How best to regulate 3.7V battery to power two small 3V motors?

Discussion in 'Power Electronics' started by summersab, Aug 22, 2016.

  1. summersab

    Thread Starter Active Member

    Apr 8, 2010
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    So, I have a 3.7V lithium ion battery and two of the following vibration motors:

    http://www.vibration-motor.com/prod...capsulated-water-resistant-Z7AL2B1692082.html

    They are rated for a max of 3.6V, so I can't just run them straight from the battery. I am trying to make a haptic feedback device, and I want a strong yet constant vibration, so I'm wanting to supply 3.0-3.3V (a 3.3V circuit may be easier to find since it's more common). I've looked at linear, switching, and buck/boost regulators, but I'm not too clear what would work best for this scenario. Linear regulators would probably drop out once the battery's voltage gets too low. Then, there's the issue of efficiency of the circuit based on the battery's voltage. What type of regulator would be appropriate? Any suggestions for a particular model?

    (I'm sure that someone will suggest a LiFePo4 cell, but I'm trying to keep things inexpensive and small since I'm wanting this to be a wearable. I'm using some prismatic lithium ion cells because they are compact and thin).
     
  2. paulktreg

    Distinguished Member

    Jun 2, 2008
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    I'd try a LDO 3V3 regulator, many available or try your motor at 2.5V (LDO's available) and see if that would be enough?
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    How about a simple diode to drop 0.6V?
     
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  4. summersab

    Thread Starter Active Member

    Apr 8, 2010
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    @paulktreg The motors are pretty weak at 2.5V, so I'm hoping to keep this at least at 3V. I'll go back to DigiKey/Mouser and try filtering again to see what I find. Since my background is ME and not EE, I ALWAYS overlook some detail in a datasheet and wind up getting something that won't work (I recently got some TI 3.3V linear regulators for this project, but I didn't look closely enough at the dropout voltage - boo!)

    @Alec_t Diodes sound promising, but how inefficient would they be? I figure that it wouldn't be TOO bad since this is only a 1V drop max. I'm fine with a solution that has a good cost/efficiency trade-off as long as it's reasonable, so I just have to ask.
     
  5. summersab

    Thread Starter Active Member

    Apr 8, 2010
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    After filtering regulators, I narrowed it down to these. Would they work?

    As for Zener diodes, how about the 1N5333B? Would that work, or is there some detail in the spec sheet I've overlooked?
     
  6. SLK001

    Well-Known Member

    Nov 29, 2011
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    I would bet that you could operate them at 3.7V without any problems.
     
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  7. summersab

    Thread Starter Active Member

    Apr 8, 2010
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    @SLK001 to the rescue! :) My only concern is that lithium batteries are 3.7V nominal - 4.2V at full charge is pushing it a bit for these little motors, I'd think. Also, while it's not necessary that the motors vibrate at a consistent rate for my application, it would be nice if they did.
     
  8. SLK001

    Well-Known Member

    Nov 29, 2011
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    You don't plan on operating these at 100% duty cycle, do you? I don't think that they are built for that kind of operation. They are built for short duration indicator operation. The specs don't directly mention duty cycle, except in the LIFETIME section (which has a 50% duty cycle). Any motor with that kind of speed and unbalance can't be slated to last long at 100%.
     
  9. summersab

    Thread Starter Active Member

    Apr 8, 2010
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    Hmm... hadn't looked as closely as I should have at that. I plan to run these for a few minutes at a time, yes. Considering their usual application in, uh . . . "toys," I believe I plan to use them at a duty cycle that is within their intended design. This was the most powerful sub-mini vibration motor I could find, though, but I don't recommend going on eBay and searching for "vibration motor." What has been seen cannot be unseen . . .
     
  10. crutschow

    Expert

    Mar 14, 2008
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    One of those should work.
    How much current does the motor draw?
     
  11. summersab

    Thread Starter Active Member

    Apr 8, 2010
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    Per the spec sheet, the max stall current is 680mA, 250mA nominal, so not a whole lot. I'm tempted to go the diode route since they're smaller, cheaper, and simpler. Thoughts?
     
  12. crutschow

    Expert

    Mar 14, 2008
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    A diode is certainly a simple way to drop some voltage, but of course it has no regulation.
    So the tradeoff is up to you. ;)
     
  13. summersab

    Thread Starter Active Member

    Apr 8, 2010
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    Could you explain that further? A Zener will produce a steady voltage as long as it is above dropout voltage and below breakdown limits, right? I mean, they aren't incredibly efficient, but shouldn't the output be reasonably constant?
     
  14. crutschow

    Expert

    Mar 14, 2008
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    I misunderstood. :oops:
    I thought you were talking about a diode in series.
    A zener shunt circuit does regulate but it's very inefficient.
    Aren't you concerned about efficiency?
     
  15. summersab

    Thread Starter Active Member

    Apr 8, 2010
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    I am to a degree, but I'm also concerned with cost and size (again, to a degree - I'll make tradeoffs in either direction if it makes sense). Since I'm only going from a max of 4.2V (fully charged battery) to 3.3V for the motors, how inefficient is inefficient, here?

    (In case anyone saw it, I deleted a post that followed this where I literally said, "Oh, I forgot size." I'm to bed, now - just thought I would explain that I'm tired, not insane...)
     
    Last edited: Aug 23, 2016
  16. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If you drop 0.8V from 3.7V with a series silicon diode the diode wastes 0.8V * I, where I is the motor current at 3V. The motor uses 3V * I Watts, so the efficiency is 3*I/(3*I + 0.8*I) *100 = 79%.
     
  17. summersab

    Thread Starter Active Member

    Apr 8, 2010
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    ...of course it would be that easy.

    That's worth it, in my opinion. Would a 1N5333B be a good choice, or should I look at some different?
     
  18. Papabravo

    Expert

    Feb 24, 2006
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    Why do you think it is necessary to run the motor at a constant voltage? Motors don't generally care about the voltage, only the current.
     
  19. crutschow

    Expert

    Mar 14, 2008
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    What??
     
  20. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    No. You don't want a zener diode; just an odinary silicon diode rated to carry about twice the motor's maximum (stalled) current.
     
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