PWM help needed

Discussion in 'The Projects Forum' started by Jacob J, Jun 18, 2009.

  1. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    Hello

    I was looking for a very simple PWM circuit to control a single 6V DC motor. I found this project, wich I think looks somehow simple.

    http://forum.allaboutcircuits.com/showthread.php?t=9016

    I have made only small electrical circuits, so I am not very familiar with schematics and stuff. I am from Denmark, so my english is limited, but i'll try to make everything as understandable as I can.

    Well back to the circuit in the link above.

    I have found these parts at my local electronic store:

    1. C4093 CMOS NAND DIP14
    2. Si-rectifier 50V 1A DO41 diodes
    3. Potentiometer 470k
    4. Cer Capacitor 3.3nF 500V P5,08 20% Z5U
    5. Resistor 3W 5% 3.3K Taped

    I have calculated the resistor to 3.3K. He says; "multiple your Vdd with 500" = 6V x 500 = 3000 ohms = 3.3K ohm. Hope it is right.

    I have not been able to find a capacitor with 3.9nF as he uses in his circuit, but as he says, I can decrease it and get a higher freq, so I think it will work with the 3.3nF, or am I wrong about that?

    Well now for the tricky part. He says that the MOSFET (I think that its something like a transmitter) needs 10V before the circuit runs well. I only have 6V, so what can I do about that? He says that if I use something else than a MOSFET, then I need a 10K resistor, but where do I put that and what is "something else"?

    Hope you can help me with this

    /Jacob J

    PS. Do I need other parts than those 6 and the motor and power supply?
     
  2. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    Okay, so I just replace the MOSFET with the ULN2803 (shortet and connected as you discripe) and the system should work fine?

    I can only find ULN2803A and ULN2803LW-SMD on my electric stores website. Is one of them good enough or do I have to find one without and A or LW-SMD?

    Does it make any difference with the changes ive made. Have I calculated my resistor right and does the 3.3nF capacitor work?

    And what about the diodes, are they fine too?
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    The ULN2003A is rated for 500mA peak current, but continuous is 350mA, package max is 2.5A.

    The 4093 will not supply enough current to drive the ULN2003A.

    You'd be better off using Bill Marsden's circuit; a bit further down the page. He's using a 555 timer.
     
  4. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    Can you give me the link, cant find it down below. Are there different 555 timers avalible? I cant seem to find the right one on my electronic stores website, maybe you could give me some model-numbers or maybe take a look at the website. You can search in the right side.

    http://elektronik-lavpris.dk/
     
  5. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    I have found this guide. I dont know, why he calls it dirty, but it seems to work.

    http://www.instructables.com/id/S8Q5N9QFHTAIQK7/

    I have found these parts, do they fit into the guide linked above:

    1. Cer Capasitors 100nF 50V P5,08-20/80% Y5V
    2. Si-diode 75V 200mA DO35
    3. Potentiometer 100k
    4. Timer NMOS DIP8 - LM555CN
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    It is called dirty because they use the discharge transistor to drive the motor.
    I have not found the maximum allowed current in the datasheets.
    If the current gets to high the chip will go hot.

    Greetings,
    Bertus

    PS this is my post 2500
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    It's in the link you posted when you started the thread; halfway down the page:
    http://forum.allaboutcircuits.com/showthread.php?t=9016

    Yes, two basic types:
    1) bjt (bipolar junction transistor)
    2) CMOS (these have limited current sink/source capability)

    It seems that all of the timers that your store carries are of the CMOS variety.
    http://elektronik-lavpris.dk/index.php?cPath=5384_5402

    This is a LM555CN single timer:
    http://elektronik-lavpris.dk/product_info.php?products_id=81770
    It is CMOS, so it will have a lower current source capacity than a bjt 555 timer.

    You could use it to drive a TIP122:
    http://elektronik-lavpris.dk/product_info.php?products_id=77696
    ...using an 8k2 (8.2k) resistor between the LM555CN timer output and the transistor's base.
     
  8. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    Now I am a bit confused. SgtWookie, shall I use the circuit Bill Marsden has posted? And shall I use the LM555CN timer and a TIP122 transistor, with a 8.2k resistor inbetween?

    Bill Marsden mentions something about that the post is wrong or doesn't have something to do with the thread.. What is that about?

    Can you please tell me what parts I need to buy. I am going to the city where my electronic store is located tomorrow and its kind of a long trip, so if I could get a list with all the parts with me, then I could buy everything needed to make such a PWM controller for my motor. Then I can figure out (maybe together with you guys) how I can put it together, when I get back.

