Relay

Discussion in 'General Electronics Chat' started by tobyw, May 30, 2013.

  1. tobyw

    Thread Starter Member

    Mar 21, 2013
    37
    4
    I'd like to wire up a bedside table type lamp to a relay so that I can control it with a picaxe chip and an infrared remote.

    I have never built anything with a relay before, and naturally I have a healthy respect for mains voltage.

    So my question is...would this relay be suitable?

    http://www.rapidonline.com/Electronic-Components/12v-Rudh-Series-12a-Spdt-Relay-60-4680

    It seems to be rated for 250VAC and 12A, which seems ok.

    My other questions is about wiring it up. Can I just solder the four pins to some stripboard, solder in some screw terminals, wire up the lamp cable to the screw terminals and put the whole thing into a sturdy plastic enclosure? Would the copper strip on the stripboard be capable of handling that current?


    Thanks
     
  2. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    Actually a 100 W bulb would have about
    amps = 100 W / 115 V
    amps = 0.9 amps
    So your question is "Can the copper strips on the stripboard handle a 1 amp current?"

    I'm not certain, but I would think that it can. Check with the datasheet for the product.
    If you switch to compact fluorescence, it would be about 25 W (or less). The stripboard should be able to handle 250 milliamps.
     
  3. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    I just realized that most of the current is only through the relay (which may not even use the stripboard). The voltage on most of the stripboard would be the reduced voltage (5V), so the current should not be more than 100 mA.
     
  4. Georacer

    Moderator

    Nov 25, 2009
    5,142
    1,266
    I can't imagine a bedside lamp drawing 100A. Even if that was the case though, the stripboard wires can carry several amps. I 'd bet up to 3A safely. We need thickness ratings to be sure, but calculators with standard values return that 6A need a 1.8mm wide wire..
     
  5. gerty

    AAC Fanatic!

    Aug 30, 2007
    1,153
    304
    tobyw likes this.
  6. Stuntman

    Active Member

    Mar 28, 2011
    181
    47

    FTW

    Easy, safe, and gets you away from powering a relay coil with an mcu (you'll thank us later).
     
    Georacer likes this.
  7. tobyw

    Thread Starter Member

    Mar 21, 2013
    37
    4
    I was thinking that the mains voltage / current would be through the copper strip joining the load pins of the relay to the terminals for the mains connected cables.

    I'm in the UK, so mains voltage is 240VAC. So does that mean the amps will be lower since the wattage is the same? Ie. 100w/240VAC rather than 1002/110VAC
     
  8. tobyw

    Thread Starter Member

    Mar 21, 2013
    37
    4

    I would love to use that..but it's getting a bit expensive for me. Why is it a bad idea to power a relay from a microcontroller?
     
  9. Stuntman

    Active Member

    Mar 28, 2011
    181
    47
    It is not a bad idea, but just not as simple as powering an LED for an SSR.

    Consider this (and know that i am not familiar with PicAxe so there could be some variations on the themes):

    The relay you listed has a 12V coil, which means you are not going to be able to supply it with the same power rail as your MCU. So count on a second power rail from somewhere (which could end up being quite pricey and add quite a bit of circuitry/bulk.)

    After getting this 12V rail setup, know that you have alleviated this being battery operated unless you wanted a car battery along for the ride. Even if you did line up a nice 12V small battery, relays like the one you listed often pull around 30-50mA at 12V. Let's be kind and say you could magically power this with an alkaline 9Vbattery. At 30ma, a 9V battery would last roughly 17hrs. Bottom line power requirements are hefty.

    Finally, now that you are having to control a 12V signal, you'll have to have additional circuitry (such as a FET between the microcontroller and the relay line) to actually turn the relay on and off.

    Yes, there are lower voltage coils, even 5V, but again, you will still run into power constraints, and I find it VERY hard to believe you could find a 5V coil that could be powered directly from an MCU pin.

    Finally, no matter WHICH coil you pick, you will then have to deal with inductive flyback voltages (If you do not know what that means, do a quick google search for an explanation). At a minimum, this requires an extra diode in your circuit to protect your mcu and other components.

    Juxtapose all this to an SSR, which can be powered (like an LED) directly from an MCU pin, uses very little current (10-15ma), has no flyback voltages, needs no additional power supply, has great isolation properties (between the mains power and LV power), and theoretically will last longer than your electromechanical relay.

    Look on fleabay for cheap SSR's. They can be had. There are also much lower power versions available like this:

    http://www.mouser.com/ProductDetail/Panasonic/AQH1223/?qs=g5ciJ0jwZaELofdq37SNHQ%3D%3D&gclid=CPX9v5bpvrcCFa9aMgodcgoAKQ
     
    tobyw likes this.
  10. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    yes, since the formula is Power = Voltage x Amperage
     
  11. tobyw

    Thread Starter Member

    Mar 21, 2013
    37
    4
    Thanks for the detailed explanation, Stuntman. I didn't really know much about the practical side of how the relay is going to work.

    I bought some 5v relays before I saw your reply. I was going to try using these with a transistor so the MCU doesn't need to supply the whole 5v. I will add in a diode to prevent back voltage problems.

    It sounds like SSR doesn't have to be as expensive as that first link though - so might be easier to stick with those. The link you posted seems to be a simple IC - is that right? You just stick an IC on a stripboard, wire one set of legs to your MCU and the other set to the mains?

    Otherwise, there seem to be items which are more sturdy looking

    http://www.ebay.co.uk/itm/Solid-Sta...ial_Automation_Control_ET&hash=item53f85ee7ca

    Is that the sort of thing I could use to run a mains lamp?
     
    Last edited: May 31, 2013
  12. Stuntman

    Active Member

    Mar 28, 2011
    181
    47

    tobyw,

    Yes, you are on the right track with that SSR. It will run a lamp with ease.

    That small unit I posted was just an example of what smaller SSR's are available. When people think of SSR's, they generally think of the ones like you (and gerty) posted, which are industrial rack mount units. There are smaller packages that do a similar job, but realize that the industrial units are cheap because of their abundant supply, and built to withstand lots of abuse.
     
Loading...