Expert PWM controller - Page 2 - Electronic Circuits Projects Diagrams Free

Someone Electro · 29th June 2006, 02:46 PM

Well a slow diode will also do fine.Anyway a slow diode dosent mean it thurn on slow all diodes have very fast thurn on times the slow discribes its thun off time the diode stays conductive for some time after there is no curent.If you have like 100khz switching freq, a fast diode i needed.

I used normal diodes on my coilgun but a single one could only take a few shots after shorting it self and since kiloamps are avaliable blowing the diode a part.3 parallel diodes work like a treat.I also had no reverse charge in the caps.

Dr.EM · 29th June 2006, 09:38 PM

Ok, I tried it out today. It worked suprisingly well, providing full control over the motor with fast switching times throughout and no voltage spikes from the induction (just 2 antiparallel diodes for clamping). The motor is at least 20A at 12v, it runs from a a lead acid battery and used to power a lawnmower (yes, a battery powered lawnmower!). The high current becomes clearer when after just a few seconds of use, the thinner white wires I used for some of the connections are rather warm (they arn't exactly thin, but the supplied wires are very thick in comparison). I have a video of it in use, contact me if you want to see it, nowhere seems to do a good free video hosting. I also realise moreso now why most motor controls switch at over 20Khz

Hero999 · 29th June 2006, 09:59 PM

Quote:

Originally Posted by Dr.EM

(just 2 antiparallel diodes for clamping)

A common myth, the diodes aren't used for clamping but to improve the efficiency, and for efficiency's sake you really need to be using a higher frequency >20kHz is best to avoid audiable noise.

Someone Electro · 30th June 2006, 08:12 AM

This huge motor is probobly very loud by it self.

Video hosting? Try putfile.com (25MB)or youtube.com (100MB).

**Nigel Goodwin** · 30th June 2006, 08:17 AM

Quote:

Originally Posted by Hero999

A common myth, the diodes aren't used for clamping but to improve the efficiency, and for efficiency's sake you really need to be using a higher frequency >20kHz is best to avoid audiable noise.

What about the inductance of the motor?, as the frequency gets higher the impedance of the motor increases so it gets less power. This is why PMW for motors is normally low audio frequencies.

Someone Electro · 30th June 2006, 08:40 AM

Thats what basicly makes DC motors lose tork at high speeds.The inductance of there coil.

I dont see how higher frq. are more eficent anyway. I know that 500Hz is more eficent then 50Hz in AC motors.

Dr.EM · 30th June 2006, 09:11 AM

Yeah, I made the mistake of touching a clamping diode after a few seconds of use and it was burning hot. That was a 6A one, so it really needs a bigger one (the practical solution would be a 10A TO220 diode I think, heatsinked).

I tried both the 120Hz and 3Khz frequencies on it and found the 3Khz to be more effective. For one thing, the 120Hz really was too loud, and it had the typical stalling on lower RPM. 3Khz gave perfectly good control, but like I say the whistling noise is irratating. After a while, you can hear the motor noise over it, and of course on full, it dissapears altogether.

My attempt at hosting the video:

http://media.putfile.com/Motor-PWM-control

Someone Electro · 30th June 2006, 10:45 AM

Ouch this motor give ot a lot of sound when PWM controled!

If it has high enugh resistace try conecting this to a audio amp it would act as a speaker quite good.Hehe

audioguru · 30th June 2006, 02:16 PM

Hi Dr. EM,
I made a motor speed control very similar to yours for an electric powered model airplane. I also used a switching frequency of about 3kHz and it whistles loudly when 1st turned on then the air noise from the propeller drowns it out.
I didn't use a diode across the motor, maybe I should for better efficiency. Instead I rely on the zener diode in the Mosfet to protect it by "wasting" the voltage spike when the controller switches off the current.

Hero999 · 1st July 2006, 10:59 AM

audioguru, you're also falling into the all to common trap of believing the protection myth believed by many nubes, the diode isn't used for protection but for efficiency's sake.

Quote:

Originally Posted by Nigel Goodwin

What about the inductance of the motor?, as the frequency gets higher the impedance of the motor increases so it gets less power. This is why PMW for motors is normally low audio frequencies.

Another myth, it actually works better at higher frequenies as the self-inductance of the motor is enough to sustain the current flow through the diode in between pulses, think of it like an inductor in a SMPS.
http://www.4qdtec.com/pwm-01.html

**Nigel Goodwin** · 1st July 2006, 12:02 PM

Quote:

Originally Posted by Hero999

Another myth, it actually works better at higher frequenies as the self-inductance of the motor is enough to sustain the current flow through the diode in between pulses, think of it like an inductor in a SMPS.

I did tests when I wrote my PIC PWM tutorial, trying different frequencies driving a 6V DC motor - in the end I settled on 2.5KHz, this gave the best performance - moving to 20KHz produced hardly any power at all.

