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Re: [beam] Re: Beamish Stepper Motor Driver

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  • Duane C. Johnson
    Hi Wilf; ... When bread boarding the circuits I found there was little difference in timing when high valued input resisters were used. So, when I design with
    Message 1 of 27 , Nov 11, 2002
      Hi Wilf;

      Wilf Rigter wrote:

      > Now a question on the Beamish Stepper Motor Circuit:

      > Since the values of the series input resistors are the
      > same order of magnitude as the RC timing  resistors and
      > their effect on the time constant can not be ignored,
      > I am curious how you decided on using those particular
      > values.

      When bread boarding the circuits I found there was little
      difference in timing when high valued input resisters
      were used. So, when I design with CMOS I usually try to
      use lower values when appropriate. Mainly to prevent
      board leakage currents from disrupting operation.

      My solar trackers must operate in severe environments
      where moisture may condense.

      Secondly, lower input values can decrease power
      requirements by increasing switching speed.

      Thirdly, I don't have a complete set of resisters to
      experiment with so I used what I have.

      On the subject of limiting the input and output currents.
      One should not rely on even the 20mA value as the CMOS
      circuit structures can sometimes latchup. This problem
      is lessened in modern CMOS but can occur. I occasionally
      observe this when bread boarding so I usually have a
      power supply input protection resister to prevent
      damage when experimenting.

       Hello Duane et al, The input resistors and actual component values were omitted for clarity and to show the similarity to monocore circuit. The modfied circuit using the input resistors and the same component values as the original is shown here inline or attached:  When 74HC or AC devices used in applications as relaxation oscillators, the external input resistors are often omitted by design and the input diodes are intentionally used to clamp the input voltage of the timing capacitor. The literature specifies 20ma as the absolute maximum clamping current. While I cannot recommend  exceeding this, 20ma  is quite conservative and higher peak input currents are usually tolerated. 
       
        Oscillators or networks using 74HC/AC parts in quasi-linear applications, like the microcore and bicore, use capacitive coupling. The switching of partially charged capacitors generates potential overvoltages in excess of Vcc/2 at the inputs. If no external series input resistor the transient at the input will be current limited by the internal 100 ohm series polysilicon resistor and will be voltage limited by driver output voltage drop in series with the input internal resistor  and the dynamic impedance of the clamping diodes. For Vcc= 5V  the transient potential overvoltage is 2.5V and the combined internal output and input resistance plus the diode drop would limit the current to less than 20ma even if no external input resistor is used. However, there are other good reasons to include the series resistor related to frequency and dutycyle stability. Applications notes often suggest using a series resistor value of 10x the feedback resistor value to avoid clamping the AC coupled feedback signal. Not calmping the feedback signal effectively increases the RC time constant, decreases power consumption and, importantly, averages the DC level at the input near the threshold.  The latter tends to move the dutycyle of the oscilator automatically toward a symmetrical waveform.  One of my earliest posts to this list described an using series input resistors in an article called Belted and Suspended Bicores including a  method to control of the average dc voltage at the input which can be used to adjust the dutycycle. To complicate matters a little bit, the Beamish Stepper Motor circuit outputs directly drive the stepper coils. The inductive load generates its own transients at the outputs which may exceed the output diode ratings Moreover motor loading of the output causes a voltage drop which together with  switching transients can be coupled back through the feedback capacitor to the inputs and can cause timing instability.  This motor load volatge drop is proportional to motor current and can be put to some good use in other applications to truncate the oscillator cycle and reverse a heavily overloaded motor.  For some applications it is desirable to control the duty cycle of a slave bicore (e.g. turning in a bicore walker by injecting a dc current into the input node) but the averaging effect of adding series input resistors would oppose the dc control signal and must be taken into account in such a design. Now a question on the Beamish Stepper Motor Circuit: Since the values of the series input resistors are the same order of magnitude as the RC timing  resistors and their effect on the time constant can not be ignored,  I am curious how you decided on using those particular values. best regard wilf
      ----- Original Message -----
      Sent: Sunday, November 10, 2002 7:30 AM
      Subject: Re: [beam] Re: Beamish Stepper Motor Driver
       Hi All;

      I find that input protection resisters are required for safety of the inputs
      in AC gates.
      The spec limits the input or out protection diodes to 20mA.
      The outputs can drive several hundred mAs. Clearly this can
      damage the inputs with current fed back through the capacitors.

      The protection resistors weren't as important with the lower
      powered CMOS families.

      The minimum resistance is, in this case, based on VCC and
      the worst case threshold votage.
      ( VCC - 30% * VCC ) / 20mA = R
      ( 7V - 30% * 7V ) / 20mA = 245 ohms

      Prudent design calls for a minimum of about 10K.

      BTW, this is not just academic. I did blow of a couple of AC ICs
      because of this. Remember these are powerful chips.
       


      http://www.redrok.com/images/beamstepper7f.gif

      Neat! Now there are about 5 distinct variations of this basic design.

      Duane

      Wilf Rigter wrote:

      Here is yet another variation of the stepper circuit, somewhat easier to read and clearly shows the master slave monocore topology. Note that in this case the resistors for the slave monocores are connected to the complementary outputs of the master monocore.---- Original Message -----
      Sent: Saturday, November 09, 2002 9:08 AM
      Subject: Re: [beam] Re: Beamish Stepper Motor Driver
       Hi All;

      Wilf and I have been developing another solar tracker
      that is based on a 74AC240 Dual Quad Tristate Buffer.
      There have been a number of variations. This is
      the results. See:
      http://www.redrok.com/images/beamstepper7e.gif

      The 74AC240 stepper driver works by enabling each half
      of the buffer. Only one half can be enabled at a time.

