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LDR Motor Circuit

Discussion in 'Sensors and Actuators' started by pizoman, Mar 10, 2012.

  1. BobK

    BobK

    7,599
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    Jan 5, 2010
    You need a resistor between the '741 output and the base of the transistor. Try a 1K.

    Bob
     
  2. pizoman

    pizoman

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    Mar 10, 2012
    I made some changes to the circuit. Added in a 1K resister in between the output of the 741 and base of the transister as per BobK's tip. Also added in a diode in parralel with the motor and switched the POT from pin 3 to pin 2 of the 741 thus moving the LDR part of the circuit to pin 2.

    Now the circuit seems to be affected by the LDR when I cover it up. It doesn't stop entirely but just slows down the motor noticeably. I'm wondering if my POT (5K) resistance is not high enough to completely cut off the circuit. Also the transister seems to heat up as well, I think I need to switch it out for one with a higher current tolerance. Suggestions?
     
  3. john monks

    john monks

    693
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    Mar 9, 2012
    Sounds like the transistor and op-amp are destroyed. The transistor is only rated for 100mA. So I suggest replacing the op-amp and the transistor that is much more substantial. And you might try placing a 1kohm resistor between pin 6 of the op-amp and the base of the transistor and another 1kohm resistor between the base and emitter of the transistor. Pin 6 is probably two close to the negative rail. Most motors draw lots of current at start-up. That may be your problem. LED's normally have a series resistor with them. You should try a 330 ohm resistor in series with the LED.
     
  4. pizoman

    pizoman

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    Mar 10, 2012
    Hey guys,

    Tweeked around with the circuit again. Finally got it to work. The final problem was that the motor wasn't hooked up properly, I had positive to negative.

    The only problem now is the transister. I replaced the old BC108 with a BD139. The BD139 still heats up after 10 seconds of operation. The motor is still drawing too much current. I'm looking for another NPN transister that can handle more current. Maybe one of the TIP (132) series?

    (The motor has a stall current of 4.6A and nomimal current at 0.4A)
     
  5. jackorocko

    jackorocko

    1,284
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    Apr 4, 2010
    the collector current for a BD139 is 1.5A, that should be plenty for that motor. It could be heating up because you are running the transistor in its linear region. What exactly is the motor running?
     
  6. pizoman

    pizoman

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    Mar 10, 2012
    The motor is just running alone by itself right now. I plan to use it later in a model toy train.

    You said that the transistor (BD139) is running "in its linear region" which may be causing it to overheat, what exactly do you mean by that and how could I adjust the circuit to get the transistor to stop overheating?
     
  7. Rleo6965

    Rleo6965

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    Jan 22, 2012
    You should connect base resistor to limit current of base. This is to avoid to much current on collector to emitter. Try place 4.7K resistor.
     
  8. pizoman

    pizoman

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    Mar 10, 2012
    I already added in a base resistor from the transistor to the 741 output. It was a 1K resistor. I'll try a higher resistance.

    EDIT: Tried changing the base resistance from 1K to 4.7K. Results, motor didn't run, resistance was probably too high.
     
    Last edited: Mar 14, 2012
  9. Rleo6965

    Rleo6965

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    Jan 22, 2012
    Please try to measure current draw to dc motor. Apply dc voltage to the motor and place ammeter in series to ground. This will be our current reference for selecting transistor and base resistor.
     
  10. pizoman

    pizoman

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    Mar 10, 2012
    I get a fluctuating value from 2.8-4Amps so I'd say an average of 3.5Amps.
     
  11. Rleo6965

    Rleo6965

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    Jan 22, 2012
    That current reading was without load. You need around 10A of npn transistor to drive the motor.

    This was reason BD139 got hot. BD139 have only 1.5A collector current.

    Let's wait for *steve* to recommend required transistor.:)
     
    Last edited: Mar 14, 2012
  12. pizoman

    pizoman

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    Mar 10, 2012
    Who's "steve" by the way? Does he come on the forum often?
     
  13. Rleo6965

    Rleo6965

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    Jan 22, 2012
    He is a "super moderator" here in Electronics Points.
     
  14. Rleo6965

    Rleo6965

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    Jan 22, 2012
    The reason I recommend steve because I'm not really an electronic engineer. I'm only a Radio TV technician with 40 yrs experience in electronics and computer repair.:D

    I'm not expert in interpreting datasheets.:)
     
  15. davenn

    davenn Moderator

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    Sep 5, 2009
    Thats a long long way from the 0.4A you originally stated!!!
    the transistor I suggested was ideal for that, but definately not for ~ 3 Amps

    exactly how did you measure the current drawn by the motor ?

    you should be doing it like this .....

    [​IMG]

    The "A" in the circle is your multimeter in Amps range ... to be safe 10A range

    Dave
     

    Attached Files:

  16. davenn

    davenn Moderator

    13,247
    1,745
    Sep 5, 2009
    Realistically if its drawing ~ 4 amps or so, you are probably better off using a FET rather than a bipolar transistor

    Dave
     
  17. jackorocko

    jackorocko

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    Apr 4, 2010
    doubly agreed
     
  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
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    Jan 21, 2010
    Thanks for the flattering comments, but please jump in. I'm not an EE either :)

    I'd just be looking at voltage, current, power dissipation and drive requirements to get a short list and then (generally) suggesting the one that happened to be on the top of the list.

    Looks like davenn has offered a suggestion, and I'd normally not second-guess him.

    So I'd go to digikey, look up mosfets, select something capable of switching say 10A at 30V, and capable of at least a couple of watts of dissipation, and pick one. (oh, obviously N channel, and a logic level device sounds like a good idea.

    I get suggestions like

    NTD5867NL-1G, IPS105N03L G, FQU13N10LTU, and RFD3055LE.

    Really, there are hundreds of devices that would suit this purpose.

    I would also think very seriously about wiring the op-amp up as a Schmitt trigger in this application
     
  19. Rleo6965

    Rleo6965

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    Jan 22, 2012

    This is the reason I'm sticking here in Electronics Point. I'm really learning new things to me from you guys.
     
    Last edited: Mar 15, 2012
  20. pizoman

    pizoman

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    Mar 10, 2012
    Sorry guys, I retested the current from my motor as per Dave's instructions and get about 8 @ the 200m DCA setting on my multimeter.
     
    Last edited: Mar 15, 2012
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