that I can attach once I know it's okay.

Thanks in advance!

EDIT: The transistor does not reach high temps immediately. It heats up over time.

Here are the the parts I'm using:

  1. Small motors, rated at 3.7v 100ma

  2. 2n2222 transistors

  3. 1n4001 as a flyback diode

  4. 5v power supply (but I'll use a 3.7v battery on the final build)

  5. Arduino nano (PWM control)

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    \$\begingroup\$ very hot is to be expected, but very very hot is not good. \$\endgroup\$ – PlasmaHH Jul 2 '15 at 11:17
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    \$\begingroup\$ @PandaLion98: plastic burns only at extremely hot, not very very hot, and by far not at very hot. Are you sure its only very hot? \$\endgroup\$ – PlasmaHH Jul 2 '15 at 11:20
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    \$\begingroup\$ If you can't hold your finger on it, it's pretty hot (60°C+). If your wet finger sizzles when you touch it, it is way too hot and will fail shortly. \$\endgroup\$ – Spehro Pefhany Jul 2 '15 at 11:28
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    \$\begingroup\$ @SpehroPefhany the transistor fail, or the finger? \$\endgroup\$ – pjc50 Jul 2 '15 at 11:31
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    \$\begingroup\$ Measure the collector-emitter voltage when the motor is running. If it's more than 0.5V, more base current will help, not less. \$\endgroup\$ – Brian Drummond Jul 2 '15 at 11:33

To answer your question: Yes, it is normal for (power) transistors under load to become very hot while operating. Most are rated for temperatures well above 100 deg C.

Even 60 deg C is too hot to touch, at least for exposed metal tabs and such.

Note that it is only normal for transistors to become hot when operated with substantial current. The transistors used in the question are rated at 1A, and 100mA is substantial enough to cause heating for them.

When operating very small current, for instance for low-speed logic, a hot transistor is indicative of a fault.

Note that the transistors in a modern CPU DO get very warm, but that is not because of high current per transistor but because there are so very many of them in a small constrained package.

When designing a circuit, of course it is desirable that it not run too hot. Keeping the temperature down increases the longevity of the device. However, keeping temperature down can mean choosing beefier transistors, and this costs more. For a quad copter it can also be noted that bigger transistors lead to smaller losses, but also higher weight and cost.

For optimal performance, the transistors shouldn't be too small (short life, danger to operator, high power loss) and not too big (heavy weight, high cost).

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    \$\begingroup\$ No, it is not "normal" for transistors to become very hot while operating - it is an exception for power transistors. \$\endgroup\$ – Andy aka Jul 2 '15 at 14:24
  • \$\begingroup\$ Yes, and these are power transistors. Just 100mA, but it's for a small quad copter, so everything is small, including the current. All I'm saying is that there isn't necessarily anything wrong with running transistors at 100 deg C. \$\endgroup\$ – avl_sweden Jul 2 '15 at 14:33
  • \$\begingroup\$ The transistors are only rated at 1A, and that is at the maximum junction temperature of 150 deg. So even at 100mA it could very well be that they become too hot to touch. That said, of course there could be something wrong with PandaLion's circuit! \$\endgroup\$ – avl_sweden Jul 2 '15 at 14:35
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    \$\begingroup\$ It's wrong to say "it is normal for transistors to become very hot while operating" because other readers might think that all transistors (whatever the application) should become hot and that is not true. \$\endgroup\$ – Andy aka Jul 2 '15 at 14:46
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    \$\begingroup\$ I've attempted to clarify this so as to not lead anyone to think their breadboard should become hot when building a LED flasher from discrete components (for example) :-). \$\endgroup\$ – avl_sweden Jul 2 '15 at 14:51

I wouldn't expect a 2N2222 running at 100mA to get hot if the circuit is designed properly.

Are you giving it enough base drive? You need about 10mA base drive. If the processor is running off 3.7V you will need a 330 ohm base resistor (not 1k as in the link) to provide enough drive.

I'm surprised that the motors only take 100mA - have you measured them? They may be be taking more.

From the datasheet the collector voltage should only be ~0.3V so you will only get 30mW dissipation, that would not make the transistor hot.

The 1N4001 is not suitable as a flyback diode - it is much too slow. Something like a 1N5819 would do, but there are many types that would be better than an 1N4001. A shottky would be preferable but a faster silicon junction diode would be better.

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  • \$\begingroup\$ Thanks for the additional tips. Still not done with the project. Someone messed up shipping, and it's not me. \$\endgroup\$ – PNDA Aug 17 '15 at 14:17
  • \$\begingroup\$ Currently 4 years into my Electronics Engineering degree. I'm having flashbacks haha \$\endgroup\$ – PNDA May 29 '19 at 17:11

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