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PWM controller

Discussion in 'Electronic Projects' started by Dr.EM, Jun 3, 2006.

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  1. Dr.EM

    Dr.EM New Member

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    This is a PWM controller design that I have built and which works. As shown, it should function that an increasing voltage applied to the comparator gives an increasing duty cycle at the output, this is variable fully from 0-100%. Swap the inputs on the comparator to give negative correlation if this is desired.

    The design uses op-amps to generate a triangle wave which can run at either 120hz or 3khz, this is fed into a comparator input and the reference voltage sets the pulse width. The output of this goes to what is essentially a discrete NOT gate, which is used as the driver for the power MOSFET. I have included control by either a simple control knob, or by an external voltage source. The power MOSFET is heatsinked to the casing which is extruded aluminium (which should make the case live, but when I checked that, it didn't seem to be, mabye the coating is insulating). Tests show it to work quite well, I had to add protection diodes very quickly, even for use with a coiled heater, but thats sorted now. Its all pretty small in its 103x120x30mm case, and should be good for around 10A, tested to 6A with no problems, didn't even get warm.
     

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    Last edited: Jun 3, 2006
  2. ThermalRunaway

    ThermalRunaway New Member

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    I'm currently looking at PWM controller circuits because I need to design a DC-DC converter which I've never done as part of a home project before. I'll take a look at your circuit - thanks :)

    Bri
     
  3. Someone Electro

    Someone Electro New Member

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    A corstuit like this is no use for a switching power suply.Go loock at ICs for this job they normaly do the "hard work" or generatng the PWM signal for smaller ones even the switching is intergrated
     
  4. dave

    Dave New Member

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  5. ThermalRunaway

    ThermalRunaway New Member

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    Yes it was PWM ICs that I've been looking at, and it's probably the better idea, but Dr.EM's circuit is good from an Electronics Interest point of view, and there's no reason to believe that it'd be no good. I think I'll experiment with it.

    Brian
     
  6. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    It's a speed controller, NOT a switch-mode PSU!.
     
  7. Dr.EM

    Dr.EM New Member

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    That's right, this PWM is just useful for controlling motor speed, lamp brightness, heater output or driving transformers. If you need a circuit to do any of those, then this should work fine up to around 6A. I might try it with a 20A 12V motor I have, but the inductance might be a bit leathal for its fairly simple diode protection (which i've just noticed isn't shown there. You must add diodes for use with anything other than lamps which have inherently have very low inductance, non coiled heaters are also ok).
     
  8. Someone Electro

    Someone Electro New Member

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    A fast diode amtiparallel whith the load should make it drive inductive loads no problem.
     
  9. ThermalRunaway

    ThermalRunaway New Member

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    Ok, I can understand that you use the circuit to control motor speeds and that my application is completely different, but surely the PWM controller part is common? In a DC-DC converter which is designed around a PWM controller IC, the pulse-width is altered by a voltage range at the feedback pin on the IC itself. This feedback voltage is varied by changes in output load, and therefore the pulse-width is changed to compensate for this effect. From reading your description, it seems that those properties fit your PWM controller quite well? What I'm saying is, surely your PWM controller circuit could be adapted to my application?

    Or am I missing something completely here...

    Brian
     
    Last edited: Jun 27, 2006
  10. Someone Electro

    Someone Electro New Member

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    Esayest way to make a switchmode powersuply is to buy a SMPS controler IC this can be a very small IC like SOT 5 or up to 28 pins.Then you need a external trasnsistor and the proper cirucit around it.All datasheets containg multiple cirucits that use the chip.
     
  11. Hero999

    Hero999 Banned

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    This is fine for lamps and LEDs but if you want to controll motors you need a higher frequency and a diode in reverse paralell with the motor.
     
  12. ThermalRunaway

    ThermalRunaway New Member

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    Hi Someone Electro, I completely agree that it's the easiest way, infact I've already chosen a PWM IC that I was going to use for the project, but because the projects I do are home projects for my own use, the design is not governed by cost, size limitations or even the easiest solution. More often, the way I design a project is governed by how I feel I'll learn the most. This circuit seemed to offer me an opportunity to experiment with PWM controllers at the discrete component level, which I felt I would learn much more from.

    I think I'll still go down the road of using a PWM IC now because I've managed to get my prototype working and it seems to be performing quite well, but I think I'll experiment with that circuit!

    Brian
     
  13. Someone Electro

    Someone Electro New Member

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    In all of my project i try to to it the easyest way posible wich is also mostly the cheapest and easyest to get working.Also less complicated means less thing can go worng.

    For instane im curetly making a audio amp for two 20W speakers.Im usinf the TDA 1554 it needs next to no extrenal compomemts and has safety fetures.To make it out of transistors i would need a lot of them and the cost would be much higher.It may also not sound as nice.For the filter i was trying out in the simulator im using the cheap LM741 and i made a "ground" whith a resistor divider circuit to make a single suply op amp unnessery.Becuse i have no dual suply since the amp needs only single suply
     
  14. giaosucan

    giaosucan New Member

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    I think you can design H-bridge circuit to control motor speed by PWM. I have just designed this circuit since monday.
    my circuit was used in robo contest .(do you know this contest)
    In my circuit , i am also use MOSFET ,such as IRF540 .
    it works very well.
     
  15. Someone Electro

    Someone Electro New Member

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    Yea this circuit only drives curent one way so it cant reverse a motor.

    the IRF3205 are good if you want a big motor driver.They have a chanel resistance of 0,008 Ohm when saturated and thats the same as if there was a wire.They are $2,50 bucks each here.But have a prety low voltage of 55V

    They can run a 10 Amp motor whith no heatsink,but its recomended.Also need a very good gate driver to run as cool.
     
  16. Dr.EM

    Dr.EM New Member

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    Yep, the MOSFETs I used had 0.022ohm ON resistance, so the IRF3205 you mentioned are a better choice (depending on saturation voltage at the gate). Use the lowest Rds ON mosfets you can find for using this circuit. I think i'm going to have to try this with a 20A motor I have, though my diodes aren't extra fast.
     
  17. Someone Electro

    Someone Electro New Member

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    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.
     
  18. Dr.EM

    Dr.EM New Member

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    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 :D
     

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  19. Hero999

    Hero999 Banned

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    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.
     
  20. Someone Electro

    Someone Electro New Member

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    This huge motor is probobly very loud by it self.

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

    Nigel Goodwin Super Moderator Most Helpful Member

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    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.
     
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