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I am using a small 6V DC motor with a rotary encoder and TB6612FNG controller to drive it. I can start / stop / brake the motor using the controller no problem.

What I want to achieve is the following: When the motor is stopped and there is no electrical power to the controller, I want the motor to stay in "braked" state by default. For example, if the power is lost, I don't want the motor to be allowed to spin freely. I assume this would (probably) be achieved by short circuiting the terminals.

My questions are:

  1. Can TB6612FNG do it for me? I tried reading the specs sheet, but honestly it was a bit too technical for me.
  2. If not, what would be the best method to achieve this goal?

I am looking for the easiest and most reliable solution, have no problem to buy other controller for the task. I am also a programmer without much electrical engineering background, so I hope this question makes sense :)

thanks!

Karolis
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    1. Yes. Page 4. Look for short break. – winny Sep 03 '20 at 09:33
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    @winny Found it, but I am probably too dumb to read that diagram. Can you explain more how to get that controller leave motor terminals in shorted state by default? – Karolis Sep 03 '20 at 09:43
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    Depends what you mean by default. Before your (I assume you have one) MCU has booted? Weak pull-up on both IN1, IN2 and STBY. Default once your MCU has booted, make sure your program defaults to HIGH on said pins. – winny Sep 03 '20 at 09:46
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    @vinny Wouldn't it require power to be supplied to the controller for your suggestion to work? – Karolis Sep 03 '20 at 09:49
  • @Karolis, lazy me also don't like reading datasheets. I think the answer is NO, because ***no electrical power no brake***. AFAIK, only a step motor can give you a small brake, because a step motor stores magnetic energy in its permanent magnet steps/teeth which like to "'align" together and refuse to move, therefore sort of weak stops/brakes. But you can still use you big hand to move it, in other words, the brake force is small. – tlfong01 Sep 03 '20 at 09:54
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    Oh, THAT default :-) An NC relay? A depleation mode MOSFET? – winny Sep 03 '20 at 10:08
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    @tlfong01, I have manually shorted the motor terminals and I can definitely say that it does "brake" the motor. It is not 100% "locked", but it needs several times more force to be moved. This kind of braking would be sufficient for my needs, as the motor also has a 1:40 reduction ratio. – Karolis Sep 03 '20 at 10:10
  • @Karolis, lazily me reluctantly read the datasheet and found it hard to see what is going on in the short brake situation. https://i.imgur.com/QuQX00m.jpg. I see two arrows go inside the motor, but then what? Will they have a fight and crash the rotor and stator (don't ask me what they are, I only remembered something to scribble in my EE diploma exam paper, and I got my rusty diploma!) so motor is permanently damaged, can't move and so stop/brake for ever? Scratching my rusty head for already 3 minutes but got nothing. Ah jogging time, I will keep scratching my rusty head when I jog. Cheers. – tlfong01 Sep 03 '20 at 10:21
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    You can't load up a true DC motor without active drive to counteract the external torque. If shorted winding is good enough, your problems are greatly reduced. – winny Sep 03 '20 at 10:24
  • @Karolis, so it is not electrical force that brakes, but between gears' frictional force that opposites your big hands's biological and mechanical force that try to move the motor. So change gear ratio can change brake force. Now let me see what is going on: (1) Your head orders your hand to turn the motor. (2) The motor gears' frictional force fights back, generates heat and smoke. (1) It is like a car driver uses this foot to press the pedal, and the between wheel and brake drum friction that brakes. Good, no need to scratch my heard any more, not an EE problem, not my business! – tlfong01 Sep 03 '20 at 10:33
  • @winny, but what is the reason of short circuiting the motor? It it to generate heat, so the gears expand and cannot lock in smoothly and therefore brakes by mechanical friction? Dear me, I need to scratch me rusty heard again. Will come back after jogging to hopefully learn (or confuse) more about DC motor braking. – tlfong01 Sep 03 '20 at 10:39
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    @tlfong01 could you please stop spamming this thread with your head scratching / confusing comments? I am looking for replies from people who actually know the answer. – Karolis Sep 03 '20 at 10:43
