Circuit explanation/discussion..

Discussion in 'The Projects Forum' started by squelch, Apr 6, 2010.

1. squelch Thread Starter New Member

Mar 22, 2010
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like: Q1: how does R1 and R2 limits the voltage input ?

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2. Engr Member

Mar 17, 2010
114
5
R1 and R2 will act as a voltage divider of the input voltage.

3. squelch Thread Starter New Member

Mar 22, 2010
6
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tnx sir,
sir Engr., this circuit is a bidirectional dc speed motor controller,
Q:is there any reasons why the R1=100k and R2=10k resist. are used in this design ?

4. ifixit Distinguished Member

Nov 20, 2008
639
110
Yes. This resistor string ensures the pot has a center position where the motor is off, and an upper speed limit for motor clockwise, and counter clockwise rotation.

Details...
IC1b is an oscillator, which has variable duty cycle and frequency depending on the voltage set at the output from IC1a. The output from ICb-6 is triangular with the average center voltage being adjustable with P1.

IC1a is a X1 buffer of a voltage set by the R1, P1, R2 resistive divider. P1 also sets the DC offset value of the triangle output of IC1b-6.R2 sets; a minimum duty cycle value to some nominal value like 10%, and the lowest triangle wave offset.

R1 limits; the dutycycle to approximately 50%, and the highest triangle wave offset. The frequency is around 200Hz. IC1c & d make up a window comparator circuit. R6,7, & 8 set reference voltages for the window comparator. The triangle wave will clip the upper, or lower window boundaries depending on where the P1 pot is set.

Neat circuit. Does it work well?

5. squelch Thread Starter New Member

Mar 22, 2010
6
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THANK U very much sir ifixit,,
,yes the circuit is working properly,
sir, can i ask some more questions ?
Q1: what is resistor string ?
Q2: what formula did you use on finding the frequency (200Hz) ?
> how about for R3=470K what is the purpose of it ?

6. ifixit Distinguished Member

Nov 20, 2008
639
110
• A "string" of resistors is just a way of referring to many resistor connected in series. In this case; R1, P1, & R2.
• The frequency is approximately equal to 1/(C1 X R5). However, since the switching thershold can be changed by the Pot, the actual frequency will be a little lower or higher depending on the P1 setting and the value of R3 and R4.
Regards,
Ifixit

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7. squelch Thread Starter New Member

Mar 22, 2010
6
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im Back ,!
again, tnx for the response sir ifixit..

:another questions;
Q1: how about for C3 and C2 whats their purpose ? (are they the one that filter frequencies ? not sure 'bout that)
Q2: is there any further explanations/computations why the values of (R6=33K,R7=10K,R8=12K) are used on the design ?

-tnx,

>others can also give comments/discussions on the circuit.. tnx..

8. ifixit Distinguished Member

Nov 20, 2008
639
110
1. C2 & C3 are for power supply decoupling, which means they prevent motor switching noise from interferring with the PWM circuits.
2. R6,7, & 8 set reference voltages for the window comparator. To find the current in the string do this: I=Vc/(R6+R7+R8). Now the voltage across R8 is R8 x I, which is the lower window thershold voltage, and the voltage across R7+R8 is (R8+R7) x I, which is the upper window thershold voltage.
Try to calculate these voltages yourself, but first measure Vc at the top of the string. Let us know how it goes.

Good Luck,
Ifixit

9. squelch Thread Starter New Member

Mar 22, 2010
6
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sir, what is responsible for the reversing of the rotation of the motor ?

10. ifixit Distinguished Member

Nov 20, 2008
639
110
The pot position determines the motor direction and off mode.

When the signal from IC1D-13,10 goes more positive than IC1D-12, the motor rotates one way, and when the signal goes more negative than IC1D-9, the motor rotates the other way. When the signal is neither then the motor doesn't rotate at all.

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11. squelch Thread Starter New Member

Mar 22, 2010
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>so, in other words the op-amps as window/voltage comparators
are the one that acts/dectates what rotation it will go ? (not sure )

12. ifixit Distinguished Member

Nov 20, 2008
639
110
Your Welcome, and Yes that's right. The operator determines the motor direction by turning the pot and then the window comparators activate the motor driver.

13. professorslk New Member

Apr 30, 2010
3
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Hi Mr. Fixit,
I am currently building the same circuit: Bi-Directional motor control circuit. Your replies to Squelch questions have helped me a lot too, thanks. How ever, I still have one question too, I know the purpose of the transistors is to assist with switching the MOSFETS, how ever, the transistors are reasonably switching slowly. What else can I use to replace the transistors, or is there a way I can change my drive circuit (window comparator) in order to avoid using transistors. If you have a circuit depicting this, it will be appreciated.

14. ifixit Distinguished Member

Nov 20, 2008
639
110
Hi,

Without re-designing the entire driver, you could try changing the value of R10, R11, R12, & R13 from 10K to 1K. You should see a good emprovement

The gates of the MOSFET drivers are capacitive and need to be driven harder to avoid slow rise and fall times.