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I am trying to direct a bi-directional dc motor that is about 1.5v (i got it from a micro servo and it is about half the size of a 130 dc motor at 1.5 - 3v) with the smallest power source/battery possible. I'll be controlling it all from a micrcontroller.

I was told that MOSFET transistors would be the best thing for this project as it would draw the least amount of power and current for itself and the motors. Could someone tell me if this is the best solution or if there is a better way to establish an H-bridge at very low power? Any and all references, explanations, and links to tutorials would be much appreciated as I am still unclear if MOSFET is the combination of certain transistors to make the h-bridge or if the transistors themselves are considered MOSFETs...

I was using a L293d but that cost too much power and the batteries needed would have taken up too much space.

KellysOnTop23
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  • How much current does it draw? Can you run it directly from the IO pins of the microcontroller? – Bryan Boettcher Dec 17 '15 at 15:33
  • Why don't you just use the electronics from the micro servo as well? – Icy Dec 17 '15 at 15:34
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    Have you measured the motors resistance at rest? Some of those little servos have huge stall currents, so it might be worth making some measurements. What did you mean by "I was using a L293d but that cost too much power"? – gbulmer Dec 17 '15 at 15:36
  • @gbulmer - the L293d has volt drops on each section of the H bridge of about 1.5 volts. – Andy aka Dec 17 '15 at 15:37
  • What power supply voltage are you using for the H bridge? – Andy aka Dec 17 '15 at 15:38
  • What's the microcontroller voltage and what's it being powered from? Pair of AA batteries? 3.7V lithium (available in tiny sizes)? – pjc50 Dec 17 '15 at 15:39
  • @insta i read that running a dc motor directly from the micro controller's pins was not a good idea. – KellysOnTop23 Dec 17 '15 at 15:54
  • @Andyaka - That may be *exactly* what the OP meant, however, I always like to discover what it is the OP actually meant. – gbulmer Dec 17 '15 at 15:55
  • @icy i considered that as well but every time i tried to find out what was on that little board inside the servo so that i could recreate it i couldn't find anything. – KellysOnTop23 Dec 17 '15 at 15:59
  • @andyaka i was using a 5v pin from the micro controller board – KellysOnTop23 Dec 17 '15 at 15:59
  • @pjc50 im using the nordic dk, it was a 5v pin from the board going to the motor controller. I wanted to use coin cells but quickly realized that wasn't happening. i wanted to try like a cr2 lithium battery or something that little – KellysOnTop23 Dec 17 '15 at 16:00
  • @gbulmer the l293d, i was told, does have volt drops and didn't seem to work with any of the batteries i would be able to use considering the size f this project. Thanks for the help! – KellysOnTop23 Dec 17 '15 at 16:01
  • What is on that board will be a 'H' bridge and a variable width pulse generator controlled by the variable resistor in the servo. It will compare the input pulse with the generated pulse; longer and it will drive the motor one way, shorter and it will drive it the other way. Just provide a fixed resistor instead of the variable one and drive it with a PWM from your micro and *et voila* – Icy Dec 17 '15 at 16:04
  • @icy is there anyway i could recreate that on a breadboard to test it. Do you know the parts or a tutorial i could follow?! I already have the micro controller sending pwm so this sounds like a great fix, if it works with a low power source – KellysOnTop23 Dec 17 '15 at 16:08
  • There are lots of threads on rcgroups where people doing diy micro rc stuff have evaluated various h bridges for single (lipo) cell applications. Your desire for 1.5v seems to be asking a lot though. – Chris Stratton Dec 17 '15 at 16:31
  • 40 odd years ago RC control stuff was done with discrete components, nowadays its likely to be pretty much a single IC, if you can find the part number of the IC used on your board you might be able to get more information. – Icy Dec 17 '15 at 16:34

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MOSFET transistors do not need a current to stay "ON" so potentially they can be lower power. Although you could build your own H-bridge using 2 NMOS and 2 PMOS transistors, I would not recommend that since there are ICs that will be easier to use and have everything you need included.

I found an example of such an IC, the L9110 of course there are more but this a cheap one I could find easily. You can buy ready-made modules using this IC on ebay.

