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Transistor Question

RagtopRagtop Posts: 406
edited 2011-05-29 12:28 in General Discussion
I am using a 2N4401 transistor with 5v going to the collector pin through a 101ohm resister and 3.3v going to the base pin with a 101ohm resister.

I was expecting the same voltage on the collector and emitter pins but instead I am getting
Collector 3.88
Base 2.56
Emitter 2.61

I was thinking you could control a slightly higher powered circuit with a lower powered one with a transistor, but am I misunderstanding what transistors do?
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Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2011-05-28 07:50
    What's connected to the emitter (other than the voltmeter)?

    A transistor is a current amplifier. You supply current going from base to emitter and a corresponding greater current will flow from collector to emitter. The series base resistor allows you to supply a voltage to the base and the resistor determines the current proportional to the voltage (base current = (base voltage - base-emitter voltage) / base resistor). The base-emitter voltage is typically 0.7V which is determined mostly by the material used (silicon) to make the transistor. See the Wikipedia or a basic electronics text for details.
  • RagtopRagtop Posts: 406
    edited 2011-05-28 09:02
    a resister/led are connected to the emitter.

    "Current is the rate at which electric charge flows past a point in a circuit. Voltage is the electrical force that would drive an electric current between two points."

    Argh....for some reason I can't keep my head straight on these electrical terms.

    So, even though the voltage for base and emitter are the same. The current from the emitter is being amped higher then what is connected to the collector.

    Hmmm. would a 5v device operate correctly connected to the transistor with the increased current even though it is receiving less voltage?
  • LeonLeon Posts: 7,620
    edited 2011-05-28 09:16
    If the transistor is connected properly, the base and emitter can't have the same voltage. Vbe must be about 0.7V, as Mike said.
  • Mike GreenMike Green Posts: 23,101
    edited 2011-05-28 09:28
    Putting a resistor and LED in the emitter lead really complicates things because the base current has to flow that way and there's a voltage drop across the LED and the resistor. Don't do that. You can put the LED and resistor in the collector lead, substituting for the 101 Ohm resistor already there. Just connect the emitter to circuit ground.
  • vaclav_salvaclav_sal Posts: 451
    edited 2011-05-28 11:49
    This link may help with some basic
    http://www.electronics-tutorials.ws/transistor/tran_4.html

    As Mike said put the load - LED plus its limiting resistor - into collector circuit.
    Start with required current to turn the LED on and using the transistor spec determine the base bias resistor.
  • RagtopRagtop Posts: 406
    edited 2011-05-28 13:47
    Leon wrote: »
    If the transistor is connected properly, the base and emitter can't have the same voltage. Vbe must be about 0.7V, as Mike said.

    Well, not exactly the same. There is a .05v difference. Which with my 10 dollar multimeter is pretty good.

    I just got volts/current reversed in my mind. I was thinking you would get the same voltage on collector and emitter when base was "pressed". I read "Make: Electronics page 73 on" but still managed to get confused on what the transistor was actually doing.

    The led circuit is unimportant (not part of a project) I was just trying to understand what was going on as I was planning on interfacing the prop with a 6v servo and I thought transistors were part of doing that.
  • RagtopRagtop Posts: 406
    edited 2011-05-28 13:53
    vaclav_sal wrote: »
    This link may help with some basic
    http://www.electronics-tutorials.ws/transistor/tran_4.html

    As Mike said put the load - LED plus its limiting resistor - into collector circuit.
    Start with required current to turn the LED on and using the transistor spec determine the base bias resistor.

    Make: Electronics at least starting out on the subject, had the load (led) on the emitter side. But now I am remembering having
    trouble (lots of hair pulling) with darington arrays until someone here told me to put the leds on the power side instead of the ground side.

    Would darington arrays be better for interfacing higher voltage servos?
  • Mike GreenMike Green Posts: 23,101
    edited 2011-05-28 14:18
    Before you just "throw" hardware at some situation, how about first understanding what you're planning to do?

