Need help with this circuit

Discussion in 'The Projects Forum' started by barrage, Nov 7, 2009.

  1. barrage

    Thread Starter New Member

    Nov 7, 2009
    3
    0
    Hi, need help with this thermostat circuit. I'm using a lm311n to try to activate the relay, but seem to have an output voltage reguardless of the voltage diff at the inputs. The circuit is below, except inputs at pin 2&3 are swapped, 500k hysteresis still at pin 2. Any ideas what might be going on? Thanks


    [​IMG]
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Are you certain that the 1N4148 is installed with the cathode towards +V?
    What is your +V?
    What is the voltage rating of the relay coil, and what is it's resistance?
     
  3. barrage

    Thread Starter New Member

    Nov 7, 2009
    3
    0
    Supply voltage is ~12V. The output at pin 7 ranges from ~9.5-12V. I might have the cathode backwards - I had trouble deciphering which end was which on these diodes... And here's the info on the relay.10 AMP PC RELAYContact Res.: 50mΩ max.
    Operate Time: 10ms max.
    Release Time: 4ms max.
    Insulation Res.: 100MΩ min.
    Mechanical Life: 100M cycles
    Electrical Life: 100K cycles
    Resistive Load: 10A@28VDC or 10A@125VAC
    Inductive Load: 3A@28VDC or 3A@125VAC[/B][IMG]http://www.mode-elec.com/main/getpicture.php?id=536&type=img_med2[/IMG]Circuitry SPDTOper. Voltage 12VDCCoil Resistance 400Ω
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    OK, 12v/400 Ohms = 30mA. That's well within the sink current limit of an LM311.
    However, if you're using an LM339 comparator instead, you'll have problems - an LM339 can only sink up to about 6-8mA current.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Try disconnecting pins 5 and 6 on your 311. It may be causing too much current in the inputs.
     
  6. barrage

    Thread Starter New Member

    Nov 7, 2009
    3
    0
    Got it. Rookie mistake on the diode. Tested the diode, had it flipped around.

    Thanks SgtWookie.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Glad you figured it out. :)

    Just remember that the end with the band on it is the cathode.
     
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