Need confirmation on whether circuit works

Discussion in 'Homework Help' started by kyousouhiro, Nov 27, 2009.

  1. kyousouhiro

    Thread Starter New Member

    Oct 13, 2009
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    Here is a circuit I drawn. From the picture, the left part is a PWM motor driver, running on a 12V 40A lead-acid battery. The right part is a VU meter but I thought of using this to act as a speed meter. When the potentiometer is turned, then the LEDs will light according to the voltage being sent to the motor. The question is, will this circuit work? I have two 82 ohms resistor to limit the current that is flowing into the meter circuit but I have doubts. The motor is used to push a transporting platform. It could take more than 10A if the load on the platform is heavy.
    [​IMG]
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Well, the LM3915 won't work very well for that. It's actually a logarithmic display driver; the LM3916 is the VU meter display. The LM3914 is the linear display.

    Actually - no. The 555 timer is controlling the gates of the power MOSFETs, which switch the motor ground path on and off.

    Sorry, no.

    D3 will get fried to a crisp in moments. A 1N4001 has a rating of 1A. It can take brief bursts of higher current for longer periods, but the average needs to be around 1A or less.

    If the motor has 10A or more current flowing through it, you should use a diode rated for over 10A. If the diode gets fried, the MOSFETs will die soon afterwards.
     
  3. kyousouhiro

    Thread Starter New Member

    Oct 13, 2009
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    oh, i see. thanks for the info. so i'll need to change D3.
    So meaning, I can't use the 3915 part and I can only use the motor driver part?
    Is there any other way I can use lower rating components for such an application? My lecturer said I can't get good grades with just a motor driver and the platform.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Yes. Also, your IRFZ44 MOSFETs will have a very, very long turn-on time due to the way they're being driven by only a 10k pull-up resistor. They will likely burn up very quickly.

    They should be controlled by pin 3, which is the output of the 555. It can source or sink up to 200mA when powered by 15v.

    Oh, I guess you want someone to do your homework for you then?

    This really belongs in the "Homework Help" forum.

    We can't do your homework for you, but can point you in the right direction.

    You won't be able to measure the speed of the motor directly.

    You might give an indication of the percentage of PWM though.

    Oh, there should be a 1k resistor between the 555 output pin 3 and the 100k pot R1.

    You will also need 100 Ohm resistors from the output of the 555 to each MOSFET gate.
     
  5. kyousouhiro

    Thread Starter New Member

    Oct 13, 2009
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    This is my final year project, so I thought it'll be in the project section.
    I've tried running the PWM and it seems quite fine for a while. Then there are smokes and I quickly turned it off. I assume it's the wire that's being burnt up. I was thinking of using the 3915 to indicate the percentage of the PWM as the motor receives 0-12V depending on the pot. I thought paralleling the motor with the 3915 would work. But the high current gave me some doubts and that led me here.

    Thanks for your help. Guess I'll post in the Homework Section now.
     
  6. BMorse

    Senior Member

    Sep 26, 2009
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    234

    You can use an op-amp to drive the input of a LM3916 VU Meter IC to get the "average" speed display. Use the PWM output of the 555 as an input to the op-amp..... you might need more passive components to get it to "average" correctly..... I used a similar circuit with my HDD activity Meter based on the LM3916 VU Meter IC..... But your display will only display what you have the speed set at, not the actual speed of the motor, you would be better off using a tachometer circuit to figure out actual motor speeds, also use 7 segment LED displays to display RPM values (this could get you a + on your points.).....

    My .02
     
  7. kyousouhiro

    Thread Starter New Member

    Oct 13, 2009
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    I'm not gonna show the actual speed. just the percentage of the PWM output voltage that goes into the motor. So, the user will know whether the motor is going fast, or slow. Hm... I'll research on what you suggested. Perhaps that may do. Thanks
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    I see. An understandable error. No, as long as it's homework, it belongs in the "Homework Help" forum.
    That's because you're using a 10k resistor to turn the MOSFETs on instead of what I recommended.
    No, the smell is from the MOSFETs burning up.
    I see. Well, you're trying to get input from +12v for the LM3915; that just won't work.

    Do some research on "differentiator and integrator circuits".

    You need an integrator circuit. This could also be described as a low-pass filter. It could be active (requiring semiconductors or ICs) or passive (only R,L,C components).
     
  9. kyousouhiro

    Thread Starter New Member

    Oct 13, 2009
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    Hm... I put quite large heatsinks with the MOSFETs and they are not hot, but I can't guaranty they won't burn.
    Alright, I'll research on what you suggested. Thanks a lot for your help.
     
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