restricying the current to a motor

Discussion in 'General Electronics Chat' started by domingojordan, Aug 17, 2014.

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  1. domingojordan

    Thread Starter New Member

    Aug 11, 2014
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    [​IMG]

    In the above circuit, how can i limit the voltage and the current that the mosfet provides to the motor. Is that done with the PWM?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Yes, it's controlled by the duty-cycle of the PWM signal.
     
  3. Sensacell

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    Jun 19, 2012
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    I see an Arduino connected directly to the AC mains- a very dangerous practice.

    Use an isolation transformer.
     
  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    The format of the circuit is not the best.

    But as far as I can see, Arduino is not connected to mains. They are sourcing 5 volts and providing ground from Arduino, but that is for circuit that comes after the rectifier.
     
  5. MikeML

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    Without transformer isolation, or optical isolation, it is a death trap!
     
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  6. tom_s

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    Jun 27, 2014
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    translation :) - the circuit is at mains potential meaning touching anything while plugged into mains can be lethal. we recently had a girl (AUS) that got killed talking on an iphone while it was being charged (electrocuted - 240vac).
     
  7. KMoffett

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    Dec 19, 2007
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    Sounds like an urban myth. Cellphone chargers are isolated switch mode power supplies. Do you have link to an "official" story?

    Ken
     
  8. Alec_t

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    The Arduino ground is only one diode drop away from mains potential, Shteii01!
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    May I remind you that the Terms of Service do not allow transformerless powersupplies.
    Use a transformer to isolate the circuit fom the mains.

    Bertus
     
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  10. shteii01

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    Ah, ok, yes, I can see that. I actually have/had very little to do with designing/studying power supplies so I need all the pointers and guidance I can get.
     
  11. tom_s

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  12. KMoffett

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    It would be interesting to see the coroner's forensics report. In those links one said it was a cellphone and another it was a laptop. I'm trying to imagine the electrical path from a failed charger to a laptop/cellphone (exposed conductive materials?) to a pair of headphones (never seen ones with conductive cases). Seems like there's something missing there...like she was sitting in the bath tub.

    Ken
     
  13. atferrari

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    Jan 6, 2004
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    Why there are no more people dying because of that?! Myself, I use the phone while charging quite often.
     
  14. domingojordan

    Thread Starter New Member

    Aug 11, 2014
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    In the belove circuit they are not using a transformer.

     
  15. domingojordan

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    Aug 11, 2014
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    Do I need to use a transformer?
     
  16. domingojordan

    Thread Starter New Member

    Aug 11, 2014
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    In the belove circuit they are not using a transformer. Do I need to use one?

     
  17. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I'm suspicious too. It would be incredibly difficult to get this current path (as shown in the news webpage);

    [​IMG]

    Because even if the cellphone charger went live the laptop chargers are fully isolated SMPS.

    Then like you said the earphones and wires are all insulated.

    And in Australia we are required to have ground fault leakage protectors in every home too. So that safely shuts off the 240v AC mains if more than a few mA flows to ground.

    For that scenario to have happened the cellphone charge would have needed to fault to live (AC mains active) and the laptop charger would need to fault to AC mains neutral, with no ground faults, and then the wiring or earbuds would need to be exposed and have an insulation fail. Sounds suspicious.
     
  18. tom_s

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    lots of varying reports of what happened and a lot of 'cheap' unapproved iphone chargers around. apparently got burns to her ears and chest so something did go live and she completed the circuit. looks like the property may not have had an earth leakage / RCD device on the mains. probably pre 1996 when it became compulsory.
     
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  19. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    YES, YES, YES

    First, read this Wiki, which discusses the power distribution grid. There are differences in different countries, but this description likely covers your specific situation in your country...

    I have prepared the following diagram and simulation. The box on the left represents the Electrical Distribution Grid. I show it as a 230V AC voltage source, one side of which (doesn't matter which) is referenced to earth ground. This is intrinsic in your country's grid. All voltages shown are with respect to earth ground.

    Look at the voltage at the node "hv" V(hv), green trace. Note that is always positive. It is a half-wave rectified wave, with a peak voltage of 325V (1.414*230V). Now look at V(arduinognd), red trace. Note that it is the mirror image of V(hv), but delayed by 180 degrees. Finally, look at the current that flows through R1, I(R1) blue trace. It is as you expect, the normal full-wave rectified waveform.

    The reason that this circuit is a death trap is because the instantaneous voltage on the ArduinoGnd side of the circuit goes to a peak of -325V. Imagine that you have the USB cable plugged between the Arduino and your computer. Now either of two very bad things can happen:

    All desktop computers I have ever worked with have a direct connection from their metal box to earth ground. This is shown in schematic as the dashed line. The USB plug 0V pin is connected to the computer's metal box. The instant you plug this in, there will be a huge explosion, with a huge current flowing through the Arduino, along the USB cable, through the computer to earth ground. You will blow something up...

    If your computer is a laptop, which might have a transformer-coupled isolated power supply, you may avoid the explosion described above, but you have now connected the exposed metal parts (d-sub shell, video connector shell, modem cable, usb cable, network cable, phone line, touch-pad) of the lap-top computer to -325V.

    If there are any grounded peripherals plugged into the laptop, chances are they get back to earth ground some other way, and you are back to the explosion. If nothing touches earth ground, then the metal parts of the laptop, and all thing plugged into it are hot... Imagine your kid coming to use your laptop to play a game...

    Sorry to be blunt, but if I have to explain this to you, you have no business playing with transformer-less, off-line circuits...
     
    Last edited: Aug 19, 2014
  20. Alec_t

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    Sep 17, 2013
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    As MikeML says, most definitely YES!
     
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