    There's a good Wikipedia article on servo motors here. There's also an existing Propeller Servo Controller (PSC) with links to schematics, documentation, and firmware on the web-store page here. There are also links to a pair of articles in Nuts and Volts Magazine on an even simpler Propeller controller for servos (the "Propeller Multi-Controller") that uses just a serial resistor on the Propeller I/O pins rather than the bidirectional driver chip used on the PSC.
  • RagtopRagtop Posts: 406
    edited 2011-05-29 05:12
    Hmm, well the idea would be to control a 6v servo with a 3.3v prop. I am assuming the control line going to the servo needs to be jacked up to 6v. I have a servo working with the propeller at 3.3v but need more torque.

    I looked through the links mentioned. I looked at the Multi-Controller link but could not find any detail on the serial resister and parts 2 and 3 are not there.

    The "hardware" part of the PSC looks great for throwing at the problem, but I don't see the 8-BIT BIDIRECTIONAL VOLTAGE-LEVEL TRANSLATOR listed on my favorite sites. Maybe, digi-keys, I just don't like their website and have never ordered from them.
  • RagtopRagtop Posts: 406
    edited 2011-05-29 06:00
    Leon wrote: »
    If the transistor is connected properly, the base and emitter can't have the same voltage. Vbe must be about 0.7V, as Mike said.

    My head keeps getting spun round and round one this. The voltage drop from collector to emitter should be about .05v according to this Nuts and Bolts article. So the results I was getting were wrong not just my reverse thinking of voltage/current.

    transister.jpg

    So you should be able to drive a higher voltage load (minus the .05v transistor drop) with a transistor?
  • LeonLeon Posts: 7,620
    edited 2011-05-29 06:04
    Yes. You need to check the transistor dissipation and current against the data sheet, to make sure that it isn't overloaded.
  • RagtopRagtop Posts: 406
    edited 2011-05-29 06:26
    Leon wrote: »
    Yes. You need to check the transistor dissipation and current against the data sheet, to make sure that it isn't overloaded.

    Does the prop demoboard put out 2a out its 5v and 3.3v pins?
  • LeonLeon Posts: 7,620
    edited 2011-05-29 06:31
    I doubt it. Check the data sheets for the regulators.
  • RagtopRagtop Posts: 406
    edited 2011-05-29 06:32
    trans.jpg

    Switching the led to the power side of the transistor I am now getting these numbers when placing the black probe on the ground post and the red probe to each of the transistor pins. The led is much brighter
    and my understanding is much dimmer.
  • LeonLeon Posts: 7,620
    edited 2011-05-29 06:38
    Identify those leads.
  • RagtopRagtop Posts: 406
    edited 2011-05-29 06:44
    The transistor leads? or the probes. The probes come with the cheapo Cen-tech digital multimeter.

    The transistor leads have the flat side facing so, left to right emitter, base, collector.

    The resisters I thought were all the same but on exam the collector and emitter resisters are 101ohm and the base is 100k.
  • LeonLeon Posts: 7,620
    edited 2011-05-29 06:47
    I meant E, B, and C.

    If the emitter is grounded, as it should be, it can't have 0.39V on it! The base should be about 0.7V with the emitter grounded.

    Base resistor should be about 1k. That will give a base current of about 4.3mA, ensuring the transistor bottoms.
  • RagtopRagtop Posts: 406
    edited 2011-05-29 07:03
    Ok. Seeing one of many misconceptions on my part, thinking the voltage would show the same on collector/emitter. The emitter being grounded should show 0?

    Didn't have a 470r, but put a 510r from emitter to ground and the base pin has come up to .7v, but so has the emitter pin.
  • LeonLeon Posts: 7,620
    edited 2011-05-29 07:18
    510R is fine. It will give you a base current of about 5 mA with 5V on the input, which will turn the transistor fully on.