Bimpelrekkie
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  • ON also makes some like this, e.g. [LB1938FA](http://www.onsemi.com/PowerSolutions/product.do?id=LB1938FA); typically these get dubbed "low power, low saturation" or something like that. – Fizz Dec 17 '15 at 16:05
  • And this would be able to drive a dc motor from a micro controller from a battery like the cr20 (it is 3v, 800mah)? – KellysOnTop23 Dec 17 '15 at 16:06
  • @respawnedFluff that looks great!! is there going to be a voltage drop after the current goes through one of these IC's because i need to direct 2 dc motors which means ill need 2 of these – KellysOnTop23 Dec 17 '15 at 16:20
  • Good conduction performance MOSFETs do however tend to need a substantial gate voltage which can be a challenge here. The L9110 looks to have extremely high internal loss. H-bridge losses are per channel, so if you have two each driving its own motor they do not add. But any sag of your supply battery is common! – Chris Stratton Dec 17 '15 at 16:30
  • Hey @chrisStratton , I'm the same dope from [this](http://electronics.stackexchange.com/questions/206280/replacing-5v-pin-with-battery-not-working/206281?noredirect=1#comment435006_206281) question. That LB1938FA from RespawnedFluff 2 comments above looked promising. It had low voltage (i think it said 2 would get it going) and 800mah which i think some lower batteries can handle. You think i should try that out or should this project just be scrapped cause it seems like bi directional motor control in a small container is asking too much for what im capable of... – KellysOnTop23 Dec 17 '15 at 16:46
  • @KellysOnTop23: have you figured out how much current your motor needs to run? Without knowing that the discussion is rather academic. – Fizz Dec 17 '15 at 17:16
  • @RespawnedFluff So the little motor i want to use has a stall current draw of 500 mili amps and the voltage can't be more than 1.5. Thanks again for the help!! (and this is after its been put through the l293d motor driver with the desired PWM) – KellysOnTop23 Dec 17 '15 at 18:22
  • @KellysOnTop23: If you plan to run the motor near that, you need at least [AAA cells](http://data.energizer.com/PDFs/E92.pdf). CR2032 are pretty much out of the question; at 30mA their voltage drops precipitously. See th 100ohm load test in http://data.energizer.com/PDFs/cr2032.pdf – Fizz Dec 17 '15 at 18:31
  • @respawnedFluff what about something like [this](http://data.energizer.com/PDFs/CR2_EU.pdf) little guy to go through [this](http://www.farnell.com/datasheets/1339874.pdf) motor driver for 2 of those little motors mentioned above? – KellysOnTop23 Dec 17 '15 at 18:57
  • @KellysOnTop23 the CR2 battery is fine but LV8549M needs 4V supply for itself. Also Rdson of 1ohm is not so great. At 0.5A you'll lose 0.25W in that, whereas the 0.35V drop of the LM1938 will only lose you 0.175W. If you really want a FET-based driver try [LV8417CS](http://www.onsemi.com/PowerSolutions/product.do?id=LV8417CS), which has 0.27 ohms [and works at 2V], but don't try to solder that by hand (WLP needs reflow). Also, ON [bought Sanyo's driver business](http://www.onsemi.com/PowerSolutions/newsItem.do?article=2458) in case you didn't know. – Fizz Dec 17 '15 at 19:18
  • Got it! last thing....i hope. The LV8417CS you suggested is one channel and i need 2 (two dc motors). would this one be ok, the [LB1909M](http://media.digikey.com/pdf/Data%20Sheets/ON%20Semiconductor%20PDFs/LB1909M.pdf). Then this one here is also 2 channel and looks about the same [LB1948MC](http://www.onsemi.com/pub_link/Collateral/ENA2015-D.PDF). They both have slightly higher voltages but the drop you mention, would i still be able to power everything properly? – KellysOnTop23 Dec 17 '15 at 19:35
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This is too late, but I did find this chip: https://www.pololu.com/file/0J570/drv8835.pdf and the carrier: https://www.pololu.com/product/2135 Motor supply voltage can go to zero.

Burgess_Shale
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