Hero999 · 1st July 2006, 01:07 PM

Interesting, possibly the diode wasn't switching fast enough.

Someone Electro · 1st July 2006, 02:32 PM

Then the diode would overheat.The inductance of the motor is restricing the curent flow a lot at such a high freq.

Dr.EM · 1st July 2006, 02:58 PM

I tried this circuit at over 20Khz in prototyping and found it worked very poorly operating a small computer fan.

antpsai · 5th July 2006, 09:09 AM

Hi all. The terms like "efficiency" and "protection" applied to the diode across the motor in a PWM circuit are both correct and both misleading. Its like saying that the wheels in a car are there for efficiency (rolling) and protection (suspension?), yes but the wheel is there to make the car run. Same thing with the diode - it is there to make the circuit run by letting the current circulate (free wheel is the term) through the motor -without the diode the circuit would be a BAD circuit. BTW the diode can be replaced by a transistor to improve efficiency (lower drop) when it is suitably switched.

29th June 2006, 02:46 PM	(permalink)
Someone Electro Experienced Member	Well a slow diode will also do fine.Anyway a slow diode dosent mean it thurn on slow all diodes have very fast thurn on times the slow discribes its thun off time the diode stays conductive for some time after there is no curent.If you have like 100khz switching freq, a fast diode i needed. I used normal diodes on my coilgun but a single one could only take a few shots after shorting it self and since kiloamps are avaliable blowing the diode a part.3 parallel diodes work like a treat.I also had no reverse charge in the caps. __________________ Il give you shocking experience.

30th June 2006, 08:12 AM	(permalink)
Someone Electro Experienced Member	This huge motor is probobly very loud by it self. Video hosting? Try putfile.com (25MB)or youtube.com (100MB). __________________ Il give you shocking experience.

30th June 2006, 08:40 AM	(permalink)
Someone Electro Experienced Member	Thats what basicly makes DC motors lose tork at high speeds.The inductance of there coil. I dont see how higher frq. are more eficent anyway. I know that 500Hz is more eficent then 50Hz in AC motors. __________________ Il give you shocking experience.

30th June 2006, 09:11 AM	(permalink)
Dr.EM Experienced Member	Yeah, I made the mistake of touching a clamping diode after a few seconds of use and it was burning hot. That was a 6A one, so it really needs a bigger one (the practical solution would be a 10A TO220 diode I think, heatsinked). I tried both the 120Hz and 3Khz frequencies on it and found the 3Khz to be more effective. For one thing, the 120Hz really was too loud, and it had the typical stalling on lower RPM. 3Khz gave perfectly good control, but like I say the whistling noise is irratating. After a while, you can hear the motor noise over it, and of course on full, it dissapears altogether. My attempt at hosting the video: http://media.putfile.com/Motor-PWM-control __________________ Angry!? I'm absolutely electrolytic! Will have to make do with myspace now I guess...

30th June 2006, 10:45 AM	(permalink)
Someone Electro Experienced Member	Ouch this motor give ot a lot of sound when PWM controled! If it has high enugh resistace try conecting this to a audio amp it would act as a speaker quite good.Hehe __________________ Il give you shocking experience.

30th June 2006, 02:16 PM	(permalink)
audioguru Experienced Member	Hi Dr. EM, I made a motor speed control very similar to yours for an electric powered model airplane. I also used a switching frequency of about 3kHz and it whistles loudly when 1st turned on then the air noise from the propeller drowns it out. I didn't use a diode across the motor, maybe I should for better efficiency. Instead I rely on the zener diode in the Mosfet to protect it by "wasting" the voltage spike when the controller switches off the current. __________________ Uncle $crooge

1st July 2006, 01:07 PM	(permalink)
Hero999 Experienced Member	Interesting, possibly the diode wasn't switching fast enough. __________________ What's so bad about Microsoft? Get Opera it's simply a superb browser.

1st July 2006, 02:32 PM	(permalink)
Someone Electro Experienced Member	Then the diode would overheat.The inductance of the motor is restricing the curent flow a lot at such a high freq. __________________ Il give you shocking experience.

1st July 2006, 02:58 PM	(permalink)
Dr.EM Experienced Member	I tried this circuit at over 20Khz in prototyping and found it worked very poorly operating a small computer fan. __________________ Angry!? I'm absolutely electrolytic! Will have to make do with myspace now I guess...

5th July 2006, 09:09 AM	(permalink)
antpsai New Member	Hi all. The terms like "efficiency" and "protection" applied to the diode across the motor in a PWM circuit are both correct and both misleading. Its like saying that the wheels in a car are there for efficiency (rolling) and protection (suspension?), yes but the wheel is there to make the car run. Same thing with the diode - it is there to make the circuit run by letting the current circulate (free wheel is the term) through the motor -without the diode the circuit would be a BAD circuit. BTW the diode can be replaced by a transistor to improve efficiency (lower drop) when it is suitably switched.

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