36mosfets, no space =(

Scarecrow

Hello! its been a while hehe.....

i have to work in this project for a robotic arm that has 6 very small sensorless brushless motors rated at 26ma to do the trick i use pwm signals and a 6 mosfet bridge for the 3 signals needed but the problem is that i have very limited space for the bridges and i was thinking about using some kind of logic chip instead the mosfets to do the trick......is there any solution for this or is it just crazy? =P

cheers

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"The Only Thing Im Sure Of Is That Im Never Sure Of Anything"........

Phil Pilgrim (PhiPi)

Just so I understand, each of the six motors uses six MOSFETs (three half-bridges) to drive the three phases, each phase requiring PWM?

-Phil

Scarecrow

yes exactly but i have very limited space (Around 5x5cm board) and its killing me =S i dont want to solder 36 smd fets either =(

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"The Only Thing Im Sure Of Is That Im Never Sure Of Anything"........

Phil Pilgrim (PhiPi)

What voltage do these motors require?

-Phil

D Faust

Try using multiple h-bridge ICs.

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Scarecrow

they need 3v 26ma no load current and i think i cant use h bridges but 3 half bridges? also power consumption is kind of a prob too so i dont know if a buffer or nor gate might do it?

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"The Only Thing Im Sure Of Is That Im Never Sure Of Anything"........

D Faust

An h bridge is made of half bridges, I think. I'd check out a schematic for one to see if it would work.

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Phil Pilgrim (PhiPi)

The outputs from an SX or AVR micro could easily handle the current, and each can be run from 3V. (You may need to run at a higher voltage to account for the internal voltage drop at 26mA — about 1V for the AVR.) These outputs are basically tri-stateable half bridges, since they both source and sink current and can be turned off. However, the number of motors each microcontroller can handle is limited by the overall maximum current into Vdd and out of Vss, which is the sum of the motor currents, plus whatever the micro needs. This would preclude using, say, an SX28 for all six motors, even though it has enough pins to do so. Six AVR ATTINY13s (8-SOIC) could probably do the job and would fit your space, as could six SX20s (20-SSOP). You'd have to write code for them, of course.

-Phil

deno

I don't know if this will work for you, but I us the L293E H bridge alot for various application.· It is very hardy, and forgiving.· Do a google search on it to get the data sheet.

Deno

Phil Pilgrim (PhiPi)

An H-bridge is designed to drive two-phase motors. Brushless DC motors are three-phase devices and require three half-bridges rather than a full H-bridge. Also, most commercial full- and half-bridge drivers provide way more current than the 26mA required and take up more room than the one square inch available for six motor drivers. If the separate microcontrollers I suggested above are overkill, it may be possible to use simpler logic-level drivers in their place.

-Phil

Scarecrow

problem is that when theyre gonna get something to move it probably goes up to 40 ma maybe more and if they stall it would burn everything and putting all those mc sure costs more that some p-n mosfets in a small package i was thinking of something like this: www.vishay.com/docs/74592/sia511dj.pdf but maybe there was some low level logic chip that packs many gates that coul do the job? do you know if i could use something like some octal or hex nor gates?

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"The Only Thing Im Sure Of Is That Im Never Sure Of Anything"........

Post Edited (Scarecrow) : 9/11/2007 7:02:36 AM GMT

Lawson

Some of the Inverter and Buffer chips can put out a lot of current. ( 74AC245 - Octal Buffer is used as a motor driver in BEAM robots) What about transistor packs? Still 36 transistors, but at least it's no longer 36 packages to solder.

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Lunch cures all problems! have you had lunch?

Beau Schwabe

Scarecrow,

You need to be careful when looking at the IC specs for Source and Sink I/O currents. Many times you might find something that can source or sink what you need, but if you use several of those pins at the same time the current will exceed the rated current limit for the IC itself.

If a standard H-Bridge is too much overkill, you might be able to compromise with an Op-Amp that can handle low voltage and "higher" currents. If you have 6 three phase motors, then I think that 5 of these IC's would work just fine with 2 Op-Amps to spare.

www.ortodoxism.ro/datasheets/texasinstruments/opa4684.pdf

This is a QUAD Op-Amp that has an output drive of 120mA..... Each motor would require 3/4 of the IC, so 5 IC's would be required.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.