PWM help needed

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Engineer1138
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Re: PWM help needed

Post by Engineer1138 » Tue Feb 22, 2005 11:26 am

Camino:
if you're controlling a motor that runs at 30A (7.2V?) and starts up at 400A, then the difference between PWM from a 555 or from a PIC will be the least of your problems. You're dealing with *serious* heating problems for starters. Your FET is going to need to get throught the linear region as fast as possible, and that means good FET drivers. Even when fully on, an IRLZ44V (RdsON of 0.017ohm) will dissipate about 15W at 30A, and that requires a gate drive of about 10V. Considering that you'll be PWMing it, that power dissipation is going to go up quite a bit. Better transistors are available, but you still need to understand the control problems first.<p>If I were you, I'd start by visiting http://www.irf.com/technical-info/appnotes.htm and read appnotes AN-944, AN-937 and AN-949 for a start. Once you have a grounding in actually SWITCHING the transistors quickly, then you can address the issue of just how to generate the PWM.<p>It sounds like a fun project, but it can get expensive real fast when you start burning through transistors.

Rodney
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Re: PWM help needed

Post by Rodney » Tue Feb 22, 2005 12:58 pm

These are readily available commercially. Go to any hobby shop that handles electric powered planes and ask about the ESC (electonic speed control). They make them for brushed motors and also for brushless motors and range from handling a few milliwatts to several hundred watts of power. You can not make one as cheap as you can buy a ready made unit. Most all utilize FET's that are switched either full on or full off to minimize heat loss (power loss) in the speed control itself.

camino75080
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Re: PWM help needed

Post by camino75080 » Tue Feb 22, 2005 7:15 pm

Wow, you guys rock!<p>Chris, by brake a mean when the signal from the reciever is less than 1.5 ms brake means that a transistor will short across the motor with PWM and at 1 ms, the motor should get no power or braking.<p>Dr. when, I will look into the assembaly language. thanks for the idea.<p>Engineer1138, I plan on using two or three IGBT in parallel.<p>Rodney, I know I can buy them, I've been racing RC cars for a few years and want to build one so I have full control over how it switchs the motor.<p>Thanks again!

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Chris Smith
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Re: PWM help needed

Post by Chris Smith » Tue Feb 22, 2005 7:32 pm

Your complexity has grown exponentially. <p>There are at least five or six units, each control a separate function.

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