Short Circuit problems... Driving me nuts!!

Discussion in 'The Projects Forum' started by Goldfields, Jun 23, 2011.

  1. Goldfields

    Thread Starter New Member

    Jun 14, 2011
    3
    0
    HI All,

    Im designing a test system for a control panel used in wind turbines, i need an LED to light from an LM324n once the voltage received from the output of a relay switch is is above the deseired 21v. no problem, nice and easy. now the problem.
    I have only one power supply. that power supply feeds the power and GND to both the LM324n and the Relay switch.
    In order for the relay switch to latch and drive power to the LM324n it needs 24v and 0v., but what seems to be happening is the switching rail on the relay switch is shorting (for a tiny amount of time), which in turn causes my LM to fry. without using a circuit breaker (which would render my sense circuit usless) and without using a second power supply how do i solve this very annoying issue?

    N.B sorry about shoddy diagram.
     
  2. #12

    Expert

    Nov 30, 2010
    16,257
    6,759
    First, the 324 can only be trusted if it has more than (input + 2 volts) for its power supply.
    Second, relays are inductive and can kick a very nasty voltage so you need to protect the inputs and the power supply pin of the 324.
    Third, the drawing doesn't really tell how the 24V is picked up for the measurement.
    Fourth, you have the input polarities backwards on the 324.

    My opinion is to cut the input voltages to the 324 by about half.
    Use a resistor and a 12v zener diode to set the switch point.
    Use resistors to cut the 21 volts down to 12 volts and put a small capacitor at their junction to quench voltage spikes. A thousand ohms per volt would work. Even 10,000 ohms per volt will work. Try 10,000 ohms per volt.
    Use a diode and capacitor to filter the voltage on the supply of the 324 so inductive pulses don't hit it like a lightening bolt.

    That's a start. Reply for clarification.
     
    Last edited: Jun 23, 2011
  3. Goldfields

    Thread Starter New Member

    Jun 14, 2011
    3
    0
    All makes sense. will the diode and Cap help protect? my office is a genoicde of LM324s.
    How does the the zener help with switch point? why not just stick with VD?
     
  4. #12

    Expert

    Nov 30, 2010
    16,257
    6,759
    I'm assuming the 24 volts in question is DC, right?
    All this is BS if it's AC.

    The diode and capacitor on the power supply pin will help the Vcc stay as high as practical if the voltage wobbles on the way up and the capacitor will absorb spikes. It might even help to put maybe 100 ohms in series with the diode to limit the spike current going into the capacitor. (I'm talking about dozens of microfarads.) If you give the LED 10ma and the 324 uses 3ma for its own power, the 100 ohm resistor will cost 1.3 volts of power supply voltage. Not a problem if you do the switching at about 12 volts on the inputs. You could even go lower if you want.

    The 324 smokes at 32 volts, so a 27 to 30 volt zener after the 100 ohm resistor might be a reasonable safety feature. It depends on how much you can trust the turbine to stop at 24 volts and how powerful the inductance is.

    You can use only resistors and capacitors to do 2 voltage dividers on the inputs, but the 12V zener provides another layer of spike protection. It is important to get some series resistance between the voltage and BOTH of the inputs, and a capacitor to absorb spikes is necessary. The trade off is how much capacitance you can use without slowing the response time too much. It might even be advantageous to delay the switching by a few milliseconds. I don't know enough to be sure about that aspect.

    Are we making progress?
     
  5. castley

    Member

    Jul 17, 2011
    31
    0
    Some relay contacts are adjusted to make before break. Check this.
     
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