Question about relay capacity

Discussion in 'General Electronics Chat' started by urbaud1, Nov 10, 2010.

  1. urbaud1

    Thread Starter New Member

    Oct 17, 2010
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    I recently purchased an electronic hobby kit to build an Infrared Toggle Switch (Circuit Specialists, Kit 58) which I want to use to turn on/off a standard 100 watt incandescent light bulb. I have assembled the kit and it works perfectly. In the instructions it says: “The maximum contact current is 1A. This on-board relay should be used to control an off-board relay or switch. It should not be used to switch high currents or high voltages directly.” However, the rating on the relay says: 3A 120VAC 24VDC (Songle SRSZ-12VDC-SH). Also, my understanding is that a 100 watt bulb draws .83 amps (100/120).

    I have two questions: 1) because the relay indicates the capacity to handle up to 3A and a light bulb draws considerably less (.83 amps) could I not use the relay directly? And 2) if I have to use an off-board relay or switch, can someone provide me detailed instructions on how to do this?

    This is a link to Circuit Specialists:

    http://www.circuitspecialists.com/prod.itml/icOid/4030

    urbaud1
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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    The relay contacts are rated for 3A 120VAC into a resistive load, which a light bulb would qualify as. You're fine.

    Were it an inductive load you probably wouldn't want to exceed an amp or so and also place a small capacitor across the contacts.
     
  3. urbaud1

    Thread Starter New Member

    Oct 17, 2010
    13
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    Hi marshallf3,

    Thanks for replying to my post. So you think I could hook the 100 watt bulb to the relay directly. What is an inductive load? I think I understand what a resistive load is. Also, would I need a capacitor with the resistive load? Just curious, what kind of small capacitor? Can I assume you are an electronic engineer?

    Thanks again,
    Dan
     
  4. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Yes, an old engineer but some things don't change except you all do everything with processors nowadays. :)

    An inductive load would be something like a motor or a transformer; anything inductive in nature.

    No, you won't need a capacitor with the light bulb or any plain resistive load.
    If you put a small motor on there you'd want something like a 0.01 or 0.1 uF capacitor rated well above the voltage the load would normally be seeing. This can help with suppressing any arcs on the relay contacts.

    Most inductive loads are designed to be powered by AC, most resistive loads can be powered by AC or DC.
     
  5. urbaud1

    Thread Starter New Member

    Oct 17, 2010
    13
    0
    Hi marshallf3,

    Yes, it's me again. A question I forgot to ask you. Why do you suppose the instructions for the IR toggle switch say not to connect the load directly to the relay? Do they know something we don't?

    Thanks again for your replies and the info about the relay.
    Dan
     
  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    It could be one of two reasons. Perhaps the writers of the instructions were being overly cautious, or perhaps the PCB tracks from the relay contacts to the terminal block are too small for more than 1 amp.
     
  7. DigitalReaper

    Member

    Aug 7, 2010
    70
    2
    It may be that the design of the PCB is unsafe for high voltages (eg. high and low voltages running side by side), driving a second relay removes that problem.
     
  8. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
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    Carefully consider the last two posts, as well, the board when mounted must be secure, of good clearance to it's surroundings, power wires secured/fused, and any metalic housings grounded.

    Claiming abiltiy to switch power loads would have required a UL rating.
     
  9. urbaud1

    Thread Starter New Member

    Oct 17, 2010
    13
    0
    Hi Tracecom, DigitalReaper, GetDeviceInfo,

    Thanks for your replies. I assume you mean printed circuit board when you use the term PCB. It's small, 1" X 3" and all of the tracks are 1mm in width. FYI-I did hook up a standard 100 watt bulb to it, via the terminal block and it appears to work fine. I am using a 12V battery to power the PCB. Each time I click a button on the TV remote the light goes on and off when clicked again. I have not left it on for any length of time, so I don't know if the relay will get hot.

    DigitalReaper how would I run a second relay? What kind, how is it powered and what is the circuitry? I'm not an engineer so I don't have the knowledge you do. Given what all three of you say, perhaps I should run the second relay. I'd appreciate any comments about the second relay or the fact that the PCB appears to handle the 100 watt bulb.

    Thanks again.
    Dan
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    A 1mm trace will handle an amp easily if it's standard 1 oz copper, the main concern is that you've got live mains (house) voltage loose on something that may be unsafe to do so on. Caution, and treating it as an experiment instead of a permanent circuit, you should be fine.
     
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