Create a short between a dc motor pins TRIAC

Discussion in 'General Electronics Chat' started by byemi, Jun 21, 2016.

  1. byemi

    Thread Starter New Member

    Jun 21, 2016
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    Hello all,

    Did anyone made an experiment by using a TRIAC to create a short between the pins of a DC motor ? This kind of situation it is needed in an application where the pins of a DC motor need to be shorted, when the RPM of the motor is increased. For the moment I don't have any data or information about the motor. All I know is that it is a DC motor, with the a coil resistance of 0.1 ohm. The main idea is to create a short between the motor pins when the RPM (or voltage) is reached. My idea was to use a TRAIC component, and this because it needs to be a compact and cheap solution.

    Waiting for ideas and probably solutions. In case some other information are needed please let me know.
    Kind regards, Emi
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    A triac is a possible solution for making a low resistance connection between two points.
    With more information we can give better answers.
     
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  3. Tonyr1084

    Active Member

    Sep 24, 2015
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    Welcome to AAC (All About Circuits).

    Can you tell us more about what you want to accomplish? From what you describe it sounds like you want a motor to reach a specific RPM then shut down. And the way you want to do that sounds like you're wanting to build a circuit that will self destruct when it does.

    Assuming your DC motor has a permanent magnet, while spinning in either direction, shorting the leads together will form an instant "Dynamic Brake" system where the short forces the motor to literally stop spinning in under a half second. Such dynamic braking systems are used in power equipment like power saws where when you release the trigger the motor stops almost immediately. The purpose is to prevent possible injury from accidentally contacting a sharp spinning blade. I've also seen dynamic braking on kids ride on cars. When the child releases the pedals the motors go into dynamic braking to stop the forward (or reverse) motion.

    If you want to limit the RPM of a motor you'll need a completely different approach. Using a triac (typically used for AC applications) - once triggered, should shut the system down. The potential is for damaging your motor control circuit. Shorting its power out will overload the power source and likely burn out components, possibly burn wires and potentially damage or destroy your power supply.

    When I was young (about 13 I think) I had an old car battery. My dad had removed it because it was no longer able to start the car. So experimenting with it I took a 22 gauge wire and connected it directly across its terminals. The wire immediately burned my hand and absolutely burned all the wire inside the insulation to tiny balls. That day I learned about the power of a 12 volt car battery. And that was with a dead battery. Imagine what a good battery would have done.

    Tell us what you have and what your goal is and perhaps we can help you find a solution.
     
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  4. byemi

    Thread Starter New Member

    Jun 21, 2016
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    Hello everyone and thank you very much for helping

    Thank you Tony for the keyword you just mentioned..."dynamic brake"...This is what I want to obtain. The purpose of this test is to test a spring by compressing and then let the spring extend it self. The system is as follows : A dc motor, a gear box, a threaded spindle which passes along the spring. I want to compress the spring until it reaches it's maximum compression length, by using the dc motor with the entire system ( gearbox + threaded spindle) and then decouple the power supply from the motor and here the problem appears. The spring is very powerful 50N/mm...this means that with with every millimeter the forces are added...when the spring is maximum compressed the forces on the spring is somewhere about 1kN, and I remove the power supply from the motor destroys the gear box and everything from this system. The cheap way of doing this test was to somehow short the pins on the motor, in other words make a "dynamic brake "...

    I hope that I've been understood what I want to do. I didn't made any sketches to show you, but Tony i think you know what I'm trying to reach or what is my goal.

    Thank you again for the help and waiting further. Kind regards, Emi
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Have you considered using a relay or contactor (depending on the motor current)?
     
  6. byemi

    Thread Starter New Member

    Jun 21, 2016
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    0
    Hi... I want to use small components, for example like the triac I've mentioned from the strat of this topic...thank you anyway for your idea and answer.
    Kind regards , Emi
     
  7. Tonyr1084

    Active Member

    Sep 24, 2015
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    86
    As for the Dynamic Brake (DB) it's not an absolute brake. It will resist movement. The greater (faster) the movement the greater resistance. But as it approaches zero RPM its braking will approach zero too. In other words, it will still turn.

    Not sure I understand what you meant when it sounded like you said your gear box will self destruct when you disconnect power. Sorry, I can't grasp that. Would you elaborate on that? And I understand you're trying to measure springs. Against what criteria? What standard? Are you looking for two springs with similar characteristics? I suspect there's a better way to achieve your goal than running up a motor and gear box then switching off power and switching on the DB and measuring the time it takes to return to zero compression. Still would like to know more about what you're trying to accomplish.
     
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  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If you have a powerful motor then much of its rotational energy has to be transferred to the dynamic brake. The triac would likely require a large heatsink.
     
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  9. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Hydraulic pressure would be much easier to work with, direct reading, and much more accurate if you get a precise calibrated pressure gage.
    Motor AND load cell would be the next suggestion.

    You are trying to get spring force at a specific interval of compression, right?
     
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