Clever way, energy efficient way, to drop ~3.7V down to ~1.5V?

[Jacob Rosenthal]

Im using 3V vibration motors in a current haptic project. The normal current usage for these is 80-100ma, generally 100ma if you leave them run open.

I'm adding A LOT more of these to the project and my goal is 30 motors running concurrently, which with these motors would put me at 3Amps. I have cable and connector constraints at ~1amp. BUT, I've foun several of the manufacturers are selling 1.5 volt solutions now that use 30ma! I'm ordering samples to se if the magnitude feels the same.

Im still running a ~3V System so Eric Ose and I brainstormed to find a clever way to drop that voltage. Obvious options came up:

Dropping entire system voltage to 1.5V - Actually possible with my microcontroller and speed, but I'm running potentially long busses and that doesn't leave ANY room for dropout

Voltage divider - obviously wasteful

Voltage reg adds lots of parts and cost to my design

Diodes - seems easiest..

Then we came up with using an LED to drop the voltage. I already have an LED in parallel with motor to indiciate vibration! What if I put it in series and dropped across that?. The LED current limiting the motor is the obvious problem but I've found some possibilities on digikey for a 2.1V drop at 35ma usage so seems doable.

Anyone have a better idea or a reason to talk me out of this?

[Jasper Nance]

normally you would use a zener diode here. smaller form factor and better performance than an LED. you're still burning up I*Vdiode no matter how you look at a linear solution though.

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[Jasper Nance]

how does a linear reg add more parts? cost ok, but parts not neccesarily.

[Jacob Rosenthal]

Support stuff. Generally want 2 caps at least, if not a resistor to set voltage, etc.

Since I CAN run my device at 1.5 volts, I could still push 3.7 through my bus then reg my entire device down to 1.5.. Im not actually reging the micro currently anyway... just letting run whatever speed it runs based on input...

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[Jasper Nance]

TI makes these tiny SC70 LDO regs that can push 50ma and dont need any caps if you can stand some oscillations (which you can it sounds like). But why not just need it overvoltage and current limit with an appropriate resistance?

[Tony Brenke]

is there a problem running two 1.5V motors in series? that would be the simplest way to drop the volts across each to a usable level.

next question.  What type of motor?  brushed or brushless?  brushed I would assume.

GENERALLY a brushed motor rated for 1.5V will run for a while at 3V. heat will be the killing factor.  

what I would do is to place a current limiting active circuit in each 3V line running to the 1.5V motors.  limit the 1.5V motors to its std current rating running at 1.5 and power that fro the 3V you have available.

experiment in this prior to using it in your final solution.

[Dillon]

Motor's usually die from heat of over-current caused by excessive voltage.

Not doing the math but PWM'ing them so they see average 1.5V might achieve what you want. You already have all the circuitry you need implemented.
Need to keep the PWM frequency up or they'll burn out during that high pulse and torque will ripple.

[Jacob Rosenthal]

Tony,

Hadn't considered it, but its a lot of parts. Anyone sell an integrated one?

Dillon,

Good catch. Forgot to add that one. Trunk came up with it too. 

I'm kinda leaning towards regulating my  whole circuit. 

Sent from my iPhone

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[Tony Brenke]

A 50% pwm can be done with very few parts.
The active current limit can be done with few parts also. ( per motor).  With that many motors youj may as well bight the bullet and change your circut to use the 1.5 to start with.

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[Jacob Rosenthal]

Im already doing PWM on that motor with the micro, So a code change is definitely one of the quickest and least parts count ways. I was able to fit a lin reg on the circuit as well, which might be a good idea to finally get on there too...

[Kim Schuttenberg]

How are you switching the motor current?  You may be able to build a simple buck converter using a DA-port and a digital output (+1 inductor and 1 diode).

This also gives to a software controllable voltage.

[Jacob Rosenthal]

Switching motor current? IE turning it on? PWM from a uC

On Wednesday, April 24, 2013, Kim Schuttenberg wrote:

How are you switching the motor current?  You may be able to build a simple buck converter using a DA-port and a digital output (+1 inductor and 1 diode).

This also gives to a software controllable voltage.

[Tony Brenke]

put a ceramic capacitor across the motor leads and never run the PWM above 50%. that is a very rude way to make your 1.5V and it will work.  company did this with 12V heaters at 48V.   lasted a few months before they burned out.  up the PWM freq if you can, they cap will be more effective that way.

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[Kim Schuttenberg]

I was wondering if you had an external switching transistor, or were just using the uC output drivers.

On Wednesday, April 24, 2013 11:24:05 PM UTC-7, Jacob Rosenthal wrote:

Switching motor current? IE turning it on? PWM from a uC

On Wednesday, April 24, 2013, Kim Schuttenberg wrote:

How are you switching the motor current?  You may be able to build a simple buck converter using a DA-port and a digital output (+1 inductor and 1 diode).

This also gives to a software controllable voltage.

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[Jacob Rosenthal]

yes, switching fet

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