Linear resonant actuator and back EMF

Discussion in 'General Electronics Chat' started by Midge, Jun 4, 2015.

  1. Midge

    Thread Starter New Member

    Jun 4, 2015
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    Hi - I'm using a 7555 CMOS timer as an astable, driving a 2N3906 transistor, which in turn is switching a mobile phone vibration motor on for a short burst of a few seconds, and then back off for several minutes. I have this working nicely.

    The motor is a tiny linear resonant actuator (LRA) about 10mm across and 3mm thick and consumes about 60mA. Voltage across it is around 3V DC.

    If I was driving a small relay in a circuit like this, I'd put a freewheel diode across it.

    My question is do I need to do the same with an LRA - where I don't quite understand how it operates and whether back EMF is an issue.

    My working assumption is its like a tiny solenoid with a return spring, which breaks its own circuit as soon as the armature moves, so I'm thinking there could be some back emf there for a fraction of a second, but the field is going to be much smaller than a relay or motor.

    Thoughts appreciated. Thanks!
     
  2. Midge

    Thread Starter New Member

    Jun 4, 2015
    13
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    PS - a 1N4148 across certainly doesn't seem to harm it.
     
  3. john*michael

    Member

    Sep 18, 2014
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    An LRA is a voice coil and works exactly like a speaker, except that there is a mass being vibrated. There is also a spring, so the LRA will resonate at a specific frequency, and you will get much more motion at the "sweet spot."

    It is more analogous to a motor than a solenoid, because the motion is based on creating a magnetic field to move a magnet and not on magnetizing a piece of soft magnetic iron. This means that a magnetic field is always present, so that motion of the mass produces a voltage on the output even when power is not present. So depending on the applied voltage, it is possible that a catch diode would conduct during the "off" period of your timer and turn on the diode, as the magnet moves back past its original position. This will dampen the motion of the mass.

    The inductance is less than you would have with a solenoid, so I don't think you would need a catch diode - you could check with a scope to see if you needed it. There may be an excursion below ground as well, which would tend to cause the output of the timer to conduct. If you think you need protection, I would consider a Zener rated at the supply voltage.
     
  4. john*michael

    Member

    Sep 18, 2014
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  5. john*michael

    Member

    Sep 18, 2014
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    Should have said back-to-back zener
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Never used one, but keep in mind that a BEMF diode provides power to the coil between pulses, this is often a problem with fast acting inductive devices such as dispensing solenoid valves etc in keeping them retained.
    Another reason that DC relays can be ran off of unsmoothed DC (120° ripple) without vibration.
    Max.
     
  7. Midge

    Thread Starter New Member

    Jun 4, 2015
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    Thanks for the replies guys. It's got me wondering whether the device is actually an LRA (I just assumed it was). i.e. if it operates like a speaker coil (John), it's going to want an AC driver signal, whereas this thing operates on DC. I've found a video on YouTube of what looks like the identical item.

    https://www.youtube.com/watch?feature=player_detailpage&v=5avUvKLEiLs#t=407
     
  8. john*michael

    Member

    Sep 18, 2014
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    Midge,

    It does look like an LRA, but you are correct that an AC drive would be required. I had assumed you were using the timer to generate the vibration frequency. If you are running it with DC, you must have an "ERM" (Eccentric Rotating Mass) which is a kind of DC brush motor with an off-center mass on the shaft. You can treat it just like any small brush-style motor.
     
    Midge likes this.
  9. john*michael

    Member

    Sep 18, 2014
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    You can check this by seeing if the vibration frequency changes with voltage.
     
  10. Midge

    Thread Starter New Member

    Jun 4, 2015
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    Last edited: Jun 4, 2015
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