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Topic: Comparator and false triggering. (Read 4931 times) previous topic - next topic

Undert9281

I have it there because after this I will need to add another comparator in the collector of that NPN. I am looking for a constant speed and torque from the motor when the motor becomes loaded I wish to use negative feedback and another pot to adjust gain values. I have achieved a constant smooth speed. I need to add current control, I have it drawn out on a different schematic. I need to solve this problem first.

Paul__B

I have it there because after this I will need to add another comparator in the collector of that NPN. I am looking for a constant speed and torque from the motor when the motor becomes loaded I wish to use negative feedback and another pot to adjust gain values. I have achieved a constant smooth speed. I need to add current control, I have it drawn out on a different schematic. I need to solve this problem first.
The motor goes in the collector circuit.  To monitor the motor current, you put a resistor from the emitter to ground, and measure the voltage across that resistor.

weedpharma

On re reading the OP, I do not know which of the op amps you have the CRO connected to.

Weedpharma

MarkT

#18
Jan 04, 2015, 02:36 pm Last Edit: Jan 04, 2015, 02:39 pm by MarkT
Your circuit shows no decoupling - until you fix that anything could be happening.
The symptoms you describe are typical of an unintended oscillation. 

Rule for comparators: 
1) Decouple well
2) Use positive feedback.
3) for high speed comparators use a ground plane.
All three rules are to prevent unintended oscillation / jitters.

[ An opamp in normal modes uses negative feedback to tame the gain to a small
value like 10 or 50.  Without negative feedback you get gains of 100,000 to 1,000,000,
so issues of instability and oscillation are suddenly very common!]

[ Also - why not use a comparator to do a comparator's job? ]
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Undert9281

What do you mean by decouple? Shall I put a small capacitor say .1uf to the power pin?
What is the ground plane, true 0 volts? What do you mean by using a comparator to do a comparators job?

P.S why not put the motor at the bottom where the negative lead can truly be grounded? It will be drawing 8 amps at 24 volts...that will all sit at the collector no? I would rather have it go directly to ground from the emitters side? A new schematic is provided. The op amp with the A is for current control.

Undert9281

Damn can't find how to upload the schematic on the iphon

MarkT

I use decoupling in the same sense as everyone else - google will put you straight,
searching these forums will put you straight.  Basically the supply can be a feedback
path around an amplifier - you don't want this, you decouple....

A ground plane is a plane that's grounded.  Simple.  Once you get to high speed or
high gain (or both) circuitry then oscillation is all to easy to happen by mistake,
the simplest thing is to follow rules and guidelines to reduce unintended feedback,
or learn more about electromagnetism (Kirchoff's rules break down here, they only
really work at low frequencies anyway).  There is stray capacitance in all circuits,
and its often the culprit.  Ground-planes suck all the stray fields to ground, reducing
mutual coupling.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Undert9281

I noticed it was only the Ping sensor that was affecting the output of the op amp, tied a 10k in parallel and then to ground. Problem solved. The 24V motor only turns on when you break the beam supplied by the Sensor

Undert9281

Circuit is complete, just need to solder it all together. The motor runs at a constant speed and when the motor experiences a load it compensates and maintains a constant speed. I have a constant current as well as current adjustment pots.

raschemmel

See this tutorial for example of previous comment.
Driving motor with transistor
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

polymorph

The way it is wired up, the Op Amp output only goes up to 7.5V (an LM324 can only pull to Vcc - 1.5V), then the Emitter can only go up to about 7V.

You -must- put the motor in the collector of the circuit. And have a resistor between the Op Amp output and the Base lead.

The resistor must be selected so that the Base current is no less than 1/20th of the short circuit current of the motor, but be no more than 1/2 the rated output current of the LM324. The transistor must be rated for about 1.5x the short circuit current of the motor.

BTW, an LM324 is not a comparator, it is an analog Op Amp. You may run into problems using it as a comparator. The LM339 is an open collector comparator.

I haven't assessed the rest of the circuit.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

polymorph

This is -way- to complex. Is there a reason for that?

There is an Enable pin on the 555 timer, and the output of the 555 timer itself can serve as the PWM signal.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

polymorph

Oh, and -any- unused sections of an Op Amp or Comparator -must- be wired to something.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

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