high current output pins? - Page 1
Forum section: electronics / general

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 08, 2014, 02:23:32 am
content: I wish to drive a small brushed motor with PWM directly from the pins of a microcontroller. The object is to keep it lightweight for my foam airplane project.I'm thinking a SOIC-8 PIC, glued to the battery pack, with one input pin from the radio and the rest of the GPIO pins shorted together to drive the motor.(I'll take the 20ms frame throttle signal from the radio, capture it and output PWM, also I can add low-battery throttling.)Thing is the motor easily sucks 200-300mA, which exceeds all kinds of max ratings for the PIC, like the 12F629.eg 25mA per GPIO pin abs max, I'd have max 5 pins free = 125mA max current outputwhich also just happens to be the abs max total current for all GPIO pinsand in any case you can only pull 250mA abs max into the VDD pin.Three reasons it won't work.Is there a microcontroller in that form factor, from any company, that would have stronger output pins, and higher current limits through its power pin?And no, a brushless motor is not the answer, since they don't typically make brushless motors that small, and the extra weight of the 6 FETs+PCB would be substantial. (There's a reason all the small toy RCs use brushed motors still)

author: vvanders
author_rank: Regular Contributor
timestamp: August 08, 2014, 03:17:54 am
content: I think some of the earlier psocs were meant for lighting solutions and have high output drivers built in.

author: kolbep
author_rank: Frequent Contributor
timestamp: August 08, 2014, 04:09:54 am
content: Cant you also stick a transistor on the micro as well. I am sure the added weight will not be too much. Or maybe a uln2003 darlington array, so that you can parallel the outputs?

author: westfw
author_rank: Super Contributor
timestamp: August 08, 2014, 08:32:56 am
content: Add one transistor. You're not doing bidirectional for a plane, are you?http://img04.shop-pro.jp/PA01025/432/etc/262ms.jpg

author: fcb
author_rank: Super Contributor
timestamp: August 08, 2014, 09:00:17 am
content: Add a small N-channel MOSFET (SOT23 package) - some of them exceed 1A, you'd probably get away with a GPIO>GATE without any sort of resistor, and if you measure & choose the FET carefully you might even get away without a back emf diode. Even a 2N7002 is better than using a microcontroller pin.

author: Dago
author_rank: Frequent Contributor
timestamp: August 08, 2014, 10:04:35 am
content: Sounds like begging for trouble. Just put an SMD transistor, the added 100mg or something cannot make a difference even with a foam plane.

author: dannyf
author_rank: Super Contributor
timestamp: August 08, 2014, 12:02:46 pm
content: QuoteYou basically designed a circuit that uses a non-existent part.Two options:1) make that non-existent part yourself;2) change your design.

author: amyk
author_rank: Super Contributor
timestamp: August 08, 2014, 12:38:07 pm
content: They do make things like this:http://www.freescale.com/files/analog/doc/data_sheet/MM908E625.pdfNot quite the package you're looking for, but it is a MCU with integrated H-bridge that's rated for >1A.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 08, 2014, 10:01:26 pm
content: Quote from: Dago on August 08, 2014, 10:04:35 amDepends, every weight you add I might have to add more weight to maintain the CG.It's a silly project anyhow, it's a dollar store foam glider. When I was a kid there were these small foam gliders and I always wanted to make an RC one.Back then I used the motor from a Stomper (http://en.wikipedia.org/wiki/Stompers_%28toy%29) but of course, nothing ever came of the project, except a broken stomper and a foam plane that never flew...I know they make super mini 4 channel planes like thesehttp://www.ebay.com/itm/Indoorfly-No-9-Micro-Balsa-Model-Airplane-Kit-w-Motor-prop-Prop-Saver/111421484393?_trksid=p2047675.c100011.m1850&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20140602152332%26meid%3D8897458513827498465%26pid%3D100011%26prg%3D20140602152332%26rk%3D2%26rkt%3D10%26sd%3D111394491618so it's not a problem, but this mini plane ends up being close to 100$ when you add the radio.

author: fcb
author_rank: Super Contributor
timestamp: August 09, 2014, 09:22:13 am
content: Use a SOT23-6 packaged PIC then (10Fxxx), much smaller than a SO8...

author: Psi
author_rank: Super Contributor
timestamp: August 09, 2014, 09:43:14 am
content: mosfet mosfet mosfetYou can glue a to92 package mosfet to the top of the IC if you want to keep everything compact.