What diode to prevent damage to a motor shield wired with a switch between the battery +ve ? Regene

Discussion in 'General Electronics Chat' started by marcusob, Jun 22, 2016.

  1. marcusob

    Thread Starter New Member

    Jun 22, 2016
    7
    0
    Hi,

    Ive tried to find what rating of power diode to wire in series with an emergency shut off button, I'm going to be using a DPDT as I have 2 batteries on 2 circuits - the 11 volt battery to the motors via a robo claw motor shield (2X30A) and a 7 volt battery for an Arduino and logic power on the roboclaw.

    So I'll have 2 batteries via a DPDT switch to the robo claw motor input and lb input. To feed back any regen power when the motors are not being driven I need to have a battery connected to the robo claw - BUT the DPDT switch will stop a permanent connection (switch is on the +ve) so I need a power diode in series, what rating should it be ?

    I've got a 11.1 Volt 3S - 24C rated Battery X 5000mah (5AH) = 125 Max Current Amp Draw

    My motors have 12 volt 313 RPM Brush Motors The specs are here

    Operating Specifications:
    Operating Voltage Range: 6~12VDC
    Rated Voltage: 12VDC
    Rated Load: 4.5 kgf-cm (62.5 oz-in)
    Max No-Load Current: 0.52A
    No-Load Speed: 313 RPM
    Min. Stall Torque: 30 kgf-cm (416.6 oz-in)
    Max. Stall Current: 20A @ 12VDC
    Dielectric Strength: 250 VAC
    Motor Brush Type: Graphite
    Output Power at Max. Efficiency: 13W

    What diode do you recommend, and I just connect it in series with the switch on the +ve ?

    Capture.PNG
    I have a 11.1 volt battery, these are my motor specs https://www.servocity.com/html/313_rpm_hd_precision_planetary.html

    and here is the motor shield (its the 30A version) - http://www.ionmc.com/RoboClaw-2x45A-Motor-Controller_p_26.html

    Here is my battery https://traxxas.com/products/parts/batteries/idpowercellbatteries/lipo/2872X-5000mah-111v-3S-25C

    And here is my web site !!

    http://www.roboticsfordreamers.com/

    Thanks,
    Marcus
     
  2. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,129
    266
    What does this diode do for you?

    When the switch opens, the back EMF of the motors (assuming they are spinning) will always be less than the battery voltage.
    A diode there would never be biased to conduct- unless somehow you externally spun the motors at very high speed?
     
  3. marcusob

    Thread Starter New Member

    Jun 22, 2016
    7
    0

    I presume it allows the charge produced by the motors to flow back into the batteries, allowing run-off. Bearing in mind this isn't my idea, it is mentioned on lots of motor shield manufacturers recommended installations. without it they say damage may occur to the motor shield, hereis a quote from my RoboClaw (2X30A manual - motoshield made by ion ) - see number 3, the circuit diagram is how they stipulate the shield shuld be fitted if used with an emergency stop placed between the battery and the shield on the +ve.

    "Precautions
    There are several important precautions that should be followed to avoid damage to the
    RoboClaw and connected systems.
    1. Disconnecting the negative power terminal is not the proper way to shut down a motor
    controller. If any I/O are connected to the RoboClaw a ground loop through the attached I/O pins
    will result. Which can cause damaged to the RoboClaw and any attached devices. To shut down
    a motor controller the positive power connections should be removed first after the motors have
    stopped moving.
    2. A DC brushed motor will work like a generator when spun. A robot being pushed or turned off
    with forward momentum, can create enough voltage to power RoboClaws logic which will create
    an unsafe state. Always stop the motors before powering down RoboClaw.
    3. Powering off in an emergency, a properly sized switch and/or contactor should be used. Also
    because the power may be disconnected at any time there should be a path for regeneration
    energy to get back to the battery even after the power has been disconnected. Use a power
    diode with proper ratings to provide a path across the switch/contactor"
     
  4. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,129
    266
    Use a garden-variety 5A, 100V silicon diode then.
     
  5. marcusob

    Thread Starter New Member

    Jun 22, 2016
    7
    0
    I've had lots of different answers on various sites, and it sounds like no-one agrees on this at all. Lots of people say that the energy wouldn't be enough (motor would have to rotate more than the rated RPM otherwise no charge would be generated it would be consumed), and also that the charge generated would have to be more than that in the battery.

    I've had so many conflicting and aggressive answers I didn't think forums were this unfriendly, so fuck it.

    Just out of interest if there is charge floating around on a circuit, i.e. an alternator is generating energy and the only thing connected to it is a control board which is connected to a light bulb. But nowhere in the circuit is there anything currently consuming the power being generated, as the control board is off (and therefore the light), then does this energy disperse through heat in the cables ? What happens in a circuit if energy is being generated but not consumed ?
     
  6. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,129
    266
    There are two sources for this energy to consider:

    1) Mechanical energy of the rotating parts.
    If there is no regeneration path back to the battery, there would be no current, no power dissipated in the electronics- no harm done.
    The energy dissipates via mechanical friction.

    2) Electrical energy stored in the inductance of the motor windings.
    If the current in a coil stops abruptly, the energy stored will result in a voltage spike.
    This will be a relatively small amount of energy, it should be easily absorbed by the bypass capacitors on the driver board without damaging anything.

