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555 PWM circuit

Discussion in 'Electronic Design' started by Vicente, Nov 18, 2007.

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  1. Vicente

    Vicente Guest


    I've build a PWM control following this design:

    But I made some changes:

    1. I don't know what the 10uF capacitor is used for, so I am not using
    2. As I want to use a high current motor (10A), I've changed:
    2.1: IN5408 > BY229-600
    2.2: IRF830 > BUZ11 with HEATSINK
    3. I guess that the 1K resistor in the BUZ11 gate is used to discharge
    the internal capacitor of the transistor (avoid over-heating), but if
    I use it there is no current in the MOTOR
    4. Capacitors on pin 6 and 5 are 10nF instead of 100 nF

    I will use a 12v 10A motor, and a 12v 20Ah battery, but for testing
    purposes I am using an standard battery car (12v 50Ah).

    The circuit is working fine but I am experiencing some issues:

    a) When the motor is not working at maximum I hear a high frequency
    b) With the 50Ah battery I saw a 30A peak current, and the transistor
    burned out. I am thinking about using a more powerfull transistor. Do
    I need to change it or not? Cause I will use a 20Ah battery in the
    final design.
    c) I will use this PWM in a three-wheeled electric vehicle. What do
    you think about the general project itself? It is enough with that
    battery and motor? Any other advice?

    Thanks in advance for your help.
  2. Hot Jock

    Hot Jock Guest

    If you don't understand the function of a component in someone else's
    design it would be better to keep that component in. If you didn't
    understand the purpose of the 555 in that circuit, would you have left
    that out?

    Hmmm, not entirely fine then.

    See my note above about missing out components from a working design.

    A 20Ah battery will probably peak at around 100A into a short circuit,
    so 30A is no problem. You will just get less service time from the
    smaller battery. This is probably caused by high frequency modulation.
    See my note above about missing out components from a wroking design.

    That really depends on how far and fast you wish to travel.
  3. Guest

    When I first tried experimenting with PWM circuits, my MOSFET (IRF530)
    overheated, and when it eventually failed, it acted as a short
    circuit: Motor Full Throttle On.

    This would have been... inconvenient... if this had been used as the
    speed controller of an electric vehicle.

    So put on some sort of emergency disconnect switch that's easily

  4. Vicente

    Vicente Guest

    Hi Michael,

    I´ve also noticed that :) but thanks for your advice.

    BTW Did you finished your project? Any more ideas?

    Thanks (and escuse me for my english)
  5. Vicente

    Vicente Guest


    I will try with that capacitor, but, do you know what is for? and can
    I use a "non-polraity" capacitor?

    On the other hand, what do you think about using 2 parallel BUZ11 (30
    A peak burnt out one)? How do I have to connect them?
  6. Nobody

    Nobody Guest

    The resistor between pin 3 and the gate is 10 Ohms, not 10K, right?
  7. Vicente

    Vicente Guest

    Yes, but the resistor which I had to remove was the one between the
    gate and ground (1K). If I use it, I never have current in the motor
    and I don't know if that is overheating the transistor.
  8. Vicente

    Vicente Guest

    Ouch! I had a mistake, in my circuit I am using a 10K resistor, and in
    the design it was10ohms. I will change it. THANKS!
  9. Nobody

    Nobody Guest

    It's for supply decoupling, i.e. preventing the switching current from
    causing noise on the supply lines.

    It doesn't need to be polarised, but polarised electrolytics are normally
    cheaper than the alternatives at large sizes (i.e. >1uF).
    You can connect FETs in parallel. Just connect all pins of the same type
    together (i.e. connect both gates to the 555, both drains to the motor,
    both sources to ground).

    But you need to look into why you're burning out a FET rated at 33A while
    driving a motor which you say is only supposed to draw 10A.

    If the motor is rated at 10A at no load, it could draw a lot more while
    spinning up. You should really think about adding current limiting.

    More generally, don't expect to be able to take a circuit designed for a
    couple of amps and just scale the components up to handle ten times that.
    At lower currents, some things can just be ignored, while they may need to
    be dealt with at significantly higher currents.
  10. Nobody

    Nobody Guest

    At 10 ohms, you have a voltage divider of 1010 ohms (1K+10R) total, with
    1000/1010 (i.e. almost all) of the voltage on the gate. At 10K, you have
    an 11K divider with 1/11 on the gate, so it's not turning on.

