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12v rail vs 5v rail computer psu

Discussion in 'Power Electronics' started by kellygoose, Jun 16, 2015.

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  1. kellygoose

    kellygoose

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    Jun 16, 2015
    I'm using a spare cordless drill motor to turn a threaded rod to move a project of mine. No heavy lifting here just raising and lowering a 15a mitre saw motor on a link frame I built. I got a working fine 380w psu out of a pc, new style atx. The psu is rated at 16a for 12v and 36a for 5v. The drill motor was originally powered by a 19.2 volt battery. In the application I'm using, the motor will rarely run and only for short periods. I tried to use the 12v rail to power it, and it shuts the psu off immediately after opening circuit. (I'm still using the trigger mechanism to control power to the motor, so voltage is regulated by the trigger) Doesn't matter how little or far I crack it open, the psu shuts off. So I tried the 5v rail and it works fine under all conditions. It satisfies me because it works but a bit more speed would be nice, i.e. 12v or higher.

    I'm curious as to why the 12v doesn't work and the 5v works fine. Seems to me if you do the math as far as voltsxamps the wattages are similar and even favor the 12v rail. What am i not understanding here?

    Anyways this will work fine for me until I find a microwave in the trash and can get higher voltage that will handle more current.

    Thanks for any and all replies..
     
  2. Martaine2005

    Martaine2005

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    May 12, 2015
    Hi Kellygoose,
    I think you answered your own question..
    How many amps is the 12v?
    How many amps is the 5v?
    Have a read with this link here.
    Martin
     
  3. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Are you sure you connected the +12 V and not the -12 V? The +12V on newer PSU can be good for 16A. The -12 V is only good for about 0.8 A. Just thought I would ask.
    Adam
     
    Martaine2005 likes this.
  4. Martaine2005

    Martaine2005

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    May 12, 2015
    Never thought of that...Although his chop saw would have gone down, rather fast. Instead of up!
     
  5. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Depends on which way he had his drill to start with :) Or he may of had the direction switch the other way.
     
  6. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Ah no just read the thread again. If the saw was down connecting it to -12 Volts would have pushed it further into the bed causing the drill to stall and the PSU to limit at 0.8 A, possibly.
    Adam
     
  7. Martaine2005

    Martaine2005

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    May 12, 2015
    Hi Adam, you know I love you:rolleyes:
    The switch does reverse the polarity to the motor. And also includes all the the electron thingy's. too
    My drills (Dewalt) have exactly that.Plus for forward and negative for backward.

    I would be back on for why does 5v work and not 12v?
    I apologise as my stella's are getting warm.
    Martin
     
  8. kellygoose

    kellygoose

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    Jun 16, 2015
    Thanks for replies,

    it's on +12v for sure. Regardless of up or down, the psu shuts off immediately.. Doesn't make sense to me either...


    Using all the +12v wires as well as all the +5v wires in both situations.
     
  9. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Kiss Kiss! I guess he used -12V and not +12V. So ok he has the switch in the right position. 5 Volts makes it go up, yep no probs their. So he brings the saw down again. This time he connects -12 V and the drill tries to go down, but it's already down so it stalls and the PSU current limits. If this is not the case then I guess the PSU +12 V can't supply enough current (surge current)??
    Thanks
    Spurt!
     
  10. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Bingo...... Looks like on load they can draw considerable current....... back of the net

    Adam
     
  11. Minder

    Minder

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    Apr 24, 2015
    If no kind of controller is used and the motor is connected directly across any supply, the current is only limited by the resistance of the armature, (very very low) until the generated DC comes into play, there will be very high current until the rpm becomes stabilizes the load.
    This initial hi current is enough to kick out the supply.
    M.
     
  12. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    I think most modern drills have controllers in them (PWM), and even on low speed under load I am guessing this is still too much current for the PSU.
    Adam
     
  13. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    Add a 3R3 10watt resistor and let us know
     
  14. Martaine2005

    Martaine2005

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    May 12, 2015
    Sorry Colin?
    A big resistor?
     
    Last edited: Jun 17, 2015
  15. Minder

    Minder

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    Apr 24, 2015
    ..and even with PWM, unless you bring the rpm up slowly, it can still be full voltage if maximum rpm is called for initially.
    M.
     
  16. Martaine2005

    Martaine2005

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    May 12, 2015
    I thought this forum was to learn to and ask questions?
    You Colin seem to think this is your playground . 'just do this..just do that'
    You may very well be correct! But instead of the 'I know best and do it' attitude,
    Please explain yourself. If you don't want to participate then don't...
    Do you believe this is helpful?
     
    davenn likes this.
  17. kellygoose

    kellygoose

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    Jun 16, 2015
    Thanks for all the quick replies this forum has already made a good impression on me. You can't type an entire paragraph into Google (yet) and get the help you need.

    I am using a controller, only using +DC out of the push no -DC whatsoever.

    Ok, I see what you mean, but what I'm not understanding is even with the controller barely engaged, or trigger barely squeezed in the case of the original application, it shuts off the psu. I don't yet fully understand these speed controllers, but when I test voltage after the controller, the result is simply a varied voltage output based on the amount of trigger depression. From my understanding this is not truly a speed controller but simply a regulator. A true speed controller would have some sort of input from the motor regarding its load or rpm, the type found on high end routers or machining equipment, not a cordless drill. That being said, I'm still confused why it pops with 12v when I'm easing into it.
     
  18. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    That's true enough M....a valid point.
    Adam
     
  19. Myhay

    Myhay

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    May 18, 2015
    maybe adding a consumer on 5V rail will not send the PSU into protection...
    Install a light bulb of 1-2A on +5V and then try to connect the motor on 12V rail...
     
  20. Martaine2005

    Martaine2005

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    May 12, 2015
    Hey Kellygoose, sorry I was offline.
    The point I was making is the 5v rail has more amperage. Hence the slower motor speed. As all have said here already, the drill motor probably draws a considerable amount of current to even start turning..
    The above suggestion has some merit too.
    Some power supplies (ATX) wont output some voltages until they have a load on the reference voltage. (usually the 5V rail)
    Martin
     
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