PWM motor circuit and diode

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Jesse
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PWM motor circuit and diode

Post by Jesse » Tue Aug 30, 2005 6:05 pm

Hey guys,<p>I've come to the "experts" for some advice. Let me first quickly describe my project & circuit. I'm controling the speed of a 12v fuel pump via pwm. I'm using a microcontroller, 16F876, to output a pwm signal (using a period of 250, or about 80Mhz) to a high-power MOSFET.<p>My question is concerning the circuit for the motor (fuel pump). Orignally I just hooked the positive and negative terminals directly up to the mosfet. That worked, but not too well (didn't have a lot of voltage control). So, more research showed I needed a free-wheeling diode in parallel with the motor. I did that, and I now have full control of the motor throughout the 12volt range.<p>My problem is that the diode that I'm using is getting very hot. Before using a diode, my mosfet was getting hot, but once I added a 3amp diode, the mosfet heat was much less, but the diode was very hot to the touch. So, I added a 6amp diode just now, and it too gets pretty hot, but now my mosfet is actually running very cool! I used my volt meter to measure the current draw of the motor, and it's only drawing about 2amps, therefore my 3amp diode should have been fine. Right?<p>Should my diode be getting that hot? Is there another electrical component I should be using as well? Could by PWM signal be too fast? (I've also tried 26 Mhz as well, but same diode problem).<p>Thank you very much in advance for any advice you can provide. This is probably a very simple queston for most of you, but I'm struggling here. And if you need a visual sketch of my circuit, just let me know.<p>Thanks!<p>Jesse

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Chris Smith
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Re: PWM motor circuit and diode

Post by Chris Smith » Tue Aug 30, 2005 6:39 pm

It sounds like a inductive load is being generated by the motor it self. When you power a motor, its accelerates and keeps going, generating its own power like a generator back into the circuit. The momentum of the armature and the field collapse becomes a generator between pulses. <p> I would install an industrial size diode with heat sink and not worry too much about the heat generated, unless your on a power budget?

Jesse
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Re: PWM motor circuit and diode

Post by Jesse » Tue Aug 30, 2005 6:45 pm

Hi Chris,<p>Thanks for the quick reply. I had figured in the inductive load and "generator" effect, hence the diode. From what I had read, it sounded like if you get a diode rated for the same current as the motor, you should be fine. But as you said, I may need yet a larger diode w/ heatsink. Makes sense. This unit will be powered by a large 12v battery, but having measured the entire power draw of the circuit & motor combined to be around 2amps, I should be fine. Thanks!<p>Jesse

positronicle
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Re: PWM motor circuit and diode

Post by positronicle » Tue Aug 30, 2005 6:54 pm

--Edited by Positronicle--

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sofaspud
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Re: PWM motor circuit and diode

Post by sofaspud » Tue Aug 30, 2005 7:15 pm

I was wondering if 80MHz and 26MHz were typos, with kHz the actual frequency. I'm also wondering about "a period of 250", as that doesn't seem to equate to 80Mhz or 80kHz.

Jesse
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Re: PWM motor circuit and diode

Post by Jesse » Tue Aug 30, 2005 7:15 pm

Alright, crude but hopefully effective, here's a quick drawing of the motor control part of my circuit, including MOSFET & motor. My freq. is around 26khz right now, and I can drop it more if recomended.<p>PWM Motor<p>Jesse

Jesse
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Re: PWM motor circuit and diode

Post by Jesse » Tue Aug 30, 2005 7:21 pm

Yep, 80khz. :-) Whoops, big difference! However it's down to 26khz now with a period of 750. Sorry about that.<p>Thanks!<p>Jesse

positronicle
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Re: PWM motor circuit and diode

Post by positronicle » Tue Aug 30, 2005 8:07 pm

--Edited by Positronicle--

Jesse
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Re: PWM motor circuit and diode

Post by Jesse » Tue Aug 30, 2005 8:10 pm

I'll go check it out. Just FYI, I'm using a IRL3103 mosfet. Is there a simple transistor circuit I could use to increase the power to the gate? Thanks again!<p>Jesse

positronicle
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Re: PWM motor circuit and diode

Post by positronicle » Tue Aug 30, 2005 9:16 pm

--Edited by Positronicle--

Jesse
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Re: PWM motor circuit and diode

Post by Jesse » Tue Aug 30, 2005 9:21 pm

Great suggestion, I'll give it a shot after work tomorrow, getting kinda late now to play. I will deffinitly post back my results. With your ultra clear explination, I don't think I can go wrong. :-)<p>Jesse

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Re: PWM motor circuit and diode

Post by Ron H » Wed Aug 31, 2005 1:10 pm

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by positronicle:
You probably should be using a driver for this circuit. Just open the connection coming from your pwm signal point, pass this signal through a 2.2k resistor, then to the base of a 2n3904(or other) npn transistor, connect the collector to the 12v then the emitter goes to your mosfets gate(and resistor). Add these couple of parts, then play with it again. Let us know how it goes.<p>[ August 30, 2005: Message edited by: positronicle ]<hr></blockquote>
If I read you correctly, you are recommending an emitter follower. This will only yield about 4.3V of gate drive (assuming 5V on the PIC), the 10k pulldown will result in very slow negative transitions, and the 2.2k base resistor is counterproductive. Bad idea.

Jesse
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Re: PWM motor circuit and diode

Post by Jesse » Wed Aug 31, 2005 2:40 pm

Ron,<p>Thank you for the reply. You seem to disagree with what has been said. Do you have any suggestions?

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Chris Smith
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Re: PWM motor circuit and diode

Post by Chris Smith » Wed Aug 31, 2005 3:45 pm

You could try a small fet to drive the gate of the larger fet for isolation and a higher gate voltage driver.

Jesse
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Re: PWM motor circuit and diode

Post by Jesse » Wed Aug 31, 2005 4:22 pm

Chris,<p>Isn't that the same thing the transistor could do? I'm going to dig some of those out first and give 'em a try, as positronicle suggested.<p>I've also ordered a few of those high-power TO-220 package diodes to play with. At least with them I can heat-sink.<p>Thanks once again!<p>Jesse

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