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 -----

From: Duane C. Johnson

To: beam@yahoogroups.com

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 -----

From:Duane C. Johnson

To: beam@yahoogroups.com

Cc: wilf_nv

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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   CUL8ER  \    \ \     \      \ \\   \      \  Receiver
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