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    @winny yes, apparently shorted winding is good enough for my needs, but I was hoping that there is a controller that can do it automatically without using external NC relays. With an NC relay solution, I need to open the relay before activating the controller, and I am not quite sure which one acts faster... It is probably possible to burn the controller if it acts before the relay had time to open. – Karolis Sep 03 '20 at 10:47
  • @Karolis, I am terribly sorry to have upset you. But I am very serious. I have been playing with DC motors driver for a long while, and I hope to fully understand its operation. Let me later show you my learning experience to hopefully convince you that I am not spamming at all. – tlfong01 Sep 03 '20 at 10:49
  • There you are, my a bit serious research work on motors and relays. I am indeed troubled by the potential EE pros' idea of short circuiting something to get a solution, without knowing why it works. (1) My motor learning experience: https://electronics.stackexchange.com/questions/510755/help-with-choice-of-dc-motor-speed-control-pwm-vs-regulator, (2) My relay learning experience: https://electronics.stackexchange.com/questions/505318/how-to-properly-use-a-relay-module-with-jd-vcc-from-arduino-raspberry. Happy learning. My apologies again. – tlfong01 Sep 03 '20 at 10:57
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    There may be one out there with built in depletion MOSFET for the very purpose, but in general these kind of before-startup-issues is where your EE skills come in since there is rarely off the shelf solutions for them. – winny Sep 03 '20 at 11:23
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    @tlfong01 Current is proportional to torque in a DC motor. Shorted winding with induced EMF due to external spinning -> counter torque to break whatever is trying to rotate it. – winny Sep 03 '20 at 11:28
  • @winny, I very much agree with you that if we don't understand any electrical circuit we have never seen before, the time has come to use engineering knowledge to apply to the new situation (that is what engineers are for!). Yesterday during my evening jog, I also thought about what you are inspiring us, as summarized: (1) We should not just focus on the present state, namely (a) Move forward, (b) Move backward, (c) Free running stop, (d) Forced (Short Circuit?) stop, / to continue, ... – tlfong01 Sep 04 '20 at 04:11
  • (2) We must also consider two states: the last state and current state, in our case (i) running state, (ii) stopping state. In running state, current was flowing, in next (stopping) state, MOSFET is switching off current, and there should be back EMF trying to carry on the current flow (to release enerny stored in the inductive motor) by a huge but short period current (across a flyback diode or a spark if the diode is not designed to short circuit in that direction). / to continue, ... – tlfong01 Sep 04 '20 at 04:17
  • This back EMF induced current to stop the running motor is what I never thought about, so I did not mention it in my long post on DC motor operation. I think we now have a long enough brainstorming chat and can take a break to start doing some research on the TB6612FNG driver, to see if we can solve @Karolis's difficult problem. Many thanks again for your inspiring idea on back EMF. Cheers. By the way, I think we are using an ***Agile approach**, without any final spec, but chatting every day to meddle through to a workable, but not at all fixed strategy to solve the problem. – tlfong01 Sep 04 '20 at 04:24
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    @tlfong01 I would like to kindly ask you to please stop helping me with this problem. Thank you. – Karolis Sep 04 '20 at 08:32
  • @winny, I googled and found that the short circuit design of braking a motor is not used only for motor controlled by full bridge drivers. The main idea is the following: (1) cut off current passing through the motor, (2) let the back EMF induced current passes through a forward biased NPN BJT, which is reversed biased and off in the previously state when motor passes current and moves. There is no flash back diode to short circuit the motor, otherwise the motor is always short circuited. / to continue, ... – tlfong01 Sep 04 '20 at 08:33
  • TB6612FNG datasheet says it has short circuit braking, implying that it uses this technique with perhaps some components inside the chip and is not disclosed. Anyway, I agree with Karolis I should stop at this point with the following explanation of short circuit braking design. Happy learning with you all. Cheers. Ref: Short circuit braking: https://i.imgur.com/lpMejl0.jpg. – tlfong01 Sep 04 '20 at 08:38
  • @karolis, my apologies again for upsetting you in the discussion. The discussion indeed inspired me to understand the short circuit design of braking a motor. This is my last comment. Have a nice project. Cheers. – tlfong01 Sep 04 '20 at 08:41

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