    With the 1K removed, the 10K gate resistance will greatly increase the
    FET's turn-on time, as the 555 can't source/sink more than 12V/10K = 1.2mA
    to charge/discharge the FET's gate capacitance.

    If you're using a relatively high switching frequency, the FET could
    be spending a significant amount of time in the linear region. It
    certainly isn't going to like passing 30A under those conditions.

    If you have access to an oscilloscope, examine the drain voltage. It
    should look very much like a square wave. If the rise/fall times are
    significant (i.e. it looks trapezoidal or, worse, triangular or
    sinusoidal), the FET will be dissipating significant power.
  11. Guest

    This circuit is useless for your application. There are numerous
    circuits around based on motor control pwm chips.
  12. Guest

    You've noticed that too? I thought it was just my lack of skill. And
    I thought your MOSFET was a little tougher than my IRF530...

    Put a heat sink on your MOSFET, regardless.

    I didn't finish my project - partly because of difficulty involved
    with coupling my motor to the bicycle gears - I was thinking of using
    a system of belts, then I got sidetracked (this wasn't my primary
    project, after all).

  13. Jamie

    Jamie Guest

    Insufficient Gate drive voltage.
    that's my answer..
  14. Phil Allison

    Phil Allison Guest

  15. Jamie

    Jamie Guest

    The problem is that you have used a Transistor that is requiring
    more Vgs voltage to get it in the low ohm state.

    This is the reasoning as to why it works when you remove that
    R. Also, more than likely this unit may have higher capacitance on the
    gate than the original and thus, is producing a longer propagation delay..

    The 10 Uf you removed is most likely there to help stabilize the Vcc
    line during high current pulses when the 555 output pulses on which
    also can cause slower Slew rate for the gate bias, along with causing
    the 555 to not time properly.

    On top of all that, the 555 output starts folding back around 100 ma
    if I remember correctly. If you were to view the gate positive
    transition via a scope, i'm sure you'll find the slow rate I am
    referring to.
    Also remember that a 555 sourcing on the output does not give you
    the Vcc rail voltage. I can't remember exactly what the drop is how
    ever, I can bet it's at least .6 or more volts below the Vcc.
    I guess if you were using a CMOS version you might not see this as
    much how ever, I think those versions also have lower output current
    handling. ( I could be wrong there, it's been a while since i've looked).

    When selecting the CW frequency, one should consider the raise
    and fall time of the device that is producing the PWM on it's output.
    This all adds to the problem of driving a fet..

    A nice high speed schmitt trigger driver would most likely help you
    out .
  16. Vicente

    Vicente Guest

    Ok, I wil try this new design, but can you explain me a little more
    why this design is better?

    Should I put a resistor between the gate and ground as I said in a
    previous post? (discharge the internal capacitor of the transistor)
    which value?

  17. Vicente

    Vicente Guest

    I am not sure about the motor ratings.

    I tested it and I have this values:
    No load: 1,5 A
    Normal use (conected to a gearbox and to a wheel and moving 80kg in a
    flat path): 10A

    I saw the 30A peak while trying to climb a small step.

    So what do you think is the best idea? Use a more powerfull
    transistor? Connect 2 BUZ11 in parallel? And what do you think about
    the new design that "Phil allison" posted some posts above:

  18. Phil Allison

    Phil Allison Guest


    ** The PWM frequency should be held constant ataround 1 to 5 kHz for best
    efficiency with small to medium power DC motors.

    ** LM555s can source and sink considerable current - so that resistor
    has little effect.

    I designed and built several PWM drives for DC motors for RC model boats
    using them - used BUZ11s as power switches as well, just by coincidence.

    Way back in the dim, dark, late 1980s.

    They all worked just fine - long as you kept damn salt water out of them

    ....... Phil
  19. Nobody

    Nobody Guest

    Add current limiting. If it pulls 30A while still moving, it could go even
    higher if the motor stalls. Even if you add enough FETs to handle the
    current, can the motor handle it?
    AFAICT, that's meant to produce a variable duty cycle with a constant
    frequency. Probably not a bad idea, but it doesn't affect any issues on
    the output side.
  20. ehsjr

    ehsjr Guest

    That looks like a good choice, modified with BUZ11's
    in place of the TIP31.

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