555 PWM question.

Discussion in 'The Projects Forum' started by NM2008, Sep 28, 2008.

  1. NM2008

    Thread Starter Senior Member

    Feb 9, 2008
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    Hi,
    I have just build a PWM circuit based on 555 timer.
    I have the output of the 555 connected through a 1k resistor to the base of a TIP122 transistor, which drives a motor.

    But I noticed the transistor itself drops about 1.6 of a volt regardless.
    So from a 12v supply, through the PWM cct I get a Max of I get 10.4v.
    Is possible to get output closer to 12v than 10.4v through means of a different transistor or is this just something that has to be taken into account when building, like the voltage drop across a diode?
    Thanks for your help,
    Regards NM
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    Can you provide a schematic so that we can better advise you?

    hgmjr
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    The transistor in question is a Darlington, which means two Base-Emitter drops (around 1.4 V).
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    Good point Bill. Sounds like what is called for is a mosfet.

    Still, a schematic would be very helpful in moving the discussion more swiftly forward.

    hgmjr
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Vce(sat) is generally less than Vbe(sat) (See Fig.2 in the datasheet). Vce(sat) is spec'ed when Ic/Ib=250. With a 12V supply, and a 1k base resistor from a 555, the base current will be ≈9mA. If the collector load needs more than (.009*250) 2.25A, he needs a smaller base resistor.
    Even if he has adequate base drive, a MOSFET is the way to go, as hgmjr pointed out.
     
  6. NM2008

    Thread Starter Senior Member

    Feb 9, 2008
    135
    0
    Hi,
    Sorry about the delay, but here's the circuit I have on the breadboard at the moment. It seems to work fine except for the voltage drop.
    Please feel free to comment on it, as I am only at a beginner stage, the first PWM circuit, I tried.
    Thanks Again,
    NM
    555 pwm.jpg
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    At full on, what is the current of the motor? This will have some bearing on the base resistor. I guesstimate you'd need less than ½ the resistance from the theoretical ideal (power supply voltage/collector current*beta).

    It would be a good idea to put a protection diode actoss the motor no matter what kind of drive you have, something like a 1N4001. Motors are basically modified inductors, capable of generating significant voltages (especially if an external source spins them manually).

    Truth, I'm not comfortable enough with MOSFETs to recommend or reject them. They have a low on resistance, and can handle significant current with no heating, but they also draw significant surges (from what I understand) in turning them on and require at least 10V to turn on completely (all of this I learned reading from this site). I do know the transistor setup should work for experimental purposes.

    One last thing, I'm not sure what the schematic would do if the pot was adjusted all the way down, except it is basically a nonfuntional circuit. You might want to throw a minimum resistance (as in a 10-100Ω resistor) in there. As is you can have pins 2,6,and 7 all connected with 0Ω with the pot set to zero.
     
  8. hgmjr

    Moderator

    Jan 28, 2005
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    It is possible that without the protection diode that Bill Marsden has strongly recommended, your TIP122 may have already been damaged. If you have another TIP122 handy and a 1N4001 or similar diode you may want to try it to see if you still have problems.

    Bear in mind that the TIP122 is a darlington transistor as has already been mentioned. A darlington cannot achieve the low Vce sat that a ordinary transistor can.

    hgmjr
     
  9. NM2008

    Thread Starter Senior Member

    Feb 9, 2008
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    0
    Bill,
    I see what you mean with regard to minimium resistance, it seems at the moment if the pot were to be set to 0Ω pin 7 would short to 6 and 2.

    I took the TIP 122 out of circuit, applied 12v to the base through a 1k resistor and got
    the same drop. A 1k is about as low as I can go without it drawing excessive current.
    But when switching in circuit can this be lowered, or will the speed of switching be picked up as 12vDC?
    The motor I use draws 3A at start up but runs at about 1.8A.

    This drop is definitely transistor related.

    If i were to use a MOSFET would I get the same results in motor performance as the bipolar, but with full 12v?
    Would the MOSFET be capable of switching at that speed and current?

    Thanks for having patience with my questions!
    Regards NM
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    Your 555 circuit varies the trequency. It varies the pulse width only a little.
    It needs steering diodes for the pot to vary the pulse width without changing the frequency.
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    Another way to draw something similar...

    [​IMG]
     
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