Control AC device with a DC circuit?

Discussion in 'The Projects Forum' started by vbtalent, Feb 20, 2010.

  1. vbtalent

    Thread Starter Active Member

    Feb 20, 2010
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    My first post, "Hello".

    I'm looking to do a simple project that would allow me to control a small A/C light with a 60 watt bulb with the push of a button. Simply, turn the light on when the button is pushed and turn the light off when the button is released.

    I realize they make a push button that will do this that operates safely at 125v AC but I want to do this using a 12v push button.

    I think using a relay is the only way to go and I can use a WallWart to supply the 12v needed.

    http://www.radioshack.com/product/index.jsp?productId=2062482
    http://www.radioshack.com/product/index.jsp?productId=3060978

    Does anyone have a simple schematic with this in mind?

    I've been reading about the concerns of "reverse voltage" and I'm not wanting to get a 125v surprise, some help would be appreciated.

    Thanks,
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
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    Have you considered the follow:

    A mechanical relay can only work at a fixed voltage.

    On the other hand, the input of an solid-state-relay is essentially an LED and thus can works from any voltage from a few volts up by selecting a suitable series resistor. It will last longer and operate without any noise.
     
  3. BMorse

    Senior Member

    Sep 26, 2009
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    I would suggest trying this out with low voltage. From the sounds of it you have very limited electronics experience to be messing around with AC voltage.....


    For one, why would you want to waste more energy by using a 12 volt DC device to control an AC device.... it seems wasteful, why not get an AC toggle switch for the light, it would be easier and safer for you and others who might come in contact with the device.....


    My .02
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
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    It seems odd. Why would you want to use two power supplies? You want the 120vAC for the light and a 12v from a wall wart.

    You may need to clarify what you want to do. Do you HAVE to use a 60w AC bulb? How about an equivalent DC light source? Car lamps are 12vDC. So you can use a 12v battery (or wall wart) a 12vDC rated momentary switch, and a 12vDC bulb.
     
  5. vbtalent

    Thread Starter Active Member

    Feb 20, 2010
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    Thanks for the concerns and ideas - however I would still like to try and make this circuit.

    "On the other hand, the input of an solid-state-relay is essentially an LED and thus can works from any voltage from a few volts up by selecting a suitable series resistor. It will last longer and operate without any noise."

    I've read that a SSR can fuse together and that would be bad...

    I'm open to ideas and suggestion but I still need to be able to turn on any 115v device (with low amperage) with the use of a small button. The 12v tactile button was just an example. If I plug any 115v device into a power strip I could use the switch to turn on anything plugged into it, why the difficulty in doing it with a tactile button?

    VB,
     
  6. retched

    AAC Fanatic!

    Dec 5, 2009
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    If the button is rated properly, there is no problem.

    The metal contacts are smaller in smaller rated items. It would be like using way to small of a gauge wire for a power line. It will over heat and burn.

    For relays, be it solid state or mechanical, You can get a 12vDC rated coil with 120vAC rated load, and a 12v rated switch, that will do what you want.

    Do some research on home automation circuits. People are doing a lot of this stuff. Using the 5v from their USB ports to activate relays that control lighting and such.
     
  7. vbtalent

    Thread Starter Active Member

    Feb 20, 2010
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    "The metal contacts are smaller in smaller rated items. It would be like using way to small of a gauge wire for a power line. It will over heat and burn."

    Understood.

    I realize the relay will do all of the work. Obviously getting shocked isn't the goal here which is why I was turning to the people of the forum. I would thinkthis would be a rather simple project but it seems like pulling teeth...

    I believe this relay will work: http://www.radioshack.com/product/index.jsp?productId=2062482

    It's the wiring I'm not sure of. I also have read that a diode should be used to prevent a problem when the relay kicks off or when the button is released?

    Do you have any links to some resources?

    Thanks,
     
  8. BMorse

    Senior Member

    Sep 26, 2009
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    I still do not know why you want to do this, but if all you want is to turn on an AC bulb when a switch is pressed, and turns off when the switch is released, using a 12 volt supply and a relay, well that is simple enough....


    Real Control of an AC Load.png


    You can try this circuit WITHOUT it connected to the mains, only power it with the 12 volt supply, if everything works, you will hear the relay click on and off along with the switch press, the 1N4148 Diode is in place across the relay coil leads to prevent back emf from causing any erratic problems.....

    B. Morse
     
  9. vbtalent

    Thread Starter Active Member

    Feb 20, 2010
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    Thank you.

    I will try this first without A/C voltage - good advice.

    VB,
     
  10. vbtalent

    Thread Starter Active Member

    Feb 20, 2010
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    Is this what you are referring to - SSR?

    http://cgi.ebay.com/crydom-D1202-ss...emQQptZLH_DefaultDomain_0?hash=item5190092d96

    VB,
     
  11. retched

    AAC Fanatic!

    Dec 5, 2009
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    Yes. That one in particular will handle 2.5 amps.
     
  12. vbtalent

    Thread Starter Active Member

    Feb 20, 2010
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    I assume its wired the same as what BMorse suggested using an 1N4148 Diode to alleviate any back emf from the relay, or does that not exist with a SSR?

    Thanks,
     
  13. BMorse

    Senior Member

    Sep 26, 2009
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  14. vbtalent

    Thread Starter Active Member

    Feb 20, 2010
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    OK thanks.

    3 final questions.

    1) What would you recommend for F1 in the link you supplied?

    2) When calculating AMPs, a 40 watt bulb produces .33A is this continuous?

    3) Would this work for an SSR? (http://www.newark.com/opto-22/120d10/solid-state-relay/dp/96F1081). It looks like it has control voltage of 3-32vdc and a load voltage of 12-140vac, is that correct?
     
    Last edited: Feb 22, 2010
  15. BMorse

    Senior Member

    Sep 26, 2009
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    1) Any fuse that can handle your load, (2A 250 Volts is typical in some appliances)......

    2) Yes it is a continuous draw while bulb is on.... might draw slightly more when first turned on......

    3) Yes, that would work....... I used this kind to control 60 watt bulbs .....
    [​IMG]

    and used similar ones that you have posted to control higher current loads, such as the exhaust fan, etc... (You can see them on the left lower portion of the pic below....)
    [​IMG]
     
  16. vbtalent

    Thread Starter Active Member

    Feb 20, 2010
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    Got my SSR today and since I've never used one before I thought I would check with the experts before I deemed it to be faulty...

    My DC input is 18V 3.0A... the relay doesn't complete the circuit...

    Am I doing this incorrectly?

    Thanks,
     
  17. retched

    AAC Fanatic!

    Dec 5, 2009
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    Is it a NO or NC relay?

    NO is Normally Open - meaning the circuit is open before the coil is energized.

    NC is Normally Closed - meaning the circuit is closed before the coil is energized.

    If you remove the 18v, does the load side close?
     
  18. vbtalent

    Thread Starter Active Member

    Feb 20, 2010
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  19. BMorse

    Senior Member

    Sep 26, 2009
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    You will not see any changes with a meter that way, you will have to have the AC load for the AC side to conduct properly when power is applied to the SSR..... Read this good info about SSR's >>>http://en.wikipedia.org/wiki/Solid_state_relay

    B. Morse
     
  20. gerty

    AAC Fanatic!

    Aug 30, 2007
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    The relay doesn't have contacts that you can measure in ohms, it has a triac inside. You might get a reading if you switch your meter to diode test.

    edit: just noticed you had your meter in diode test, try swapping the polarity of the meter leads.
    If that fails try your light on it..
     
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