Power supply question.

Discussion in 'General Electronics Chat' started by Hamish911, Dec 14, 2009.

  1. Hamish911

    Thread Starter New Member

    Dec 14, 2009
    2
    0
    Hello!

    I've recently built a few stepper motor drivers, this design: PICStep.
    While looking for a suitable power supply to feed the (bipolar) steppermotors (I've got 2 types, see attached datasheets for info),I found a couple of thoroid transformers. Input voltage 220v AC, output 42v AC, 60VA. After a rectifier and 2 capacitors, I am getting ~60V DC. Under load this drops a few volts.

    The Steppermotor drivers are using LMD18245's. These IC's are specified to run at 55v, 3A continuous, 6A peak. I've limited the PICStep drivers to 1A by using a 20K resistor. (seems like a safe value for these steppermotors to start with.)

    My question is, are these PSU's (I have 3 throdoid transformers + rectifying circuit) suitable? Is the 60VA rating sufficient? I feed 'm with linepower, which here 230V. (The Netherlands). What load is safe? Can I amp the stepperdrivers higher than 1A?

    Regards,
    Hamish
    Amsterdam
     
    Last edited: Dec 14, 2009
  2. steinar96

    Active Member

    Apr 18, 2009
    239
    4
    If your transformer output is 42V, then your maximum amper rating for the transformer would be 1.42A. Ofcourse you never want to run components very close to their maximum for a long period of time so i would say you could get 1A reliably from the transformer.
     
  3. Hamish911

    Thread Starter New Member

    Dec 14, 2009
    2
    0
    thanks for your insights!

    As you can imagine I would like to get as much torque and speed from the steppermotor at safely possible.

    The output voltage of the AC transformer is a little higher as the input here is 230V. That gives an output of ~44V AC. After rectification I measure ~60V DC. With the steppermotors turning it drops a volt or 2, and with a artificial load (e.g. holding the shaft tightly, can't stop it) it drops another few volts.

    The 1A value I've set the board to limit the current to the LMD IC's to seems like a safe bet (with the transformer specified 220AC/42AC/60VA).
    (60V DC max, 1A).

    I suspect that under a real load, when actually performing labour, the DC voltage will drop a little, pushing 'm towards 55V.

    Perhaps someone can explain why the voltage drops in the first place?

    I've checked manually (sensor: fingertip), to see how hot each part gets.
    Only the motors get hot, but not too hot to touch. The rest of the parts (thoroids, rectifiers, stepperdriver IC's) stay luke warm.

    As you probably can deduce I've not had the opportunity yet to see how this setup performs under working conditions. Some mechanical parts of the setup still need to be put together, and on my shoestring budget that takes a while.

    Thanks for your help so far!
     
  4. steinar96

    Active Member

    Apr 18, 2009
    239
    4
    The voltage from the transformer is 42V AC. The filter capacitor just past the diode bridge is set there to keep the DC voltage as high as possible while the AC waveform drops to about 0V between the AC voltage half waves.

    AC voltage is measured in rms values. The peak of the AC wave is actually 42*1.41 = 59.3V.
    When you initially turn on the rectifing circuit the capacitor is charged to the maximum of the AC waveform peaks which is 59.3V and pretty much stays there while there is no load.
    When you connect a load the capacitor supplies current when the AC wave is between peaks (when the AC waveform is between peaks the power it delivers is zero since there is no voltage) the capacitor alone supplies power to the load at that time. And while it supplies power to the load the voltage across it drops since its spending energy to keep the voltage up.
    Next when the AC waveform is above the capacitors voltage (when it has risen again) the capacitor charges back up and stores power which is then again used to keep the voltage up when the AC wave drops down again.

    So as you increase the load the voltage the capacitor maintains between peaks drops faster. The voltmeter measures aprox the average voltage so the effect is that your voltage reading is lower.
     
    Last edited: Dec 15, 2009
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