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Basic Electronics » NPN circuit connection quandry

September 22, 2012
by edb
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I am having some difficulty understanding how to connect an NPN as a switch. I do understand that the usual admonition seems to be "ignore the labels", but they ARE part of the data sheet diagrams so I'm trying to "get it". I should point out that despite the fact that I really don't understand how to hook these things up I was able to drive a small motor by wiring two of them in parallel and sending a signal through the base.

Here's my problem: everything I've read seems to agree that the way I wired the MPS 222A NPNs should have led to releasing trapped smoke. Unless I'm looking at the diagrams wrong on the motors 101 and squirter projects, I wired it backwards. So what did I do wrong -- er right? It seems that there are many devices that go by the name "NPN" (MOSFET, BJT, transistor) but are maybe wired differently? I'm switching to Fairchild 2222As and would like them to have a reasonably long life, so thanks for any clarification you can provide. Does the arrow that's "not pointing in" point to the emitter or collector?

Details: I have the MCU wired to the base pin (sending PWM), the "ground" from the motor wired to the emitter pin and ground wired to the collector pin, if the pins are in fact e,b,c from left to right. I do have a protection diode in the circuit as well.

September 22, 2012
by mongo
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OK, starting off, if it says NPN, it is a BJT. Or in other words, just a basic transistor.

The Emitter is the arrow and it points away from the Base in the NPN config, or toward the Base in the PNP variety.

To use it as a switch, the load is between the Collector and the positive supply. A positive signal at the Base referenced to the Emitter will cause the transistor to conduct, energizing the load.

To connect transistors in parallel is a little tricky. The Emitters can all be tied together as well as the Collectors, but unless you want unexpected consequences from mismatches and leakages, I would isolate all of the Bases through individual diodes and pull-up resistors. It would also make it resemble a TTL input stage where a negative signal turns the transistor off and the lack of a negative signal turns it on.

It might be a little hard to visualize... Maybe I'll do a quick sketch.

September 22, 2012
by Noter
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Here is a good summary of transistors and sample circuits - http://www.kpsec.freeuk.com/trancirc.htm. Good diagrams too.

September 24, 2012
by edb
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OK, I think I figured out the source of my confusion and why it does work as wired. Mongo, thanks for the arrow to the emitter bit. "Most" diagrams seem to put the pieces C-B-E (top to bottom, base is left) and the data sheet also labels C-B-E, but left to right.

The simple part is that I bend my base leg backwards and this screws it up in relation to the diagrams. I need to learn to bend the base forwards and then my connections look like the wiring diagrams.

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