Simplest circuit, driving me nuts!

Discussion in 'The Projects Forum' started by davebert, May 22, 2010.

  1. davebert

    Thread Starter New Member

    May 22, 2010
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    At the link below you will find a simple circuit (the kind I like) that I was going to quickly put together for a friend that I have since fixed his problem. It is for replacing a motorcycle flasher relay as the dealer wants way to much for one on an older Honda if you can even get one. I tried a regular heavy duty auto flasher and that didn't work so I was going to gain some control with this simple circuit so I could adjust accordingly the rate of flash. Anyway, I am not a student, its not homework, and I just want to understand why it doesn't work as per the plans, or if the plans are just plain wrong. I am using a 5 pin mini 12v relay that is SPST and has a 12V coil. It has the extra pin for the load to be switched from NO to NC. I can get it to light the bulb at varying rates by changing the polarized cap with different values. But I cannot get it to repeat the cycle on its own without reversing the cap leads manually and watch it for the one cycle once again. I have made LED flashers with 555 timers in the past but this circuit looks so simple I want it to work. Any suggestions on what is the problem.:confused:
    Thanks,
    db
    http://www.circuitstoday.com/simplest-lamp-flasher-circuit
     
    Last edited: May 22, 2010
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    What for lamp is used as load?
    In the circuit a 5 Watt lamp is specified.
    With a larger lamp the capacitor is charged much quicker.

    Bertus
     
  3. retched

    AAC Fanatic!

    Dec 5, 2009
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    Are you using a polarized capacitor? What size bulb?
    Is the relay LATCHING by any chance? It sounds like it. Thats why you need to reverse polarity to reset the relay.
     
  4. davebert

    Thread Starter New Member

    May 22, 2010
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    I don't know the wattage but that is a good point. I had a third tail light aftermarket unit laying around and used it. I will see if I can identify the bulb and wattage.
    Thanks,
    db
     
  5. davebert

    Thread Starter New Member

    May 22, 2010
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    Yes its a polarized capacitor but I need to check the bulb. I am using a 12v battery for the power and the bulb glows nicely. As far as the relay latching, I bought them off of ebay and they don't have much of a spec sheet. I will look again and see if I can answer that though as I copied what I could from the ebay sales page as the info can not be found with google.
    Thanks,
    db
     
  6. davebert

    Thread Starter New Member

    May 22, 2010
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  7. davebert

    Thread Starter New Member

    May 22, 2010
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    Ok, I took apart the aftermarket 3rd stop light and it has a GE921 bulb in it and a quick google search says it is about 18 watts. Would that make the circuit fail like described? What would be the formula to get the right resistor and capacitor if I have X amount of watts? On a motorcycle it would be 2 bulbs at a time. I figured it would be some trial and error but didn't think the wattage being incorrect would make the circuit not restart after a capacitor discharge. I did think about the cap being polarized and know I used polarized caps with other flasher circuits, and besides the schematic shows a polarized cap.
    Thanks,
    db
     
  8. retched

    AAC Fanatic!

    Dec 5, 2009
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    I was just wondering if you had the polarized cap installed backwards, by chance. And yeah, with an 18w lamp, you are almost quadrupling the cap resistor set up.

    The relay needs at a certain amount of time completely off to unlatch. You man not be achieving that. with such a draw in the lamp.

    You should probably try a resistor 4 times the resistance and see how she does.

    Do you have a 1kOhm resistor on hand? try that.
     
    Last edited: May 22, 2010
  9. davebert

    Thread Starter New Member

    May 22, 2010
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    The cap was installed correct right away as I have replaced lots of caps and know about proper orientation. I will see what I have and run some more breadboard tests. I know I have enough to run some in series to make a 1kOhm if I don't have a 1KOhm. I will try that tomorrow and run to the RatShack if necessary...
    Thanks,
    db
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Unless the relay contacts have significant resistance, the series resistor, relay coil and cap are completely shorted out when the contacts close. Lamp current should not affect discharge time. Increasing the series resistance will increase the time it takes for the cap to discharge, since the resistor is in parallel with the relay coil and the capacitor during this portion of the cycle.
    Maybe the relay contacts are getting welded together, due to the low initial lamp resistance and consequent high inrush current. I have seen numbers like 10:1 hot vs cold resistance. With an 18W lamp, this could mean 15A inrush current, which exceeds the contact rating by 50%. Also, I believe that a relay's DC current contact rating is less than the AC contact rating. This relay has an AC contact rating of 10 Amps.
     
  11. retched

    AAC Fanatic!

    Dec 5, 2009
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    What are you using as a power supply?
     
  12. davebert

    Thread Starter New Member

    May 22, 2010
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    A 12v battery in the form of one of those jumper boxes that will run all kinds of thing with all the included connectors. Its old and the battery shows a little less than 12.5 volts.
    db
     
  13. davebert

    Thread Starter New Member

    May 22, 2010
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    I don't know if it matters but I am using a separate wire to the contact for the lamp which is a different relay from the original circuit only in that respect. I will try a few things later today and see what I come up with but you are correct I believe that DC is harder on the contacts than AC. In the end I will probably try this again with a 555 timer and another switching device after I do some more research...Maybe that is why they use bimetallic strip of steel and heat to make the old flashers work. :rolleyes:
    Thanks,
    db

    p.s. I will check the resistance on the original circuit to see how many watts are used to flash 2 blinker bulbs and more than likely start over as this is a good lesson I see coming. :)
     
  14. davebert

    Thread Starter New Member

    May 22, 2010
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    Ron,
    I was thinking that when I checked with Ohm's law the current would only be about 1.5 amps. How do you figure inrush current or is that only with an amp probe?
    Thanks,
    db
     
  15. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    With an 18W lamp and 12V, steady-state current will be 1.5A. The cold resistance of an incandescent lamp is ≈10 times the hot (steady state) resistance. Wikipedia says the ratio is 15:1. Therefore, the initial current will be at least 15A.
     
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  16. davebert

    Thread Starter New Member

    May 22, 2010
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    Thanks for the info Ron. After some more thought this circuit won't do what I planned in the first place as effective as a better circuit so I am going to start over. I have some other plans and will review them and then start a new thread if I have any questions. I knew it looked too simple to be useful. :)
    db
     
    Last edited: May 24, 2010
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