    In short, I really don't think you have an issue to worry about in terms of damaging your controller.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,532
    2,369
    As per you other forum posts, I don't think you grasp the concept of a DC motor when freewheeling or being back fed, if no load is present the voltage is still present just that there is no current path for it, identical in most ways to a battery with no load connected.
    The generated voltage has to be higher that the battery voltage for any current to flow to the battery when connected.
    Which for a 12vdc P.M. motor and a 12vdc battery is at a rpm higher than it is rated for.
    Max.
     
    Last edited: Jun 24, 2016
  8. marcusob

    Thread Starter New Member

    Jun 22, 2016
    7
    0
    Ok, so you are saying there is no point in putting a diode across the switch pins to allow regen charge from the motors to flow back to the battery, as there would never be enough charge for it to get into the battery. So what should I do with this regen energy I need to get rid of then ? Otherwise when the motors are generating this charge (freewheeling) it will be powering the motor shield when I want the motor shield to be off (otherwise I wouldnt have flipped the switch to off in the first place).
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,532
    2,369
    I am not familiar with those drives to comment on them, but I am familiar with DC drives, servo or otherwise.
    I am assuming you are referring to the point that the motors are turned off? If so how you do you anticipate the EMF from the motors going back into the P.S.?
    Normally these will be controlled with some kind of PWM switching Mosfets etc.
    And I would not anticipate the amount of back EMF generated would be detrimental to the drives, Unless Really over-sped by back feeding on power off.
    And yes, I still stand by post #7.
    You could always do a test and actually measure it.
    Max.
     
  10. marcusob

    Thread Starter New Member

    Jun 22, 2016
    7
    0
    Ok thanks, I wonder why the manufacturer of the motor shield is so adamant that the pcb must be protected from this regen, he must be confused then, which is weird as he is the guy who actually designed the board, so I think he knows his onions. As you say, I don't think I will worry about it anymore. I've got the diodes now, what else could I use them for :)
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,532
    2,369
    The only other thing that can occur is BEMF at the switching point of the Mosfets and for this reason reverse diodes are a part of Mosfet internal design, if still concerned, some drive designers place discrete diodes across the devices as a part of the drive design, which I would expect him to do if he knows his 'Onions'.
    Max.
     
  12. marcusob

    Thread Starter New Member

    Jun 22, 2016
    7
    0
    Yep he said there was something to protect voltage spikes already on the PCB. Here is what he said actually ...

    I wrote
    "
    So I got some advice from an electronics web site about the diode necessary for preventing charge floating around when the battery connection is cut from a switch. The people over there (who seem to be pretty clued up), think that the circuit diagram is wrong for the diode placement. Rather than having it on the +ve between the pins on the switch, it should be between the switch output on the +ve and the -ve battery terminal. So my question is, should the run-off/regen power from running the motors when disconnected flow back to the battery on the +ve terminal or the -ve, or does it matter ?"

    He responded
    "
    That would be a diode used to clamp an over voltage(If I understand your description). You would usually use a TVS diode for that(and we have them on some models of the Roboclaw just for that purpose).

    What we want is a simple one way path back to the battery if the switch is turned off or the fuse blows while the motors are spinning and energy is moving into the battery positive terminal we dont want to prevent that(bad things happen if the regen energy cant go anywhere)."

    Then he wrote
    "
    I think I already replied to this in another post but here it is again. In most cases the regen energy will not be anywhere near the stall current of the motor(unless you are driving down hill). You can guess worse case you will see 10amps from a 20amp stall current motor(and that is very hard to do). So you need a diode than can handle worse case about 20amp. But you dont need it to handle it continously. So you need a diode with a pulse curretn capability of about 20 amps. You may not find a rating for that though. So you have to look at the power disipation capability of the diode and then calculate the disipation at the maximum amps over the period of time. If the power rating of the diode is higher than that you are good."
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,532
    2,369
    I have no idea what he is talking about, especially in the last paragraph, "regen energy will not be anywhere near the stall current of the motor(unless you are driving down hill). You can guess worse case you will see 10amps" ??
    I would also ask as to why in the 100's of drives of all descriptions I have installed, I have never seen such a convoluted description/instruction to be carried out?
    Max.
     
  14. marcusob

    Thread Starter New Member

    Jun 22, 2016
    7
    0
    Haha, ok then best to stroll away from this then ! Have you got any other ideas for what I can do to put fuses in the circuit ? I was think of putting a 20 Amp (stall rating) between the motor and the motor shield board - in case the motor shield tries to drive the motors when they are stuck, and I was thinking of putting a 3A fuse between the battery and the shield - the shield logic side - which is seperate to the battery that drives the motors (on the +ve)
     
  15. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,129
    266
    It's really a non-issue! stop worrying about it. I agree that what the manufacturer told you sounds like confused gobledee-gook.
    It's no going to self-destruct without a diode.

    Just because someone designs something, it doesn't necessarily mean they fully understand every detail of what is going on.
    It would be very embarrassing for the manufacturer to admit that they don't fully understand what is really happening, so they'll just double-down on the nonsense.
     
Loading...