    If you could please choose the components needed from the site I have provided a link to, it would be great.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Here, see these circuits instead:
    http://forum.allaboutcircuits.com/showthread.php?t=24028

    You're going to need to increase the value of the potentiometer (R3) to perhaps 100k, and decrease C1 to 10uF. Add another 10uF capacitor across the Vcc and GND pins of the 555.
    Omit D1, D2, R1, R2. R4 becomes 8.2k. Use the TIP122 for the transistor.

    I don't know what you have on hand, and I don't know the specifications of your motor.

    I'm afraid I really don't have the time to tell you everything you need to buy, because I am about ready to leave on a two-day trip home, about a 1,200 mile drive (about 1,930km).
    Perhaps someone else has the time to research this for you.

    You will also need some kind of a breadboard or circuit board to wire the components to.
     
  10. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    Well if I use the first circuit in your link, make the changes you say. Use the LM555CN and the TIP122, then it should work?

    When you say I omit the light diodes and the R1 og R2, shall I then still have the wire they where sitting on?
     
  11. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    Acording to the circuit you have linked and the changes you have suggested, I have made the following list of parts.

    1. LM555CN timer (1)
    2. Potentiometer 100k (1)
    3. TIP122 (1)
    4. Resistor 2W 8,2 taped (1)
    5. 1N4001P Diode (1)
    6. 1N4148 (2)
    7. Tantalum Capasitor 10uF 16V (1)

    Is this the list of parts I need to make the circuit in the link and to control the speed of my 6V motor?
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    Yes.

    I don't understand what you're talking about. You mean the interconnecting wires between them and Vcc, the output, and ground? :rolleyes:
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    Get a 0.1µF cap, and a 100µF cap. Maybe a 10KΩ pot. The mix of 10µF and 100KΩ will make a switching frequency of 0.7 Hz, in other words it will happen in a 1 second time frame. Not good.

    In this circuit...

    555 PWM Oscillator

    I planned on using a 100µF cap so you could see the action, and a 0.1µF for the motor. In your case the 100µF will be for the bypass, it goes across the power supply next to the 555. Thinking of that, get another 0.1µF. These parts should be extremely cheap.
     
  14. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    I am not sure I understand all this. Sorry I am so bad at this. Well first of all, ill try to get aluminium electrolytic capasitors for the uF caps.

    Okay, Bill what size capasitor must the C1 have?

    I dont understand the thing with the bypass, from where to where do I need a bypass and is that bypass on the schematics you have designed?

    I can feel that I soon will have a fully working PWM controller for my motor.. I cant tell you how greatful I am, that you provide me with all of your knowledge and circuits and tips.
     
    Last edited: Jun 18, 2009
  15. SgtWookie

    Expert

    Jul 17, 2007
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    Jacob, go even smaller with C1. Try around 10nF, which is 0.01uF.

    You could use a poly cap for that.

    You'll still need a 10uF or larger cap from pin 1 to pin 8 of the 555 timer.
     
  16. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
    159
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    Okay, thanks for the help. Ill buy all the parts you both have said and ill try it. I'll return with the result.
     
  17. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    Now I have modded the Bill Marsden circuit as descriped. Does it look alright? Expecially what about the bypass 100nF capasitor, is it draw'n corrects?

    I have a 10K potentiometer I can switch with, if the 100K doesnt work.
     
  18. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
    159
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    I have found a few errors already. The C2 shall be a 10uF as Sgtwookie said, not 100nF. And it must be rotated, so the + and - will change site.
     
    Last edited: Jun 19, 2009
  19. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
    159
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    Now I am finished with the circuit, or I thought I were. When I put the power on, I get a constant speed on my motor, but I cant change the speed by turning the potentiometer. I have attached a drawing of my breadboard, could you please take a look at it, and tell me, if I have done something wrong?
     
  20. Wendy

    Moderator

    Mar 24, 2008
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    You changed the base resistor. It is way, way too big. Try the value I suggested, or less (say 270Ω).

    Big problem! You have pins 3 and 4 shorted, that 555 is probably toast. Sorry to bear bad news.

    10µF is way too big, remember, this has to oscillate above 10 hz or so. The polarity shown in my schematic and layout is correct. 10µF and 10KΩ equals 7Hz, 0.1µF and 10KΩ equals 700Hz.

    What is the part number of the transistor?

    Nice drawings, think I recognized some of them. :D
     
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