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Author Topic: NOOB question - simple unidirectional motor driver.  (Read 3023 times)

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Offline philTopic starter

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NOOB question - simple unidirectional motor driver.
« on: March 11, 2008, 11:08:51 PM »
Let me start by saying that I am a programmer-type that is new to electrical design.  I've built 1 line follower robot where the motors are driven by an npn transistor.  So, now I have a new project and need to control the speed of the motor and would like to use MOSFETs.  However, I have am suffering from information overload and need to sort some things out...

Okay, I need to control the speed of a DC direct drive motor with a PWM signal.  I only need to turn the motor in 1 direction, so I don't need a full H bridge.  What I am not sure about is if I even need a half bridge.  Can I get away with a single MOSFET (N or P) or do I have to have both?  It seems like having both just allows you to reverse the signal but I don't need that.  If I can just use one, which type should I use?  Either way, what parameters should I look for when choosing MOSFETs?

Also, I realize I could just buy an H-Bridge IC and use it but I am kind of trying to learn the inner workings here, so I want to do it myself, just this once.  I plan to make a small motor driver board with 5 header pins: power, ground, signal (PWM), and the 2 motor connections.  I think I can just use a P-MOSFET (or N) a schottky diode, and perhaps a 5V regulator unless anyone thinks I need anything else.

Oh, one parameter for this project is that the motor draws 2A at nominal speed and up to 8A when stalled, so I need to be able to handle up to 8A, I guess.  I plan to use a 5V regulator since 8A isn't going to happen very often, but maybe I should go higher.

Offline Del

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Re: NOOB question - simple unidirectional motor driver.
« Reply #1 on: March 13, 2008, 12:46:23 PM »

Maybe this "Google books" link will help (start reading from page 100 / experiment 36):

http://books.google.com/books?id=Qq0CVfcAktUC&printsec=frontcover&dq=predko&sig=JKLZAnPDACW922AZZonbh4WkUm8#PPA100,M1

(from "123 Robotics Experiments for the Evil Genius")

HTH

Offline AndrewM

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Re: NOOB question - simple unidirectional motor driver.
« Reply #2 on: March 13, 2008, 01:41:38 PM »
You need a half H-bridge still, which means one P and one N channel MOSFET.  The main things to look for in MOSFETS are rated volts(if listed) and amps.  Figure out the highest value of each you will use and increase it by at least 50%, so for your 8A max draw you want a MOSFET rated for 12A (higher is always better).  For day to day (normal use) this 50% increase is for heat dissipation, but for that circuit failue it gives you a bit of a buffer.  Also for when, a year from now, you decide to increase the speed and torque of the bot with slightly larger motors and forget to upgrade your MOSFETS as part of it.  When you start designing higher end H-Bridges where power is critical you will want to start looking at the rest of the specs (trigger voltage, loss, etc, etc)
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Offline Admin

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Re: NOOB question - simple unidirectional motor driver.
« Reply #3 on: March 15, 2008, 07:10:19 PM »
Quote
Can I get away with a single MOSFET (N or P) or do I have to have both?
Yeap, just one MOSFET will work. And a fly back diode (google it up).

As for NPN or PNP, it matters which comes first, motor or MOSFET.

If the MOSFET is directly connected to ground for the source, then you'd want it NPN.

Offline AndrewM

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Re: NOOB question - simple unidirectional motor driver.
« Reply #4 on: March 15, 2008, 11:43:22 PM »
Quote
Can I get away with a single MOSFET (N or P) or do I have to have both?
Yeap, just one MOSFET will work. And a fly back diode (google it up).

Wouldn't your fly back diode need to be fast recovery and able to handle the same amount of current as the motor, in this case 8amps?  :-\ At that point your diode price is over that of the second FET.
« Last Edit: March 16, 2008, 12:07:31 AM by AndrewM »
blog: www.iamwhen.com
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Offline Admin

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Re: NOOB question - simple unidirectional motor driver.
« Reply #5 on: March 16, 2008, 07:35:00 AM »
Nah just get the diode power rating right and it will be fine.

Most MOSFETs today already have built in fly back diodes, but they aren't always enough for what you want to do. I typically just slap on a high power diode I have sitting around and call it a day.

 


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