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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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Aloha
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  • What exact temp are we talking about? – PlasmaHH Jul 02 '15 at 11:15
  • I didn't measure. But, qualitatively, very hot. – Aloha Jul 02 '15 at 11:16
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    very hot is to be expected, but very very hot is not good. – PlasmaHH Jul 02 '15 at 11:17
  • So... heatsinks? – Aloha Jul 02 '15 at 11:17
  • Why? You said it was only very hot, not very very hot, did you? – PlasmaHH Jul 02 '15 at 11:18
  • I smelled burning plastic, but no magic smoke. – Aloha Jul 02 '15 at 11:19
  • You may be using too much base current: what base current and motor current are you using? – pjc50 Jul 02 '15 at 11:20
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    @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? – PlasmaHH Jul 02 '15 at 11:20
  • @PlasmaHH It smells burning, therefore, it's extremely hot. – Aloha Jul 02 '15 at 11:22
  • @pjc50 more or less 100ma, but I doubt the page. Let's assume it's 250ma. – Aloha Jul 02 '15 at 11:23
  • What resistor do you have between the arduino and the base? – pjc50 Jul 02 '15 at 11:26
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    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. – Spehro Pefhany Jul 02 '15 at 11:28
  • @pjc50 none. Should I add one? – Aloha Jul 02 '15 at 11:28
  • Try a 100ohm and then a 1k. – pjc50 Jul 02 '15 at 11:30
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    @SpehroPefhany the transistor fail, or the finger? – pjc50 Jul 02 '15 at 11:31
  • @pjc50 I see what you did there. – Aloha Jul 02 '15 at 11:33
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    Measure the collector-emitter voltage when the motor is running. If it's more than 0.5V, more base current will help, not less. –  Jul 02 '15 at 11:33
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    I doubt very much that these motors are only rated for 100mA. They have a rated RPM of 48,000 and with a prop on them (a supplied) they are going to be shifting some air and if all you have in the spec is 3.7V at 100mA, something is wrong. Always buy stuff with proper data sheets. – Andy aka Jul 02 '15 at 11:36
  • @Andyaka exactly. – Aloha Jul 02 '15 at 11:39
  • @pjc50 Both- the finger once the wet boils off. ;-) – Spehro Pefhany Jul 02 '15 at 11:41
  • So yeah. I'll test out your suggestions right now. Shouldn't take more than 15 minutes. – Aloha Jul 02 '15 at 11:42
  • What is the stall current of the motors? That's how much current the transistors will be seeing when the motors are first starting up (before they really start spinning) – DerStrom8 Jul 02 '15 at 12:31
  • @derstrom8 I honestly have no idea. But they're pretty small, so I reckon it shouldn't hit more than 1A. – Aloha Jul 02 '15 at 12:44
  • You can measure it by hooking it up to a battery (preferably an SLA -- They do better with high-current loads) with an ammeter in series, and hold the shaft of the motor so that it cannot spin. The current read by your ammeter will tell you the stall current. While you're at it, you may as well measure the actual no-load current while the shaft is spinning freely – DerStrom8 Jul 02 '15 at 12:56
  • @derstrom8 500ma – Aloha Jul 02 '15 at 13:14
  • Is that the stall current? – DerStrom8 Jul 02 '15 at 13:15
  • Yes. Yes it is. – Aloha Jul 02 '15 at 13:16
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    The TO-92 2N2222 transistor is only rated for 600mA. While this is still higher than the stall current, you're definitely pushing its limits. It will get very hot. I suggest looking for different transistors rated for a higher current, or at the very least put on a clip-on heatsink – DerStrom8 Jul 02 '15 at 13:19
  • It's doing pretty cool with the heat sink. Pun intended. – Aloha Jul 02 '15 at 13:20
  • One other option, if you're still concerned, is to use a PN2222 (i.e. the PN2222ATF from Fairchild). It is rated for 1A but still uses the TO-92 package – DerStrom8 Jul 02 '15 at 13:22
  • I would move up to a TO-220 package at least. Less plastic to burn (proportionally speaking), more package to withstand the heat, and easier to strap a heatsink to. – Ignacio Vazquez-Abrams Jul 02 '15 at 13:31
  • I would really love to, and was in fact looking for that exact package. But, being in the place I am in now, TO-92A's are the norm. TO-220's are pretty rare here, or are reserved for the larger transistors. – Aloha Jul 02 '15 at 14:31

2 Answers2

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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.

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

avl_sweden
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    No, it is not "normal" for transistors to become very hot while operating - it is an exception for power transistors. – Andy aka Jul 02 '15 at 14:24
  • 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. – avl_sweden Jul 02 '15 at 14:33
  • 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! – avl_sweden Jul 02 '15 at 14:35
  • To all you downvoters: Do you expect to be able to touch the inner workings of all electronics, or just quadcopters? Would you still hold the same stance if the transistors were hidden away inside the device? Computers, power tools, stereo equipment; all contain components which become too hot to the touch. Avoiding that is simply cost for no benefit. What part of the rated temperature range of a component is it ok to design for? Even a 40 deg C metal object is too hot to touch comfortably. Should everything be kept below 37 deg C? – avl_sweden Jul 02 '15 at 14:42
  • @avl_sweden and to everyone that are are reading and will read this: The transistor does not heat up immediately. It reaches that temperature over time. Heat sinks were attached, but I still have to test. – Aloha Jul 02 '15 at 14:45
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    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. – Andy aka Jul 02 '15 at 14:46
  • PandaLion: Do you have any way to measure exactly how hot it gets? If it just a little bit too hot to touch, then it's just somewhere about 40-50 deg C, and that is completely ok for the transistor at least. – avl_sweden Jul 02 '15 at 14:47
  • Andy aka: Ah, then I understand your point :-) . – avl_sweden Jul 02 '15 at 14:47
  • Noted. Take an upvote. – Aloha Jul 02 '15 at 14:51
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    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) :-). – avl_sweden Jul 02 '15 at 14:51