Relay coil not getting 12v.. schematic included

Discussion in 'The Projects Forum' started by jwilli, Jul 16, 2009.

  1. jwilli

    Thread Starter Active Member

    Apr 21, 2009
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    I've got this schematic all built. At one time everything was working great, but now for some reason when 12v is supposed to go to the relay coils, they are only getting 4-6v or none at all. It's very fishy... As I hold my voltmeter down on the leads going to the 12v coils, the voltage slowly trickles higher.

    I can't seem to figure out why... I was thinking some issue with the supression diode or a capacitor but I thought I would get your opinion.

    *Note: The relay coil should be getting power when the red LED is on. The red LED comes on, just no power to the coil.

    Here's the schematic:

    [​IMG]
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    You may have blown U3, the IRF610 MOSFET.
    The MOSFET should be a logic level (IRL prefix) rather than standard level (IRF prefix).
    The relay should have a diode across it's coil, cathode towards +V.
    [eta]
    Pin 1 of R13 isn't connected. Did you mean for it to connect to the 24v supply?
     
  3. jwilli

    Thread Starter Active Member

    Apr 21, 2009
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    This is the IRF610 I'm using here... I tried replacing it but it didn't work:

    http://mouser.com/Search/ProductDetail.aspx?R=IRF610PBFvirtualkey61370000virtualkey844-IRF610PBF

    Maybe it's the wrong type?...

    Secondly, what kind of diode would you suggest using on the relay?
     
  4. jwilli

    Thread Starter Active Member

    Apr 21, 2009
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    R13 is actually the 24v trim, but I have the jumper set in the far right position, for 12v, so its not even being used in this case.
     
  5. mik3

    Senior Member

    Feb 4, 2008
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    As SGT said, use a logic level MOS (it turns fully on at 5V and not 10V like the one you use).

    A 1N4001 diode will do the job. Connect it in parallel with the relay's coil and with its cathode to Vcc.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Does the voltage at U3's drain alternate between nearly 0v when the LED is on and 15v when the LED is off?
    Do you get about 15v at the output of regulator U2?

    Does the output of the regulator drop quite a bit when the LED is on?

    A 1N4004 (or any 1N4002 thru 1N4007) would work just fine. Wire the cathode on terminal 2 of the coil, and the anode to terminal 3.
     
  7. jwilli

    Thread Starter Active Member

    Apr 21, 2009
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    First off, I can't begin to tell you guys how much I appreciate the help!!

    With 12v incoming, the voltage at U3's drain drops from 11.28v to around 4.5v when the LED is on and continues to slowly drop.

    The output of U2 is 11.28
     
  8. mik3

    Senior Member

    Feb 4, 2008
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    Because the PIC cannot supply enough current to charge the gate of the MOS fast and because the LED draws a significant amount of current and preventing fast charging. Also, the MOS is not a logic level one.
     
  9. jwilli

    Thread Starter Active Member

    Apr 21, 2009
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    What part would you recommend I use instead? I'm sorry for my lack of knowledge on the subject, but it would really help me out if I knew what part could substitute it.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    That's an indication that the MOSFET is not turning on fully.

    Use a logic level MOSFET instead.
    http://mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMveMCOqFR6qCPeAjNEHCd78ZYTWENsk%2bXw=

    You have 7815 shown on the schematic as U2. Is it a 7812 instead? Even then, if you're supplying it with 24v, 11.28v output is quite low. What is the voltage at the input of U2?
     
  11. jwilli

    Thread Starter Active Member

    Apr 21, 2009
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    Thanks for the link...

    The input voltage is 12v not 24. There's a jumper setting for if you're using 24v but I have the jumper set for 12v (in the schematic, the jumper all the way to the right)
     
  12. jwilli

    Thread Starter Active Member

    Apr 21, 2009
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    OK... I removed the red LED from the circuit and now the relays work fine.... Looks like the LED was pulling too much current (Thanks mik3)...

    It's a standard 5mm LED. If I change to a logic level MOS, would this fix the problem or do I need to figure something else out?
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    From a PM:
    That's somewhat surprising that disconnecting the red LED was sufficient to get the output high enough to turn on the MOSFET; the load current was only about 5mA if my calculation is correct; assuming the LED's Vf=2v: (5v - VfLED)/560 Ohms = (5v-2v)/560 = 3/560 = 5.36 mA.

    Using a correct logic-level MOSFET will certainly help, as the threshold voltage of the logic-level MOSFET will be much lower than a standard MOSFET.

    When you're using an output to control a MOSFET's gate, you are much better off if you can avoid powering anything else in addition to the gate.

    Additionally, you should limit maximum current to the MOSFET's gate in order to keep it within the limits of the uC. If the I/O limit is 20mA and your Vcc=5, then 5/20mA = 250 Ohms or more. The closest standard value is 270 Ohms.
     
  14. jwilli

    Thread Starter Active Member

    Apr 21, 2009
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    So, the IRF610 datasheet says the threshold is between 2v and 4v and it also says Rds is over 1 ohm for Vds of 10v.

    The IRL510 datasheet says the threshold is between 1v and 2v and Rds is < 1 ohm for Vds of 4v.

    Am I correct to assume that by using an IRL510 I should be able to use the relay AND led without pulling too much current?
     
  15. boriz

    New Member

    Jul 16, 2009
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    Yes. Nice forum BTW. :)
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    I suggest that you only have the gate of the IRL510 connected to the output via a resistor.

    The LED is rather useless if it prevents the voltage on the gate of the MOSFET to rise high enough to turn it on and sink current from the relay.
     
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