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Silly-con Valley, Ca, U.S.
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Last night I was playing around with a 2n2222 transistor, a push button switch and a DC motor and I kept blowing out transistors and I don't understand why. I set up 2 breadboards following the schematics below but only one worked. I thought maybe I had something backwards on the second one. I took it apart and put it back together multiple times but it just was not working. Finally I thought maybe there was some problem with the transistor in the "No work" circuit. So I got a brand new one and put it in the working"circuit. Worked fine. Then I put it in the "No Work" circuit and it didn't work as expected. Put it back in the working one and it did not work. That's when I realized I was blowing them up. I just don't get why. Can anyone explain this?

Thanks.


* works.JPG (45.34 KB, 768x1024 - viewed 11 times.)

* noWork.JPG (47.63 KB, 768x1024 - viewed 9 times.)
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20 GOTO 10

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Last night I was playing around with a 2n2222 transistor, a push button switch and a DC motor and I kept blowing out transistors and I don't understand why. I set up 2 breadboards following the schematics below but only one worked. I thought maybe I had something backwards on the second one. I took it apart and put it back together multiple times but it just was not working. Finally I thought maybe there was some problem with the transistor in the "No work" circuit. So I got a brand new one and put it in the working"circuit. Worked fine. Then I put it in the "No Work" circuit and it didn't work as expected. Put it back in the working one and it did not work. That's when I realized I was blowing them up. I just don't get why. Can anyone explain this?

Thanks.

The no work circuit is the best way to use a NPN transistor (as a 'low side' switch), however both circuits have a fatal flaw (which burns out the base emitter junction) in that you have no series current limiting resistor for the base lead.

That's just like wiring a LED to a voltage without a current limiting resistor, something has to blow as it then acts like a direct short circuit through the semiconductor junction once the voltage is higher then the forward voltage drop of the semiconductor junction.

Lefty

« Last Edit: March 01, 2011, 11:18:37 am by retrolefty » Logged

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After I drew those up I decided to try resistors. I just grabbed what was handy, 1k, which was overkill because in the "works" one it made the motor turn, but very slowly. In the second it still destroyed the transistor. Then I gave up and went to bed.

I'll have to play some more tonight.
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After I drew those up I decided to try resistors. I just grabbed what was handy, 1k, which was overkill because in the "works" one it made the motor turn, but very slowly. In the second it still destroyed the transistor. Then I gave up and went to bed.

I'll have to play some more tonight.

 The size of the base resistor sets the maximum current that can flow through the collector/emitter junction, which in turn is the same as the motor current. The collector current value is equal or greater then the base current X the minimum current gain specification for the given transistor. More base current doesn't hurt and insures that the transistor is forced into  full saturation allowing for maximum current flow determined then by just the motor resistance and motor voltage used, per ohm's law. However you must keep both the base current and collector current below their maximum rated values per the transistor's datasheet.

So your motor ran slow because the base resistor was too high a value and the transistor was not fully turning on. The second circuit should not have burned out the transistor if using the 1k base resistor. Are you double sure you have correctly identified the base, emitter, and collector leads?

Lefty

« Last Edit: March 01, 2011, 11:36:47 am by retrolefty » Logged

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Put a diode in anti-parallel over the motor, and maybe the transistor too?  I suspect spikes from this inductive load may be destroying your transistors in the "no work" circuit. Then again I'm not sure why it don't do that in the "works" circuit... Could be useful to know your transistors hFE also (gain), and how much current that motor needs.

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So your motor ran slow because the base resistor was too high a value and the transistor was not fully turning on.
Not only that, but presumably a large portion of the supply voltage is over the base resistor, which in this case is in series with the motor (and the base-emitter junction). Well not directly in series, but the effect is the same - that the motor don't get the full voltage over it (lower power), when used as an "emitter resistor". So it turned slowly.
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Silly-con Valley, Ca, U.S.
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Are you double sure you have correctly identified the base, emitter, and collector leads?

Lefty

Well now that you mention it, no. It just dawned on me. I think I read that when reading the datasheet for a transistor it's the opposite from reading an IC. That is, with an IC you assume you're looking at it from the top down to read it's pinout. When looking at a transistor datasheet you are looking at the diagram from the bottom. Can you confirm this or am I making that up? If it's true I'm all backwards.
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Are you double sure you have correctly identified the base, emitter, and collector leads?

Lefty

Well now that you mention it, no. It just dawned on me. I think I read that when reading the datasheet for a transistor it's the opposite from reading an IC. That is, with an IC you assume you're looking at it from the top down to read it's pinout. When looking at a transistor datasheet you are looking at the diagram from the bottom. Can you confirm this or am I making that up? If it's true I'm all backwards.

Unfortunately the 2N2222 has been around for decades and has been made in round package and more common these days plastic casing. One really needs to compare what you have and try and make sure you are looking at a 'correct' datasheet for your specific transistor. Most should have a picture showing lead arrangement Vs package type used.
http://www.fairchildsemi.com/ds/PN/PN2222A.pdf
http://en.wikipedia.org/wiki/2N2222

PS: I never make assumptions about semiconductor lead assignments ever sense I was rudely made aware of the different pin out of standard voltage regulators, a 7905 negative regulator is not wired up like a 7805 positive regulator, and let the magic smoke drift out if you assume they are.  smiley-grin

Lefty
« Last Edit: March 01, 2011, 12:42:22 pm by retrolefty » Logged

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I'm using TO-18 packages, exactly like the one in the Wiki link. If that picture is accurate for mine then I'm backwards.
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I think you look at transistors from below, for the simple reason that if you looked from above, you wouldn't see any wires, because the body would be in the way, unlike ICs.

I also think that you would "get away with", to a certain extent, wiring in a transistor backwards, because after all it is still NPN, right? However the base-emitter junction has heavier doping so it would work sub-optimally. In other words, the part intended for the "heavy lifting" is on the wrong side.

A useful gadget to have is the Atlas DCA55 component tester. That has three wires: red/green/blue which you clip onto components like transistors and diodes in any order. It tests the component, says what it thinks it is, and identifies the leads for you. For example, on a PN2222 transistor I got:

Code:
NPN Silicon Transistor
Red: coll, Green: emit, Blue: base
Current gain: Hfe=196
Test current Ic=2.50mA
Base-emitter Vbe=0.78V
Test current Ib=4.62mA
Leakage current Ic=0.00mA

The DCA55 is not particularly cheap (around the $US100+ mark) but is handy for checking components, identifying leads etc.
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For driving a motor you need a power transistor, not a small-signal transistor like the 2N2222.

The first circuit is an emitter follower and doesn't need a base resistor.

The second circuit is a common-emitter circuit - base resistor mandatory - you fried the transistor by shorting the supply into its base-emitter junction.

Both circuits MUST have a protection diode across the motor as it's an inductive load.

Determine the short-circuit current of your motor and use a transistor rated for that current.
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Yup. Had my collectors/emitters mixed up. Seems to have fixed it.

Also threw some 150ohm resistors onto the base. Works like a charm now.
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