      Let's assume that the top half of the driver is enabled.
      U1A & U1B along with R8, C1, & the input protection
      resister R7 form a square wave oscillator. The outputs
      of U1A & U1B directly drive one coil of a bipolar stepper
      motor.

      U1C & U1D along with R9, C2, & the input protection
      resister R10 form a 90 degree phase shift. The outputs
      of U1C & U1D directly drive the other coil of the bipolar
      stepper motor. The motor turns in one direction.

      If the second bottom half of the driver is enabled the
      oscillator using U1E & U1F work as before. U1H & U1G
      along with R12, C3, & the input protection
      resister R11 form a 90 degree phase shift. Except it's
      connected the other way around from before so it's
      actually 270 degrees. The outputs of U1H & U1G directly
      drive the other coil of the bipolar stepper motor. The
      motor turns in the other direction. Neat, Huh!

      An earlier version of the circuit didn't work well
      because the the sensors presented an analog enable
      signal. This was sometimes at the threshold voltage
      which caused the buffer to have high idle current and
      sometimes cross coupling which was a bad thing. %^(

      What was needed was a sensor that had a Schmitt trigger
      input. This could be done using a Schmitt trigger gate
      which works well. I suggest a 40106 or 74AHCT14. However,
      this needs a second IC.

      A better solution is to make the sensor have Schmitt
      action. The first version was:
      http://www.redrok.com/images/beamstepper7a.gif
      The problem was that it worked over a limited voltage
      range.

      http://www.redrok.com/images/beamstepper7e.gif
      works better. Q1 & Q3 and Q2 & Q4 each form a bistable
      latch similar in operation to an SCR.

      Let's start with the left side without the LEDs.
      Initially no current flows. The series resisters
      R5 & R2 cause a small bias current to flow in the base
      of Q1. Which pass current through R1 causing Q3 to
      conduct. Since Q3 shorts out R5 the current through
      R2 doubles. The output at the collector of Q1 snaps
      high disabling the connected buffer.

      (Note, R5 & R6 aren't actually required. It turns
      out that leakage currents in the transistors is enough
      to get started. I tried many transistors and never found
      one that didn't work as expected. Prudent circuit design
      demands that R5 & R6 be included because one might find
      a transistor that is so perfect it won't work. Bummer. )

      The now connected and lit LED1 has the ability to
      absorb the current through R2 starving Q1 which
      switches off resulting in the output snapping low.
      Q3 also switches off reducing the bias current
      in R2 to 1/2. This condition persists until the
      LED goes dark.

      You might ask where the current for the other side of
      the LED comes from. It is from base of Q2 on the right
      side. Actually, when the left side is turned off the
      right side is turned on doubly as the current from
      both R2 and R3 go through the base.

      The right side works the same way. Since the LEDs
      are connected anti parallel only one latch can
      be off at a time. This is safe for the buffers.

      When both of the quad buffers are supposed to be off
      it is essential that all inputs not be near the
      threshold to have the lowest idle current. R13 & R14
      ensure that all inputs be near ground. All inputs
      are connected to R13 or R14 either directly, through
      input resisters, or through the stepper motor. I
      added R15 & R16 for testing when the stepper motor
      is disconnected. If the motor is permanently
      connected R15 & R16 aren't needed. R13 & R14 can also
      be connected to VCC. They don't even need to be to
      the same voltage, although it operates quicker if
      they are the same.

      I have tested this circuit with about 25 different
      74AC240s. They all worked as expected.

      I ran the circuit from about 2.4V to 8.5V.
      OK, one shouldn't go past 7V to be within the specs
      of the 74AC240.

      The sensor section was tested to 40V. It still works
      well, the sensitivity is less because the bias current
      is proportional to voltage which requires brighter
      illumination to work.

      The step patterns are not perfectly symmetrical because
      this is essentially an analog circuit. Some resister
      adjustment can be done.

      To change the speed of the motor adjust the capacitor
      values. Note, all three need to be the same value.

      I have chosen the time constants of R9-C2 & R12-C3
      to be about 3/4ths of R8-C1. Try to keep these ratios.
      ( BTW, I'm not sure this is the exact ratio but it
      seams about right. )

      The 10M resisters in the sensor are the largest
      commonly available resisters in 1/8W size. I tried
      22M in 1/4W and that worked well with added
      sensitivity. I suppose if you could find 100M they
      would work even better.

      I have a variation which is even more sensitive to
      low light levels. Ask me if you want this variation.

      I have to thank Wilf for his invaluable help in the
      circuit design. Thanks Wilf.

      Have fun, Duane

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           Home of the $35 LED solar tracker.
          http://www.redrok.com/electron.htm#led3
         CUL8ER  \    \ \     \      \ \\   \      \  Receiver
        Powered by\    \ \     \      \ \\   \      \      [*]
       Thermonuclear    \ \Solar\Energy\from the Sun \ /////|
      Energy(the Sun)    \ \     \      \ \\   \ / / /\/ / /|
                     \    \ \     \      \ /\ / \/  /  /  / |
         WA0VBE       \    \ \     \ /   /\ \/   /   /  \/ /|
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      "Red Rock Energy" ===  ===\ /   \ /    \ ===  \ /    ===
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      1825 Florence St  Mirrors,Heliostats,Controls & Mounts|
      White Bear Lake, Minnesota                \ \     /   |
      USA         55110-3364                     \ \        |
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