Solenoid circuit question
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Solenoid circuit question

by dustynrobots on Mon Dec 03, 2012 10:18 pm

So I have a small solenoid from Adafruit and I hooked it up as in the diagram here and it works:
http://www.arduino.cc/playground/upload ... driver.pdf

However, I first had it hooked up with the solenoid and the transistor order reversed so inline from power it was:
power-->transistor-->solenoid-->ground
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Arduino

and it did not work. So my question is why would this NOT work? I'm happy it works now of course, but would appreciate any insight since I can't come up with a reason why it didn't work in this configuration. Thanks!
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Re: Solenoid circuit question

by adafruit on Mon Dec 03, 2012 10:22 pm

a NPN transistor is on when the the base is about .7V above the emitter. if you connect the emitter to ground, then you can pull the base up to 0.7v. if the emitter is 'floating' or connected to an inductor (which, when turning on, is an instant short but then turns into an open) then the base wont be steady. thats the short answer, for more details, check horowitz & hill, chapter 1 & 2
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