Help with amplification of current

Discussion in 'General Electronics Chat' started by amruth11, Dec 17, 2011.

  1. amruth11

    Thread Starter Member

    May 24, 2011
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    Hi all, I am making a robot which uses IR to detect objects and turn left or right accordingly.(It is not a big robot with PIC controllers and stuff ) I am using a IC 555 in monostable mode to run the motor for 5 sec. The whole thing runs on 5v and the output from the IC was amplified using 2 NPN transistors as a Darlington pair, but after that i tried to run my motor but it didnt run so i checked the amps the motor draws and the amps my IC is able to deliver and i found out that my motor takes about 14mA and my IC gives about 13mA. I need help from all of u in making my IC deliver 14 or 15mA at best, as i have already mentioned i am using a darlington pair for amplification and i need a better amplification of the current. I thought about the op-amp but i am not good at it, so i need ur help in this. Thx in advance. :)
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The transistorized 555 timer (LM555 for example) is capable of sourcing or sinking up to 200mA.

    The CMOS version of the 555 timer (TLC555 for example) is capable of sinking up to 100mA, but only sourcing 10mA.

    We need to see a schematic drawing of how you have things wired at this moment, and the exact part number of your 555 timer. We also need to know the voltage you are using to supply the power, and type of battery.

    If you are trying to use a 9v pp3 "transistor" battery, that is most likely your problem.
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    What kind of motor runs on 15mA? I would have thought that even a cellphone vibration motor would need more than that.

    The possible current level will of course be related to the type of battery and motor used, but with a properly designed driver the 555 drive current limit should not be a problem. Think of using bipolar transistors (or preferably MOSFETS), rather than op-amps.
     
  4. amruth11

    Thread Starter Member

    May 24, 2011
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    0
    The battery i am using is 5v battery made by joining 3 AA batteries in series. IC is NE555N and i have posted the schem. below and i have a CMOS IC 7555CN and TLC555 is not available in my area. The motor i am using is a motor from a toy car and i was wrong about the amps the toy car motor takes, it takes about 700mA and my IC gives 86mA. I also thought about MOSFETS but i dont which type should i use in the circuit and like ur help on that to if u think MOSFETS will eliminate the problem. And i forgot to mention that even LM555 is not available in my area.
     
    Last edited: Dec 18, 2011
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    There are quite a few things you need to sort out here.

    To begin with, an NE555 will not work for very long at all on a three-cell battery. Its minimum supply voltage is 4.5V, so as soon as the battery runs down just a bit an NE555N will not work properly.
    See this NE555N datasheet: http://datasheet.octopart.com/NE555N-Fairchild-datasheet-43442.pdf
    The CMOS 555 variants can go down to 3V. http://www.ti.com/lit/ds/symlink/lmc555.pdf

    If you really cannot get a CMOS type, you will need a higher supply voltage, perhaps 6V. Will this be too high for your motor?

    Next, you should not connect the 555 output straight into the base of a bipolar transistor like a 2N3904. The output of a bipolar 555 is capable of sourcing about 0.1A with a 5V supply, and if connected straight into the base of a transistor both the IC output and the transistor base will see too much current - a series resistor is required.

    For a single transistor, the resistor would be chosen so that the base current would be one tenth of the collector current, perhaps a hundred times less for a Darlington. No resistor is needed to drive a MOSFET, as it takes no DC gate current

    A diode, such as a 1N4001 or a 1N4004 is needed across the motor, to prevent damaging voltages coming from the motor when the transistor turns off.

    Next, your present circuit would hardly turn off the motor supply at all - the duty cycle is over 90%. I don't know hat you wanted, but I will show a circuit changed so that the motor stops for part of the time - see picture: the 6.5Ω resistor is in place of the motor.

    Finally, the little 2N3904 transistor is not capable of supplying as much as 0.7A: its maximum current is 200mA. http://www.fairchildsemi.com/ds/2N/2N3904.pdf

    You need to find either a bigger bipolar transistor rated at 1A or more (but the 2N3055 in my example is overkill), or a suitably rated MOSFET designed for logic-level operation, that is, one which can be fully turned on with 5V or less gate to source. Why don't you try searching for one. I live in England, and don't know what parts you can get.
     
    amruth11 and jaygatsby like this.
  6. amruth11

    Thread Starter Member

    May 24, 2011
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    Thx for the help.
    I can change my supply voltage to 6v or i can use a CMOS and its name is 7555CN.

    And thx for telling me about the resistor, i will add one.

    The circuit i am trying to make is when the IC sees a -ve voltage at pin 2 its output goes high and thats why iam using a 555 in monostable mode and the time it should stay on is for 3sec and so the motor should be able to run for 3sec. That's the circuit i have been trying to make and about the duty, do you think it be 50% or near that if i keep 27k instead. I am not good with MOSFETS and i dont have much knowledge about them.

    I also took a look at ur circuit and the amps it give is good but i coudnt tell for how long it will stay on but as i saw it because of the 10k resistor it will stay on for 1sec, thats my guess and if am wrong plz tell me.
     
    Last edited: Dec 19, 2011
  7. amruth11

    Thread Starter Member

    May 24, 2011
    49
    0
    I have attached a file containing all the on transistors an i think they are all available in my area. I thought about using a TIP121 with a base resistor of 2.6k. Will it work or if have any better ideas plz tell me.
     
    • NPN.doc
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    Last edited: Dec 20, 2011
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