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Transistor switch details. What am I doing wrong?

Discussion in 'General Electronics Discussion' started by dofo, Jun 27, 2014.

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  1. dofo

    dofo

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    Jun 27, 2014
    First post here. I searched here for info on this before posting and didn't find what I was looking for. I'm a college electrical engineering student working on an automated irrigation system project. We've got a lot going on in the control box, and so I'll give you the details relevant to my question. We have a circuit that employs a rotary flow-rate sensor that requires a current with a minimum 5VDC potential to run it. We want to employ a switch so that the sensor is only consuming power when it's needed. The brain of our box is an Arduino Due which will output 3.3VDC to trigger our switch. We would like to use a simple transistor switch setup to control this. We're trying to avoid a relay setup to save space on an already crowded PCB layout for our Arduino header. We're using the free version of Eagle and have a limit to our PCB size and our box also has limited space.

    We started with the following schematic:

    Flow SensorSchematic.png

    The important part for our purposes is the top section surrounding the transistor. We've breadboarded the circuit without the sensor and Arduino to test basic functionality and voltages. In this setup, we need to get 5VDC out of the emitter of the transistor, which is a BC550 NPN in a TO92 package (data sheet is attached). We have a couple of resistor decade boxes, so I've tried this setup in a number of configurations. I've varied the values of both 330Ω resistors in different combinations (methodically, yet rather desperately, I might add) from 1MΩ down to nothing, and I can't get anything more than 2.62V out of the emitter. I even ran 12VDC at a comparatively high current to the collector and regardless, the emitter output tops out at 2.62VDC. It sure seems like the transistor is saturating in its current configuration.

    So the questions: Am I doing something wrong or is this the max voltage that I can get out of this particular transistor? I've read the data sheet and it seems like I should be able to get 5VDC out of the transistor based on the specs and graphs. However, I'm only familiar with the basic functionality of transistors (that class is yet to come), so the data-sheet is a bit over my head. I've read some online tutorials on this that have not given me any answers. Is there a "sweet-spot" with the base voltage/current that opens the switch all the way? Do I simply need to use a different transistor, or is there a smarter way that I can employ this one to do my bidding? Again, if the answer is a relay, I already know how to do that, but I really would like to avoid it for such a simple purpose.

    Thanks,

    Aaron
     

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    Last edited: Jun 27, 2014
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    The problem is not the transistor, it's your circuit.
    An NPN transistor needs a voltage at the base that is ~0.6V to 0.7V higher than at the emitter. When you set Flow_trig to High (probably 3.3V on an arduino) you get 3.3V-0.7V=2.6V at the emittter, regardless of the voltage at the collector. Pretty much what you measure.

    You need a high side driver that can look e.g. like this:
    [​IMG]

    See also this article.
     

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  3. dofo

    dofo

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    Jun 27, 2014
    Thank you. With an NPN, if the voltage at the base is higher than the range of ~0.6V to 0.7V than at the emitter, can the transistor be damaged?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    10,370
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    Nov 17, 2011
    That should be: higher than +0.6V...+0.7V, not "-"

    The voltage can't be mch higher, that is why you need a resistor to limit the base current. The base-emitter path of a bipolar transistor is effectively a diode. A diode's voltage drop depends on the current, or current depends on vltage drop, whichever way you like to see it. If you look at the characteristic curve of a diode (e.g. 1N4148) you see an exponential increase in current with voltage. The resulting power dissipation will destroy the diode or in your case the transistor.
     
  5. dofo

    dofo

    3
    0
    Jun 27, 2014
    Thank you again for the help. We employed the high side driver and tuned in the resistor values for our project. It's working great on the breadboard and I learned a good bit about transistors in the process. The longer I do this, the more I realize I don't know! Thank you again!

    Screen Shot 2014-06-29 at 12.48.05 PM.png
     
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