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Problem Using Darlington ULN2803A — Parallax Forums

Problem Using Darlington ULN2803A

ItsMeNickItsMeNick Posts: 19
edited 2006-06-02 01:29 in BASIC Stamp
Hey Guys,

I'm having trouble using my ULN2803A to drive a relay.

My BS2 puts 5V to the input pin.
ULN2803A is grounded, and has 12V goto the voltage pin.

Now, I wire the output pin to the relay coil.
What do I put the other side of the coil to?

If I check with a voltmeter...

The output of the darlington & 12V... I get 12V
However, when I hook up the relay coil, i only get 4 volts across the coil.

What am I doing wrong?

Thanks!

Comments

  • NewzedNewzed Posts: 2,503
    edited 2006-05-30 23:45
    Connect the other side of the relay to 12VDC if that is what it operates on.

    Sid



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    Sid Weaver
    Do you have a Stamp Tester yet?
    http://hometown.aol.com/newzed/index.html

    ·
  • ItsMeNickItsMeNick Posts: 19
    edited 2006-05-31 00:10
    Thanks for the reply Sid...

    When I put the other sidew to 12V the relay still won't energize.
    When I put a voltmeter to both sides of the relay i only get 4-5 volts

    If I keep taking a voltage measurement and get off the relay coil, it shoots back up to 12 volts
    There is no load on the relay. And the 12V source is a big ol' regulated power supply.

    I'm stumped
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-05-31 00:55
    · Make sure that 2803's pin9 goes to Vss and your 12V's GND.
    · Also, see attached.· I used pin2 for input from the BS-x, I use a pull-up to +5 (the 1K); ·pin17 is the output.
    2803.jpg
    374 x 318 - 22K
  • ItsMeNickItsMeNick Posts: 19
    edited 2006-05-31 01:14
    I found the problem...

    It occurs when I hookup another relay to a different output on the 2803.

    So I have one relay hooked up fine... once I add another relay to a different output, the voltage drops and it no longer works.

    Any ideas why?
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-05-31 01:22
    "big ol' regulated supply"?· How big?· What's it rated?

    Also, what's the coil resistance of your relays?
  • ItsMeNickItsMeNick Posts: 19
    edited 2006-05-31 01:36
    Its regulated at 13.5 Volts 2.58Amps,
    I'm actually unsure as the resistance of the coils... I can check with an ohm meter tomorrow.
    They seem to just be regular 12v power relays.

    The supply is powering two relays currently, one directly & one through the darlington.
    The second I touch the third relay to another ouput of the darlington, the coils no longer energize.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-05-31 01:51
    · OK, now it's three relays; I'm starting to get motion-sick.
    · Maybe we can take the BS-x out of the circuit and just deal with the 2803 and the relays.· How about U/Ling a sketch of your set-up/circuit?
  • ItsMeNickItsMeNick Posts: 19
    edited 2006-05-31 13:49
    Ok Here it is.
    The point of the circuit is to get power·from the alternate source when·the basic stamp says so. (Watches for a Voltage Drop across the original 12V line, which goes to a 5V Regulator)

    The load on the third relay isn't important (and isnt present currently) It's just going to act as a momentary switch.
    schem.JPG
    610 x 371 - 15K
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-05-31 14:17
    Will the attached circuit work?·

    N.B. --·
    • the +12V is the same for the 2803 and the relays
    • it doesn't use the STAMP, and
    • you'll need a 5V source (sharing GND with the 2803 & 12V GND)
    2803-2.JPG
    660 x 399 - 27K
  • ItsMeNickItsMeNick Posts: 19
    edited 2006-05-31 15:00
    I'll give it a shot tonight when I get home... is that my problem?
    I'm thinking a HIGH Input will energize the relay? But really a LO input will energize?

    Thanks for the help!! I'll let you know how it works out
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-05-31 15:21
    · Yes -- the output is "inverted", so to say.· With a HI in, the output transistor (collector-emitter)·is in conduction ("on", saturated) and goes to Ground.· With a LO in, the output transistor is "off".

    · 2803s (and 2003s) have open-collector outputs.· So, with them, you're providing/switching a path to ground to complete (open/close) the circuit.
  • ItsMeNickItsMeNick Posts: 19
    edited 2006-05-31 15:25
    Ahhhhhh!!!!

    Well back to work for now, I'll let you know how I make out!
  • ItsMeNickItsMeNick Posts: 19
    edited 2006-05-31 23:12
    Great, your schem works!

    Now, how do I reintroduce the stamp back into things?
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-06-01 00:04
    Well, I think that if you're using the 1K pull-up resistors (hope so) -- then just run "A" to one STAMP output and "B" to another (just so.)

    Post Edit -- If you take a look at the 2803 data sheet, you'll see that each of the transistors has its own IB resistor and a pull-down resistor, too.· So, given this and the above, you're all set.

    Post Edited (PJ Allen) : 6/1/2006 12:07:29 AM GMT
  • ItsMeNickItsMeNick Posts: 19
    edited 2006-06-01 23:07
    So, do I have to keep the outputs HIGH until I'm ready to energize the relay? Then set them LO?

    Just wanna make sure i fully understand!
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-06-01 23:56
    Nick,

    ·· Perhaps I missed something in your system requirements, but the 1K pull-ups aren't a required thing.· The only effect they would have on overall system design is that when the system powers up the outputs will momentarily turn on (which may be undesired?) until the Stamp I/O pins are initialized to LOW.· If you wanted to keep the outputs on when the Stamp Module sleeps, then you may in fact want these.· I hope this helps.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-06-02 00:27
    · Another thing, about relays: they ordinarily have three terminals -- a COMmon contact, a Normally Closed contact, and a Normally Open contact.· You therefore want to switch between COM & NC or COM & NO.
    · So, you need to decide what you want your Default state to be (To pull-up or not to pull-up?· That is·The Question.) ·Where and whether to energize the coils and their effect and how to deal with that all is your call.
    ·
  • ItsMeNickItsMeNick Posts: 19
    edited 2006-06-02 00:33
    Thanks Chris, In that case I won't use the resistors...

    But I'm still having this problem... when I have both relays conneceted, I only get 4 volts at the output of the darlington, instead of 12

    :-\
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-06-02 00:40
    Nick,

    ·· The output of the Darlington is actually supposed to be switched to ground when on...Are your relays working?· Can you post a schematic of your current wiring diagram?

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • ItsMeNickItsMeNick Posts: 19
    edited 2006-06-02 01:29
    Find the schem above... weird though... I rebuilt it using other relays and now no problems.
    Odd... Thanks for the interest Chris! Really appreciated!!
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