    Is that with 5V on the input? You must have it wired up wrong, check that the emitter is actually connected to 0V.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2011-05-29 07:28
    Everybody having "fun" running this little exercise off the rails?
    He said it's a 2N4401, it's EBC.
    Why is there a resistor in the emitter lead?

    Here's the way it ought to be --

    pearlsb4swine.jpg
  • RagtopRagtop Posts: 406
    edited 2011-05-29 07:39
    The 5v and ground seem connected because when I undo either the led goes out.
  • RagtopRagtop Posts: 406
    edited 2011-05-29 07:55
    PJ Allen wrote: »
    Everybody having "fun" running this little exercise off the rails?
    He said it's a 2N4401, it's EBC.
    Why is there a resistor in the emitter lead?

    Here's the way it ought to be --

    pearlsb4swine.jpg

    With that I am getting
    Collector .68
    Base .01 (not sure if 1k or 10k resister as I have hard time seeing red from orange)
    Emitter 0
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2011-05-29 08:00
    Use your meter (ohmmeter), measure the resistors out of circuit.
    Maybe you've snuffed your transistor.
    I made a demo for a fellow a couple of years ago, had troubles similar to yours.
    If you like, take a look at it, or don't -- http://www.youtube.com/watch?v=4zFN1lsYLDY
  • RagtopRagtop Posts: 406
    edited 2011-05-29 08:01
    Back to my other question. Is there a reason not to use a transistor to PWM a servo?
  • RagtopRagtop Posts: 406
    edited 2011-05-29 08:05
    PJ Allen wrote: »
    Use your meter (ohmmeter), measure the resistors out of circuit.
    Maybe you've snuffed your transistor.
    I made a demo for a fellow a couple of years ago, had troubles similar to yours.
    If you like, take a look at it, or don't -- http://www.youtube.com/watch?v=4zFN1lsYLDY

    I had trouble getting that part of my 2.99 multimeter to work, but seems to be working now. Set at 2000 it is reading 1181 on the questionable resister, 1k.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2011-05-29 08:10
    Back to my other question. Is there a reason not to use a transistor to PWM a servo?
    I'd like to get you dialed-in on this transistor matter, but...

    a servo should be OK running with its power derived from +5V and getting a 3V signal to its control/input, if that's what you mean.
  • RagtopRagtop Posts: 406
    edited 2011-05-29 08:20
    Switched out to a new transistor that I have not used.
    5v to 510r to led to collector
    1k from 3.3v to base
    jumper wire from emitter to vss.

    C pin is .01
    B pin is .68
    E pin is 0

    Led still on and bright.
  • Mike GreenMike Green Posts: 23,101
    edited 2011-05-29 08:23
    Just a word or two of advice ...
    Never try to measure resistance with an Ohmmeter or Multimeter in a live circuit (one with power applied). You run the risk of burning out your meter.
    Don't bother trying to measure resistance with an Ohmmeter of something that's part of a larger circuit. You need to disconnect at least one lead of the device from the larger circuit first or you'll get a value that means little because you're measuring the resistance of the whole circuit.

    If you look at the schematic of the Multi-Controller, you'll see that there's a series resistor in the I/O leads from the Propeller going to the servo control wires. That's all you need for most servos. A 1K to 2.2K resistor will do nicely. It helps keep noise from the servo motor from getting back into the Propeller and helps protect the I/O pins from accidentally applied voltages above 3.3V (like from the servo supply voltage).
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2011-05-29 08:23
    It should be, the base has current from the 3V.
    Disconnecting the base from 3V turns the LED off?
  • Mike GreenMike Green Posts: 23,101
    edited 2011-05-29 08:27
    Regarding your measured voltages ... The emitter voltage makes sense because it should be zero. The base voltage makes sense because that's roughly one diode voltage drop (~0.7V) across the base to emitter junction inside the transistor. The collector voltage is a little lower than I'd expect. That's the transistor's saturation voltage (when it's switched on fully). It's usually more like a couple of tenths of a volt (often 0.3V).
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