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Thread: Would running AX-12A on 14-15V Power Supply Break It?

  1. Would running AX-12A on 14-15V Power Supply Break It?

    I want to use a 15V (resistor to 14.8V) power supply to power a MX-28T daisy chained with two AX-12As. I can't think of any good way to power the AX12A's with 12V and the MX-28Ts with 14.8V. Would it be terrible if I just ran them all on the same 15V dc power supply?

  2. #2
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    Re: Would running AX-12A on 14-15V Power Supply Break It?

    It is actually risky to power the MX-28 from >12V, so better to power everything from a single 12V source.
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  3. Re: Would running AX-12A on 14-15V Power Supply Break It?

    Quote Originally Posted by tician View Post
    It is actually risky to power the MX-28 from >12V, so better to power everything from a single 12V source.
    In this document from trossen http://www.trossenrobotics.com/image...ixel_guide.pdf it says that he nominal voltage is 11.1-14.8V

    I need every bit of torque I can get out of the MX-28T

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    Re: Would running AX-12A on 14-15V Power Supply Break It?

    Speaking from experience, the MX servos are less accommodating of higher voltages than the RX servos they replaced. If you run at >12V and there are any significant voltage fluctuations due to things like inductive kick in the power wires (rapid changes in torque), there can be severe damage to the servos. It is risky enough using a 4S LiFePO4 battery pack (14~11V during most of charge), but using a 4S LiPo battery pack (14.8V nominal; 16~14V most of charge) is asking for trouble.

    If you are hitting the torque limit of the servo, then you likely need to change the design to decrease torque requirements and/or go with a stronger servo.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
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  5. Re: Would running AX-12A on 14-15V Power Supply Break It?

    Alright, I'll try to stick with a 12V dc thanks

  6. #6

    Re: Would running AX-12A on 14-15V Power Supply Break It?

    I burned a number of MX-64s running at almost-fully charged 4S LiPos (15.8V, give or take.)

    It _can_ be done if:
    1) You make sure it's never over 16 V
    2) You make sure the servos are never forced to work harder than designed for (no stall/block)
    3) You don't rely on the over-load or over-temp for safety -- the motors will burn out way before they kick in

    Basically, you can't actually _use_ the torque that you get from the higher voltage for any more time than dozens of milliseconds.
    The motors in the MX servos are rated at 12V from the manufacturer.

    When you say you "need all the torque," I hope you're still designing for the 1:5 torque-to-stall ratio that is safe for the motors? If you stall them for any significant amount of time, or have an average torque load that's more than about 1/5th of the rated stall torque, you will damage the servos.

    Finally, you say "resistor to 14.8V."
    A resistor is a terrible way of trying to drop voltage for load circuits, because the voltage drop is proportional to current.
    When little current is flowing (such as during idle,) the voltage will drop very little.
    When a lot of current is flowing (such as during heavy load,) the voltage will drop a lot more.
    Getting a power resistor that won't burn up when you're drawing several amps through it is expensive, bulky, and warm.
    If you need to regulate voltage, use an appropriately sized regulator (linear or switching,) and large buffer capacitors. (This is basically what a power supply is.)

    I second ticians recommendation: Use a 12V power supply (ideally in parallel with a 12V battery, such as a 3S LiPo charged to 4.0V per cell, for bulk current reserves.)
    Last edited by jwatte; 11-23-2016 at 02:47 PM.

  7. Re: Would running AX-12A on 14-15V Power Supply Break It?

    How can I make sure it's never over 16V? If I'm using a 15V dc power supply, wouldn't that mean it cant ever go over 16V?

    I am designing for the 1:5. I don't want to make a complex power circuit but I anticipate it would be useful to have some torque from the 12-15V range JUST to accelerate. Would it be wrong to just buy a 15V supply like this http://www.digikey.ca/product-detail...410-ND/3094936

    Also could you explain "Bulk current reserves"? The unit is always going to be plugged into the wall, or a relatively stable power source. Is having it in parallel with a battery really necessary for this application?
    Last edited by memmerich; 11-23-2016 at 02:57 PM.

  8. #8

    Re: Would running AX-12A on 14-15V Power Supply Break It?

    If I'm using a 15V dc power supply, wouldn't that mean it cant ever go over 16V?
    Depends on the design of the power supply. Cheap switching power supplies can swing quite wildly based on load stepping. Check the data sheet to be sure. Or add your own bulk capacitance to compensate. That's what a battery is -- almost infinite bulk capacitance!
    A couple of 25V 4700 uF capacitors can help as well. Preferrably, further away from the power supply, closer to the first servo in the chain, to be better at regulating voltage swing caused by inductive kick/swing, both in wires and in motors.
    The power supply you linked does not show transient load response, but it shows "5%" for line/load regulation, and "150 mV p-p" (which is 1%) ripple. If those are absolute maximums, and you trust the Triad brand of Chinese power supply manufacturers, then the power supply will not swing above 16V.
    What will happen closer to the motor, on the other end of those thin inductors we call "wires," is a different question. The motor will put out significant noise on the wires. Large capacitors can help, but you really need to measure it with an oscilloscope under load to be sure. (And, of course, this may end up showing you that "it wasn't safe" and let out the magic smoke.)

    Anyway, AX-12s don't like 14.8 Volts at all. They were designed for 9.6V Ni-MH cells that go to 11.6V when fully charged, and were robust enough to run OK at 11.1V 3S LiPo cells that go to 12.8V when fully charged. I don't know what the voltage regulator on the board is rated at -- Many LDO regulators blow above 14V-16V input. That may be the limiting factor for the AX-12 servos.
    MX series were specified at up to 14.8V (where 14.8V is 4S LiPo nominal voltage) but we've deduced from history that the 12V precision Maxxon motors used have less margin for error than the cheaper AX motors.

    If you want to drive both AX-12 and MX-28 from the same power supply, get a linear voltage regulator to drop the AX-12 supply voltage to 12V or below. A LM-350 with sufficient heat sink and a couple of electrolytic capacitors ought to do it.

    I'd get a 12V power supply and a couple of 4700 uF capacitors and power the entire system with.

  9. #9

    Re: Would running AX-12A on 14-15V Power Supply Break It?

    Also: A MX-64 is only 25% more expensive than a MX-28, yet has 250% of the rated torque.

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