help regarding 555 connected to relay

Discussion in 'General Electronics Chat' started by devalvyas, Apr 18, 2009.

  1. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    Hi again,

    Need some help.

    I am using a 555 circuit in mono-stable mode to run a dc motor. I am supplying 6 volts to 555 from a 6 V, 4.5 Amps re-chargable battery.

    regarding connection of 555, i have attached a jpeg.

    The current drawn by motor is around 3 amps, hence i am using a 555 to operate a 6 volts relay to do the job.

    The optput pin 3 of 555 directly operates the relay connecting the motor with the 6 V, 4.5 amps battery.

    now i am facing two problems.

    1. once the motor starts or the relay is closed, it does not allow the output pulse of 555 to go down. the output pulse continuously remains high.

    2. The motor does not seem to get the same power as it does in case i directly connect it to the battery without the relay and electronics.

    can you please help?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    This sounds like some problems other people were having. You could need bypass capacitors, along with a capacitor on pin 5.

    Something similar to this, only with C1 used as an input and a filter capacitor across Pin 8 and 1?

    [​IMG]

    Whenever you ask a question it is a good idea to have a schematic, it clears up what you may be talking about.

    Have you read the 555 Monostable article?
     
  3. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    Thanks. I will try it our and post the responce.

    Will this also solve the problem of the motor getting less power?
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    Since you didn't use a diode across the relay coil, the 555 is probably destroyed.
    I have seen datasheets that use two diodes.

    The 555 does not time-out if pin 2 is still low (by the switch being held down too long). The datasheet for the 555 shows how a coupling capacitor plus one resistor can be added to your circuit to allow the switch to be held down for a long time and the capacitor creates a short pulse.

    The relay's contacts and their wiring to the battery and to the motor are powering the motor, not the 555 circuit.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    The motor getting less power is more than likely a problem with the relay. Have you checked the voltage across the relay/motor when this happens?

    I notice you're not using a capacitor with the switch. Read the 555 Monostable article, you can not leave pin 2 in the low state, it is an illegal condition. I still need to verify for myself exactly what happens, but one source states the 555 oscillator oscillates. Use a signal conditioner similar to what is shown in the article.

    AG posted the same time I did, but he is probably correct, a diode as shown in my schematic is a must. I would also recommend a transistor (as shown), to get the load away from the 555.
     
  6. zimbarak

    Active Member

    Feb 8, 2009
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    it is not a monostable circuit since there is no spst switch! to get a monostable multivibrator u need to use a switch [​IMG]between the vcc supply and the circuit ,
    and also try to adjust the duty cycle of the circuit T=h1/h1+h2
    since h1 is high state and h2 is the low state !
     
  7. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    I am not keeping pin 2 in low state....i am only momentarrily connecting it to ground to give a pulse and opeaning the switch. My mono-stable time is of few seconds and the puse i am giving is less than a second. i just touch the ground wire to pin 2 and remove it..
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    Uhh, no. Read the monostable article. None of what you said is how a 555 monostable works.

    That will work, but it isn't good design practice, since it is possible to take the 555 into an illegal design condition.

    So, have you tried what AG and I said yet, any of it?
     
  9. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    I can find no fault with your origional ckt. except for a diode in series with relay coil, with anode to P3 and another across relay coil, anode to ground.
     
  10. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    ITS WORKING NOW...

    To debug...I tried to measure the voltages..

    If i dont connect the relay:

    The output voltage of pin 3 is 4.5 to 4.6 volts during the pulse duration as expected and drops to zero on time-out.

    When i connect the relay.

    The output voltage of pin 3 is 4.5 to 4.6 volts during the pulse duration as expected BUT ON TIME OUT, the output voltage does not go below 0.9 volts. hence the relay is not able to open i believe. I had connected the output of Pin 3 to one terminal of RElay and second terminal of relay was grounded...

    Now as per relay datasheet, RELAY SC5 S DC6V, the pick up voltage of relay is 4.5 volts (max) and release voltage is 0.6 volts (Min).

    i have added diode in series with relay, with - connected to +Vcc and + connected to pin 3.

    adding capcitor at input pin 2 is not helping as the problem is not of triggering i believe... however as bill said, it good design practice...

