DC/DC convertor

Discussion in 'General Electronics Chat' started by Dritech, Dec 3, 2013.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi,

    In a project that I am doing, I will be using three DC motors that operate at 12V and two stepper motors that operate at 24V. Is it possible to buy a single switching mode power supply (either 12V or 24V) and then use a DC/DC convertor to step down or up (depending on the supply voltage) ??

    Will a DC/DC convertor work for motors? If yes, which would be the best option; to buy a 12V supply and use a DC/DC to produce the 24V or buy a 24V and use a step-down to decrease voltage to 12V?

    Thanks in advance.
     
  2. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
  3. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    Yes, but remember that when the motor starts it will draw its stall current. You need to allow for this. Possibly by having a fairly large capacitor in parallel with the supply.

    Are your stepper motors really 24V? Most steppers are actually low voltage with low resistance windings that will need some kind of driver. This driver uses the motor winding inductance to step down the voltage (step up the current) to the windings by switching. So the driver is in effect a kind of switchmode supply.

    So the stepper motor supply voltage may not be critical.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,526
    2,369
    Stepper motor (should) draw their rated current both at rest and at motion, the reason for the higher than rated voltage supply is in order to provide the rated current as the motor inductance increases with rpm.
    This was done at one time with a series resistor, now PWM drives are the drives of choice using the varying PWM to maintain a mean level of current.
    With DC brushed motor control with either a linear or PWM drive, the DC supply should be at least 10% above the motor rated voltage, it is not that critical as the drive itself can prevent over-speeding of the motor, also ideally the drive should have current limit setting feature.
    Linear supplies are common for both, it is not necessary to use regulated switching supplies for either.
    If you get either a Toroidal transformer linear supply or build using a toroidal transformer, these are very easy to modify or add the secondary winding(s) in order to tailor exactly the voltages you need, these are simple basically 3 - 6 components for the whole supply.
    Max.
    Max.
     
  5. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    So I can connect any stepper motor with a 12V power supply? I was looking for a stepper motor with a torque of approx 2Nm. When I contacted the sellers on Ebay, they told me the voltage is 24V and that I will not find steppers with that torque at 12V. Is this true?
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,140
    1,789
    One problem you will have with DC-DC converters and Switch Mode Power supplies in general is that in order to supply current for a "load transient" like turning on a motor phase the voltage will collapse. Supplying this current from a capacitor can be done but designing the SMPS or DC-DC converter to handle this situation is a better option.

    Unless you have size or space constraints you are better off using a selection of linear supplies for each required voltage. Once you understand the motor requirements you can experiment with SMPS and DC-DC converters.
     
  7. Papabravo

    Expert

    Feb 24, 2006
    10,140
    1,789
    Torque is proportional to current. With steppers there is holding torque and running torque. Running torque is a maximum at low speed and goes to near zero at the motors maximum speed which is determined by the available voltage. The higher the voltage, the faster the current gets into the coils, and the faster you can run.

    You need to understand a great deal more than you do to succeed at motor control.

    Keep studying and good luck.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,526
    2,369
    The idea with steppers is to maintain the rated current, which is identical to the stationary or holding current at rest when the rated voltage is applied.
    If this current is not maintained, then the torque will drop or taper off proportionately.
    For e.g. a typical fig for a small stepper could be 4vdc @1.2a/phase, this would be the voltage applied at rest in order to obtain the rated (holding) torque, if this current is maintained, then the optimum torque is obtained throughout the useful rpm range.
    If you go to the Geckodrive Stepper drive site, there are design criteria that you can look up in order to design around the motor you have.
    Max.
     
  9. Jibby

    New Member

    Sep 12, 2013
    15
    1
    Best is to take any PC SMPS that will have +12 ,-12,com. For +12v use common & +12v. For 24 volt use +12 v and -12V
     
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