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Am I missing something here?

I have an NPN transistor (2N2222) with a 10K resistor from collector to Vcc.

Also a 10K resistor from base to input signal. Emitter is grounded.

The base input is alternately connected to Vcc for saturation (Vce close to zero volts) and connected to ground for shutoff (Vce equal to Vcc).

The circuit works fine if VCC is less that 7 volts. Above that when the base input is grounded Vce does not increase above 7 volts. I have increased the value of Vcc to 15 volts and Vce is still 7 volts.

I want to have the transistor operate as a switch with Vcc at 12 volts, but it will not shut off. I have used several NPN transistors and they all show the same characteristic with a max Vce of 7 to 8 volts.

Is there a workaround for this?

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    \$\begingroup\$ Make sure your pins are correct. That sounds like Vbe reverse breakdown voltage not Vce \$\endgroup\$
    – D.A.S.
    Commented Apr 7, 2019 at 7:46
  • \$\begingroup\$ Another possible cause: is there nothing else connected to the collector? Are you feeding any other circuit with this one's output? \$\endgroup\$
    – carloc
    Commented Apr 7, 2019 at 8:08
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    \$\begingroup\$ @Barry: I know this is a trivial circuit but a schematic and a photo are usually a big help. You can add a schematic using the CircuitLab button on the editor toolbar. Double-click a component to edit its properties. 'R' = rotate, 'H' = horizontal flip. 'V' = vertical flip. Note that when you use the CircuitLab button on the editor toolbar an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. \$\endgroup\$
    – Transistor
    Commented Apr 7, 2019 at 8:32
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    \$\begingroup\$ As @SunnyskyguyEE75 comments, 7v is a smoking gun for a reverse VBE breakdown (breakdown sounds scary but with 10k in the way, it's likely to survive). Those TO-92 transistors are real easy to get the pins swapped around, I do it all the time. Some have base in the middle, so they wire up nicely like the schematic, some have collector in the middle, reflecting their internal construction with the collector being bonded to the lead frame, and I have to look those up three times before I get them right. Do a simple 2-diode check with a DMM on the transistors, to identify the base lead. \$\endgroup\$
    – Neil_UK
    Commented Apr 7, 2019 at 9:52
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    \$\begingroup\$ You are right Neil_UK. I was using the incorrect datasheet and swapping collector and emitter connections did the trick. Thanks everyone for your help. Those NPNs are tough - I whacked 20 volts through it backwards and it survived. \$\endgroup\$
    – Barry Thomson
    Commented Apr 7, 2019 at 10:14

1 Answer 1

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It's pretty easy to get the wrong pin diagram using a random datasheet for the generic "2N2222 part. There was a time, when these were not consistent and it seems even now, there are different datasheets enter image description here enter image description here

There were many reasons for this, which may be shared by esteemed colleagues.

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  • \$\begingroup\$ It happened to me once 20 years on my 1st order of equivalent or better LEDs for a client who made Bobcat instrument clusters, I specified a part that got delivered with exactly the same wavelength, intensity, beamwidth, size but had the cathode indicators reversed in the package. Process Engineer was not a happy camper and that was an embarrassing termination of a potentially great client which resulted in my choosing to become self-employed. \$\endgroup\$
    – D.A.S.
    Commented Apr 7, 2019 at 13:05

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