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Prop Controlling a relay — Parallax Forums

Prop Controlling a relay

CassLanCassLan Posts: 586
edited 2009-06-06 15:25 in Propeller 1
I was looking at this relay:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=PB389-ND

Data Sheet: http://relays.tycoelectronics.com/datasheets/d2n.pdf

I attached a screen shot of the info.



Can I drive this directly from the prop? it says the coil current is only 4.16mA.

Rick
screensh.JPG
998 x 169 - 44K

Comments

  • RaymanRayman Posts: 14,829
    edited 2009-06-06 01:05
    It's usually not a good idea to drive a coil directly from the Prop.· You should use a transistor or something like the ULN2803...

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    My Prop Info&Apps: ·http://www.rayslogic.com/propeller/propeller.htm
  • kwinnkwinn Posts: 8,697
    edited 2009-06-06 01:10
    With a 3V coil of 45ohms resistance it would draw about 70mA which is too much for the prop pin.
  • CassLanCassLan Posts: 586
    edited 2009-06-06 01:58
    Thanks for your help.
    I thought maybe this was a small enough one that could be used directly.
    Its funny that I need the prop to drive a transistor...to drive a relay...to drive my actual load [noparse]:)[/noparse]
    But I guess thats how it goes.

    Btw in searching for the ULN2803 I came across this arcticle thats all about this sort of thing...and what do you know..its hosted by parallax: http://www.parallax.com/dl/docs/cols/nv/vol1/col/nv6.pdf

    Rick
  • RaymanRayman Posts: 14,829
    edited 2009-06-06 02:09
    Look here and you'll see me driving a 200A contactor with the Prop:
    http://forums.parallax.com/showthread.php?p=687346


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    My Prop Info&Apps: ·http://www.rayslogic.com/propeller/propeller.htm
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2009-06-06 04:08
    Rick,
    I thought the fact the relay you referenced having a +24VDC coil is the funny part. The low current of a few milliamps is the giveaway. The 2.4VDC that is mentioned in "one" of the columns refers to the absolute minimum voltage you can expect the relay to stay in after it has been energized at a higher voltage (this spec is for designers just to make sure it turns off).

    Anyway, the Prop is a processor and not a power switch, there is no way you can drive a +24V relay directly from the Prop or any other low-voltage logic for that matter.

    *Peter*
  • MicrocontrolledMicrocontrolled Posts: 2,461
    edited 2009-06-06 10:41
    If it is a 3V relay hten you should have power on one end and the Prop swiches it off by driving the other end low, right?

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  • PraxisPraxis Posts: 333
    edited 2009-06-06 11:59
    microcontrolled said...
    If it is a 3V relay hten you should have power on one end and the Prop swiches it off by driving the other end low, right?

    Not unless you want to fork out for a new prop chip.

    Use a transistor or ULN2003 as was suggested be fore the drive the relay. If you use a transistor don't forget the back EMF diode.

    There are many examples posted on this board.

    Cheers

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  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2009-06-06 13:12
    Relays are mechanical and as the strenth of the magnetic field is inversely proportional to the diistance a large initial pull is required but a vastly lesser holding force is required. Low volts = higher current even with "high efficiency" coils. The collapsing field will do its preservation of energy bit. and pulse high volts into anything attached. I have even seen people puting resistors across the coil instead of or as well as the reverse diode which will have a finite response speed, even though they have used a driver transistor. Relays are not good on high speeds or high efficiences - very good for TOTALLY isolated switching
  • CassLanCassLan Posts: 586
    edited 2009-06-06 13:30
    I have to say the ULN2x03 packages are sweet.and cheap, they have all the protection stuff built right in..much more attractive to me than a resistor+transistor+diode.

    Which is better for use with the prop the ULN2003 or the ULN2803?
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2009-06-06 13:46
    I think they are much the same except for the number of channels, 7 or 8. uln2004 are for higher input levels and wouldn't work on less than 7-10v
  • SapiehaSapieha Posts: 2,964
    edited 2009-06-06 15:25
    Hi CassLan.

    If You ned "SOURCE DRIVERS" ity is UDN2981A thru UDN2984A

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    For every stupid question there is at least one intelligent answer.
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    If you don't ask you won't know.
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    Sapieha
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