    When i added the transistor as a switch between the relay and the pin 3, its working fine. Now the output of the pin 3 is connected to transistor switch. one terminal of this transistor switch - Collecter- is connected with relay and second terminal of relay is connected to +Vcc.

    in the earlier circuit, the output of 555 was powering the relay, in later circuit the output of 555 is powering a transistor switch.

    can you please explain why its working now...is the transistor better at switching compared to what the output of 555 can do on its own?
     
  11. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    few more questions

    1. how do i select values of R1, C1 and R2?
    2. Do i need any protection circuits like current or voltage protection for the above circuits?
    3. is this a good circuit with respect to power consumption and voltage drop or can it be further improved? my ultimate aim is to make sure that the motor receives the max power, voltage and current and also that it received steady voltage and current as the battery discharges...

    Thanks a lot...
     
    Last edited: Apr 19, 2009
  12. Wendy

    Moderator

    Mar 24, 2008
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    The values of R1, C1, and R2 are not critical. They are signal conditioners, meant to narrow the on/off pulse to an extremely quick spike. It is common practice in a lot of digital electronics.

    It is possible the 555 is walking wounded, if it was run without the diode. This means it may fail without warning at a latter date. This is because the inductive kickback from the relay coil resembles a static zap. It's not of course, but it can be high voltage and current. For more information on Walking Wounded see ElectroStatic Discharge from the AAC book.

    The 555 is a great little chip, with excellent drive, but it can't compare to an external transistor. A 555 is rated at 200ma (CMOS versions much less), while a simple 2N2222 is rated for 600ma, other transistors can have even more drive. That inductive kickback would zap a transistor first too.
     
  13. zimbarak

    Active Member

    Feb 8, 2009
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    for sure my friend devalvyas the ouptut of the 555 cant command a relay!
    u must use a transistor to activate the relay ! sorry about that didnt pay attention of ur pic posted ! i saw the 2nd one of bill marsden !!
     
  14. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Might help to post new revised ckt. If still using relay to drive motor, it is just a waste of power, use a conventional transistor rated for motor current, or MOSFET. The 555 will drive a relay just fine if it is within its ratings, @ 6V suggest 60Ω or greater for relay coil,with 2 diodes. I went thru this same exercise 40 years ago.
     
  15. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    can i use 2N3055 in place of relay?

    i guess i would still require a small traisistor like 2n222 to drive the 2n3055 . My motor current is going through collector where Ic which will be aorund 3-4 amps. the Hfe of 2N3055 is about 20, so the driving base current has to be atleast 5 times 4/20 = 1 amp. now 555 will not give more than 200 mA.

    if the driving base current is so high how will it reduce power?
     
  16. Wendy

    Moderator

    Mar 24, 2008
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    Yes, a 3055 will work if you use a 2N2222 in a Darlington Pair. If the power transistor starts to get hot you may need heat sinking.

    If you already have the relay go with it though, Darlington transistors drop .6 volts at saturation, as opposed to the normal .1V of a single transistor.
     
  17. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    The 2N3055 that I have has a gain well over 20 at 5A. [ TO220 package].Might give it a try ,then go to Bill's darlington or sziklai pair if necessary.
     
  18. Karl

    New Member

    Apr 20, 2009
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    Unless I missed it (or was already covered) the transistor probably will handle the current draw from the relay's coil better than the 555 timer, which is normally designed only to sink around 100 ma of current. Driver transistors are rather a common way around this issue.

    Also, agreed, you *need* to install a protection diode across the relay's coil in a reverse fashioned. If you install it in the wrong direction (forward biased) you'll destroy it. When a relay’s coil is de-energized, the collapsing field voltage on a coil (basically a choke/inductor) produces quite a high voltage “spike”. That’s the basic principle of automotive spark ignition from when the ‘contact’ opens on the distributor. (old school).

    Karl in Spokane
     
  19. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
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    thanks for the valuable inputs...

    now i might have to use a universal motor connected to ac 220 V, 50 Hz, in case my dc motor does not work. I believe the there will be no changes in the circuit as long as i am using a relay to control the circuit.

    I will power the control circuit with a ac-dc converter with 12 Volts, 500 mAmps rating, and I will power the universal motor saparately by ac mains. the circuit will turn on and off the relay which gives power to universal motor.

    pl comment.
     
  20. Wendy

    Moderator

    Mar 24, 2008
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    Make sure you have good filter capacitors around the 555. Motors tend to generate a lot of static.

    The spec for a 555 is 200ma, but most people limit it at 100ma to insure a long lifetime. The CMOS version runs a lot less than that, though it is a respectable drive for a CMOS chip.
     
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