Motor back emf disrupting 555 performance

Discussion in 'General Electronics Chat' started by csterman, Jan 17, 2009.

  1. csterman

    Thread Starter New Member

    Jan 12, 2009
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    0
    I am trying to build a circuit that will power a small 1A DC motor for 0.5-15s. I found a diagram in this forum and built it without a problem. It works great when I only test it with an LED.

    When I attach a motor, things go haywire. The problem is that the diagram I have includes two separate sources. 1 source for the 555 timer circuit and another separate source for the motor.

    Because this is part of a science olympiad competition, I am bound by the rules to have only 4 x 1.5V batteries. Is there a way to split the 4x1.5V source into two isolated circuits? Is there a way to keep the motor noise/back emf from disrupting the 555 timer?

    I could power the motor with 2 x 1.5V. Will the 555 run on 3V?

    Chris
     
  2. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    Could you post the schematic of your circuit?
    Are the grounds connected together?
     
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    No, the 555 needs a voltage between 4.5 and 15V to run. Use a diode in parallel with the motor to absorb the back EMF created. The cathode of the diode has to be connected to the positive side of the motor.
     
  4. csterman

    Thread Starter New Member

    Jan 12, 2009
    4
    0
    I have tried the diode in parallel with the motor. It doesn't work. Should I be using a rectifier diode or a schottky diode? Right now, I have a random diode I got from radioshack in there.

    I am working on the schematic...

    Chris
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    A shottky diode would be better because is faster but a rectifier diode will be fine. Are you sure the diode is working?
     
  6. csterman

    Thread Starter New Member

    Jan 12, 2009
    4
    0
    good point. I will put the diode on the multimeter and check.
     
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    My electric model airplane has a Cmos 555 driving a Mosfet using PWM for motor speed control. The motor draws a max of 8A from the 8.4V Ni-MH battery and it works perfectly.

    The motor has two 0.1uF ceramic capacitors from each terminal to its grounded case. The battery has a 0.1uF ceramic bypass capacitor and a 100uF bypass capacitor.
     
  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,176
    397
    Were you driving the motor directly from 555 ? If so ,555 overloaded, put 22Ω between 555 & motor; motor may still tunn ,with not much power. Hopefully with driver , might add 47 Ω between motor & 555 + input,with 6V to motor & 100μF to 555+ in. and common[-6V].
     
  9. csterman

    Thread Starter New Member

    Jan 12, 2009
    4
    0
    Thanks everyone.
    With the addition of the capacitors that audioguru suggested and a diode to avoid having the back emf trigger the 555, I think we have solved the problem. It seems to run pretty well now.

    Thanks for the help.

    Chris
     
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