Plz help Crit. my 555-controlled dc motor circuit

Discussion in 'The Projects Forum' started by emc2k99, Jun 6, 2011.

  1. emc2k99

    Thread Starter New Member

    Mar 12, 2011
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    The intent is to control a 24v motor which has a 10amp capacity. Can anyone point out any flaws or ask questions to help me ensure this schematic is solid before I spend money on the components?

    Here is the schematic...
    Any comment or constructive crit is welcome. As always, thanks!
    [​IMG]
     
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    If the goal is to PWM control the motor speed then;
    C4 is much too large
    Q4 needs a resistor to ground to ensure good turn off
    D5 (and Q4) need to be rated >10A
    D5 and Q4 need a good sized heatsink
    7812 reg needs a large input cap, say 1000uF to gnd
    7812 reg needs a series input resistor say 470 ohms to decouple motor spikes

    That's just a start...
     
  3. emc2k99

    Thread Starter New Member

    Mar 12, 2011
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    0
    RB, I can't tell you how much I appreciate you taking a look at my circuit!
    ...yeah I forgot to switch C4 back down to 47uF. The sim program kept reporting an error with the cap being so low...so i raised it to slow the charge time, and the sim ran fine afterward.

    10-4 on both the amp rating for D4 & Q4 and the heatsink.
    I attached a resistor to the emitter of Q4
    I fixed C7's capacitance
    And added two 470ohm resistors in series but I'm not totally sure if they're in the right place.

    Question, is it better practice to attach a fuse to the ac side rather than dc side of the transformer, if not and I want the motor to peak at around 9 amps, should I try to find an 11 amp fuse on the dc side?

    Here's the revised circuit:

    [​IMG]
     
  4. wayneh

    Expert

    Sep 9, 2010
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    It might make sense to fuse the motor's current path separately from the regulator and 555 current, since these components are not protected at all by a 15A fuse. Putting a fuse on the AC side makes sense only to protect against the transformer from becoming a fuse itself. Since that can only happen if/when the transformer ruins itself, and since your AC supply probably has a 15A breaker anyway, I'm not sure that's too useful.

    Anyway, fuse application strategy depends on what you want to protect. I usually protect valuable or hard-to-replace components, and the users. Less concerned about the rest.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    You still have a major problem with C4 being so large. I calculate the operating frquency as 1.4Hz. What do you want this base modulating frequency to be?
     
  6. jerseyguy1996

    Active Member

    Feb 2, 2008
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    I admit I am an amateur but does this:

    mean to connect a resistor between the emitter and ground or between the base and ground? It seems to me that if you want to guarantee a good shut off of the darlington that it should be a resistor between base and ground.
     
  7. John P

    AAC Fanatic!

    Oct 14, 2008
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    The 2N6038 is only rated to 4Amps, and the 1N4001 is only rated to 1Amp. You're risking a fire with D1 and D2 in a 9Amp circuit, and D5 not much better.

    Also, I haven't worked it out, but what is the frequency of the 555 oscillator? If it's above say 100Hz, D5 not only has inadequate current-carrying ability, but it's too slow. You want a fast or Schottky diode. Actually given that the effect of a slow diode there is to give pulses of short-circuit current to the transistor, better use a fast diode whatever the frequency is.

    C7 doesn't need to be in the circuit at all. It's a fallacy to think that large capacitance on the output of a voltage regulator is necessary. Under what circumstances will the voltage there ever change?

    C1, C2 and C3 are duplicating each other's function. I'd just use one capacitor there, on the output side of the switch. You really don't want that capacitor charging up as soon as power is connected--and once you turn off the switch, it's going to stay charged a long, long time. Also, think about whether inrush current to the capacitor(s) will blow the fuse. Make it a slow-blow type.

    R1 is probably too small. You will certainly have that LED shining brightly.

    In today's diagram R5 and R6 are in a catastrophically wrong location!

    Just noticed--what's R4 doing? 2.2Ohm carrying 9Amps is a lot of power going to waste. In fact at full load, that's almost a 20V drop; it'll prevent anything useful from happening.
     
    Last edited: Jun 7, 2011
  8. Wendy

    Moderator

    Mar 24, 2008
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    I had missed R4,R5, and R6. They do nothing good for you. I already mentioned the frequency of the oscillator, it needs bumped up at least 10 fold, probably X100. Your capacitor can be around 0.1µF and it wouldn't hurt a thing.
     
  9. jerseyguy1996

    Active Member

    Feb 2, 2008
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    I'm pretty sure he meant to put R4 between the base and ground rather than the emitter and ground.
     
  10. John P

    AAC Fanatic!

    Oct 14, 2008
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    2.2Ohms from the base to ground? He'll never get the transistor turned on.

    And regarding the frequency, it's something to be cautious about. The higher it goes, the more time the transistor spends switching between full-on and full-off modes, and that means the more power it has to dissipate. There's a limit to how high the frequency should be, but don't ask me what it is.

    Anyway, why is this being done with an inefficient old Darlington transistor and not a MOSFET?
     
  11. jerseyguy1996

    Active Member

    Feb 2, 2008
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    No. The_RB in post 2 wrote:

    Then the OP responded with this statement:

    I admit that I am an amateur but isn't the procedure to ensure a "good turn off" of a transistor to place a weak pulldown resistor between the base and ground? I think that is what The_RB meant and the OP misunderstood. Of course 2.2 ohms is too little to be a weak pulldown but I was only commenting on the placement of the resistor, not its resistance.
     
  12. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Yes Q4 needs a BASE resistor to ground, sorry I was not specific enough.
    Darlingtons in particular need to ensure good turnoff. A 3k3 from base to ground will be fine.

    I would try a PWM frequency of about 800Hz. So try a value of C4 at about 0.1uF.

    My comment about caps on the regulator was to decouple the INPUT of the regulator from the motor DC.
    Put the 1000uF cap on the Vin pin to gnd. Then put a 470 ohm resistor in front of it. This means the voltage at the Vin pin of the regulator will be pretty much unaffected by whatever the motor PWM is doing to the PSU voltage.

    Re the fuse, I would prefer it between the big PSU cap C1 and the motor.

    And lose the two 470 ohm resistors in the motor power wire and the resistor in the Q4 emitter wire! :eek:
     
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