Transistor as a switch

Discussion in 'Electronics & Electrics' started by kizzap, Sep 25, 2007.

  1. kizzap

    kizzap Member

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    Hi guys, I've got a circuit that I am trying to get working (circuit below). The idea behind the circuit is that when the switch is pressed once, the 555 outputs high for 10 seconds, as given by the 9k1 resistor and 1000 micro farad capacitor.

    I know that the circuit works up to the point where the motor is turned on. I have proved this by wiring in a resistor in at the output. However this is where I have encountered problems.

    When I connect the transistor/motor in, the circuit will not last for the full 10 seconds. I cannot find any reason for this at all. placing a LED/resistor in instead of the motor and the circuit works fine.

    Circuit:
    [​IMG]
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    Few more notes, I know that the Vcc says a 15v supply, however i have dont have access to a steady 15v supply, so I have been using either 9v batteries, or a 12v supply from a computer.

    Also there is also a 10k resistor between the output of the 555 and the base of the transistor.

    the motor itself is classed as a "4.5v" motor, however, It is running off the same supply as the chip.

    Things I have tried:

    Using a separate power supply for the motor, differentiating it from the chip circuit.

    connecting an LED in parallel with the motor, showing that the current is slow to rise and at maximum current, or when the LED is brightest, it suddenly cuts out. this leads me to believe that the transistor cannot handle the current.

    Anyone got any ideas on what I should be trying next? Any input would be appreciated.

    Thanks
     
  2. chief4

    chief4 Member

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    might be a good idea to run the motor outside of the circuit and measure the current to see if it exceeds the transistors max current rating. Also reduce the value of the base resistor and see if that makes a difference. Try 1K.
     
  3. underskore

    underskore Member

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    am i going crazy, there is no base resistor? dont you need to control the base current or else it can go into some crazy thermal breakdown situation.
     
  4. chief4

    chief4 Member

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    he said he is using a 10K base resistor. I was thinking the transistor may be overheating. reducing the value and driving it into saturation might help.
     
  5. underskore

    underskore Member

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    well besides brushing up on basic electronics it looks like i need to read posts aswell :wired:
     
  6. OP
    OP
    kizzap

    kizzap Member

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    i actually had another look at the circuit, and realized that I had a 470 ohm resistor connected to the base of the transistor, so i swapped up to the 1k resistor but still no change. I attempted to measure the current (assuming you meant from the power supply with no added resistance) and it would definitely be over 10A.

    The transistor is overheating, before and after the change of base resistor,
    would this be a sign that the motor is drawing too much current for the motor and just cutting off to protect itself? and if so, how would i go about fixing this issue? also, how would I go about dropping the voltage to the motor? just use a 5v supply to the motor instead? or would my only choice be to use a larger transistor that can handle the current?
     
  7. underskore

    underskore Member

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    have u tried a 10k as the base resistor, i might be mega rusty with this basic electronics but if you lower the base current wont it also lower the current going thru the transistor thus making it less likely to over heat?
     
  8. chief4

    chief4 Member

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    10A? Thats a pretty big motor then. Your BC547 is only rated for 0.1A. You will need a much bigger transistor. Might be worth using a relay for that much current.
     
  9. OP
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    kizzap

    kizzap Member

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    I tried with the 10k resistor and the motor wouldn't even start highest value i could get it to start was 3k8

    it shouldn't be, it is basically one of those motors you get in toy cars...about 1 inch long
     
    Last edited: Sep 25, 2007
  10. dakiller

    dakiller (Oscillating & Impeding)

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    You need enough base current to keep the transistor in saturation if you want to use it as a switch like this, meaning the transistor wants to let through more current than the motor will use, but if you lower the base current and the transistor falls out of saturation, its going to limit the current through the transistor and the motor acting like a resistive load is going to have a smaller voltage drop across it because of the lower current it now gets, this now remaining voltage that the motor doesn't get now is right on the transistor and it will get hotter

    With a 10k base resistor, your only going to let through around 150-200mA which probably isn't enough for your motor. Test the motor by itself and measure what current it uses and then set your base resistor to put the transistor into saturation by about twice the motor's current, about the 2kohm mark I'd guess
    BAD BAD BAD!!!! You don't measure the current without a load in series with the meter. You essentially shorted out your power supply there. You want the current through the motor, while it is powered
     
    Last edited: Sep 25, 2007
  11. Zevensoft

    Zevensoft Member

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    Correct me if I'm wrong, but shouldn't the load go between the emitter and the ground, in the case of using the transistor as a switch? In the current configuration it would seem you're "shorting" the transistor.
     
  12. OP
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    kizzap

    kizzap Member

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    tried a 2k and same heat issue. how should that ammeter be connected? a or b below, i was under the comprehension it should be a.

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    should it be b?

    for a pnp it should be on the other side
     
  13. dakiller

    dakiller (Oscillating & Impeding)

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    No, its correct how it is for an NPN transistor, if you used a PNP then you would connect it the way you said
     
  14. Goth

    Goth Grumpy Member

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    A for current, yes. B for measuring the voltage across it.
     
  15. Zevensoft

    Zevensoft Member

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    Ahh yes you're right, I didn't see that there, too used to PNP's.
     
  16. dakiller

    dakiller (Oscillating & Impeding)

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  17. dephilile

    dephilile Member

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    If you are switching 10amps it might be easier to use an N-channel mosfet. Instead of providing a base current all you need to do is charge and discharge the gate capacitance (in the order of pF).
     
  18. Zevensoft

    Zevensoft Member

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  19. OP
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    kizzap

    kizzap Member

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    So what you are saying is that circuit a is the correct circuit, however, if this is right, my multimeter cannot measure the current flowing as it goes above maximum range of 10A, this is with a 1ohm i think 1-2W?( i know...useless), or around 180mA with a 15ohm 1/4w resistor,
     
  20. chief4

    chief4 Member

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    if your measuring the current draw of the motor correctly (sounds like you are), then it is way too much for the transistor you have chosen. You simply need to get a bigger transistor.
     

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