need help finding the right relay (5vdc and 12vdc I think)

Discussion in 'The Projects Forum' started by julienrl, Feb 2, 2010.

  1. julienrl

    Thread Starter Member

    Aug 10, 2009
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    Hello, I am trying to find a relay if anyone can help me (5vdc and12vdc I think). I want to mod an external USB hard drive enclosure so that when the usb cable is plugged into a powered device, the hard drive's power supply is turned on. USB is normally normally a bit under 5V, I don't know if that would be an issue, and the power supply is 12v 2A .

    Thank you
     
    Last edited: Feb 2, 2010
  2. newton

    New Member

    Apr 19, 2007
    9
    0
    how does it turn on now? button? switch? if so, take the switch / button wires off, wire them through the contacts of the relay, or even a small transistor / opto if you like, then take the low side of the coil (if a relay) to 0V (you have to find it in the enclosure) and tie the otherside into vusb. make sure you don't draw too much though, the usb will quit working....
     
  3. newton

    New Member

    Apr 19, 2007
    9
    0
    oh, and if you just need a 5V coil / 12Vdc relay, radioshack is probably your friend.
     
  4. julienrl

    Thread Starter Member

    Aug 10, 2009
    21
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    Won't the 0V be the same as the ground on the USB cable? And I thought that relays did not draw any power once they were on.

    5v coil! ok, I was trying to get one off digikey but I could not figure out how to select the proper relay. There are many different options when narrowing down the searches, could someone help me with what settings to look at? I like making DIY stuff but this is the first time I use a relay and I just know nothing about them.

    Also, will the fact that USB is a little off 5v all the time be an issue for the relay?

    Thanks again!
     
    Last edited: Feb 2, 2010
  5. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    Digikey stock number PB282-ND should serve you well.
     
  6. julienrl

    Thread Starter Member

    Aug 10, 2009
    21
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    Thank you! :D

    1-A little question, the specifications say "Control On Voltage (Max)3.75 VDC" and "Control Off Voltage (Min)0.5 VDC" What does that mean? because 5v is more than 3.75 and when its off, its less than the minimum (since its 0).

    2-Also, does a relay draw power constantly?

    3-From what I can see, this relay would work too right? (cheaper shipping costs)
    http://www.sparkfun.com/commerce/product_info.php?products_id=524

    4-Finally, what happens if I use one that has a 12v coil raiting; does the coil need 12v to operate or is 12v the minimum needed for it to operate? like this one http://www.sparkfun.com/commerce/product_info.php?products_id=9508
     
    Last edited: Feb 2, 2010
  7. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    The coil voltage rating is what the relay is designed to properly operate with. If you check the data sheets of the two relays from Sparkfun, you will see that they will operate with 70% and 75% of the rated voltages. 75% of 12 volts is 8 volts, so it would not operate for you at 5 volts.

    Hope this helps. I referenced a Digikey part since your original post said you had looked for a relay there. Most suppliers will have either a complete manufacturer part number or a link to the data sheet of the devices they sell. Data sheets give all the technical information about a particular device. Usually more information than is needed for a hobby project.
     
  8. julienrl

    Thread Starter Member

    Aug 10, 2009
    21
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    Thank you so much for all that! and I use digikey as a reference site because I find their data sheets are very well organized and easy to read and they have almost everything imaginable so I did appreciate you pointing it out since from there I can find similar ones in other stores too.

    You were of great help! off for a latching relay I go look! :p (unless those latching ones also need power to be turned off somehow)
     
  9. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    Latching relays come in a couple of different varieties. One type has two coils; one to latch, another to unlatch. Another type uses one coil. Applying one polarity voltage latches it, the other polarity unlatches it. If you are using a USB cable to power the coil, do you really need a latching relay?
     
  10. julienrl

    Thread Starter Member

    Aug 10, 2009
    21
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    well I would not have the required polarity or different power sources for two coils, but I was just looking into it. I don't think that I need it since the device is powered externally so it should not effect the data transfer, but I was looking into it just in case.

    thanks again
     
  11. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    Sounds like you are well on your way. Good luck with your project. Let us know how it turns out!
     
  12. julienrl

    Thread Starter Member

    Aug 10, 2009
    21
    0
    I got my relays; I am just wondering, is there an importance on the polarity of my coil? (aka, where I put the + or - of my 5v)

    I will do an instructable with the project so I can share this with all aspiring tinkerers like me and to share the results to those who helped me.
     
  13. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    You shouldn't have any problem with polarity on the coil of the relay you talked about getting from Sparkfun. You will, however probably want to put a protective diode across the coil. Wire it so the cathode (the end with the band) toward the + wire and the anode (the end without the band) toward the negative power lead. That will help protect whatever you hook up to from back EMF on dropout.
     
  14. julienrl

    Thread Starter Member

    Aug 10, 2009
    21
    0
    *edit*

    my old message: well I managed to get it working on one of the hard drives, but the second one I can not get working for the life of me.

    I am troubleshooting and when I test for the continuity of the circuit everything seems fine, but when I plug the power in it does not spin up..... but everything is connected properly and it works on the other one! (the way I did it is I bypassed the on-off switch with the relay).

    new message: well i got it working, the switch is really strange, when its on the power is supposed to be running through the off pins or something, anyways I just changed the relay's setting... don't get why but the second one works that way :p my other two hard drive work in the logical way I had hoped.
     
    Last edited: Feb 4, 2010
  15. BillB3857

    Senior Member

    Feb 28, 2009
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    Can you draw a schematic of both versions? Seems strange. Different model hard drives?
     
  16. julienrl

    Thread Starter Member

    Aug 10, 2009
    21
    0
    yes they are 2 different models. I have it working on all 3 of my external hard drives, I am just curious as to why, but I think it is because the switch shorts rather than closes the circuit when it is on.

    Here is the instructable!


    thanks for all the help :)
     
  17. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    Very nice and detailed write up! Good Job!!!
     
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