Can not turn off transistor switch

[milin]

Hi,

I use a simple BJT transistor switch to control the flow of current through the emitter. The base of the transistor is connected to a digital output pin that comes from a PC USB I/O board. The collector is connected to a 12V battery.

My problem is that even if there is no input signal (the output pin is low) the transistor is "on" (current flows out of the emitter). When I check the voltage between base-emitter its 0.7 V so this explains why the transistor is "on".
If I connect the base of the transistor to ground it switches "off" as expected

So my questions are:
1. How come the base-emitter voltage is 0.7 V even if I completely remove the connection to the base of the emitter? Is it related to a problem with "floating" input?

2. What can I do to solve this problem?

Obviously the I/O card does not connect its output to ground when output is low since this would turn off the switch instead it just lets the output "float" !?!?

[pebe]

It is possible to get a small collector current with the base open circuit, but normally it is nowhere near enough to saturate the transistor with a 150ohm collector load.

Short out its base to emitter. If the collector voltage is still low, then the transistor has a shorted base/collector junction and is faulty.

[milin]

I found the error, its kind of embarrassing but I have put the transistor the wrong way and the pin I thought was emitter was actually the collector.

When I got the transistor I simply searched the web for "pn2222a datasheet and found this: http://alltransistors.com/pdfview.php?doc=mtp2n2222a.pdf&dire=_motorola. However the setup of the pins is not the same as found in this pdf: http://www.jameco.com/Jameco/Products/ProdDS/178511.pdf. It seems as different manufacturers use different pin configurations for equalent transistors.

Thanks for the input.

[pebe]

That was unfortunate, but it seems that it's not just a misprint. I looked up a list of manufacturers in this link and found both types of connection. It's not something I have come across before.

http://www.datasheetcatalog.com/datashe ... 222A.shtml