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Relay Driving Circuit Design Help

  1. Jun 30, 2016 #1
    Hello all! I hope I am posting this in the right place.

    I am looking for some help on the following circuit. Basically I am doing a relay application using one of TI's chips. Below is the reference design from an app note along with my actual circuit modeled in TINA. The main function is that using the comparators it will toggle the relay when an overload is detected. This is my first real circuit design application so any assistance with it I would greatly appreciate it. I am looking for tips and a walk through so I can arrive to the solution rather than being given one. Any other questions please ask!

    1. There is a supply of 110 VAC being provided through J1. The transient response shows that the voltage at VF4/VF5 does not exceed 2V. Why? my thoughts are that it is not being rectified correctly and/or there is a mistake in my design with the cap/resistor at the output.
    2. How can I verify a rectified Voltage at the output?
    3. Can the input into the comparator be AC? It seems like its just going to keep toggling and my thoughts are that everything should be rectified and supplied to the system as DC.
    If I come up with more I will post on here. I apologize if any of these questions seem remedial.

    Attached Files:

  2. jcsd
  3. Jul 1, 2016 #2
    R4 is wired incorrectly. It should connect between VF4 and U2 Vin.
    I haven't checked the rest of the simulation, their could be other errors.
    Toggling output of the comparator is valid in this particular case because the same signal is fed to the EN input of U2. Thus U2 only looks at a half cycle of the current sense signal.
  4. Jul 1, 2016 #3


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    I assume the earth symbols on the diagram are a local floating common and not connected to the power grid earth. The diode bridge would not survive an earthing.
    I gave up using Tina and went to using the better supported LTspice.
  5. Jul 1, 2016 #4
    What is it that you mean? The floating ground?

    Yes I agree TINA has been giving me issues. Sometimes the outputs are different and all I do is delete a connection and redo the same connection and it changes everything. I found that out this morning.
  6. Jul 1, 2016 #5


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    In the rectifier bridge, there is a diode connected between each of the power lines and Earth.

    Edit: I am referring to the original diagram upload_2016-6-30_9-38-34.png
    Last edited: Jul 1, 2016
  7. Jul 1, 2016 #6
    I am going to change a few things and repost with an update.
  8. Jul 2, 2016 #7
    Ref Post #4
    Many (all?) simulation programs have that characteristic. When you delete and then add the same connection or component, the order (sequence) of the internal circuit representation changes. This changes the order that the describing equations are evaluated. Since the program uses a finite resolution for the numbers, sometimes there are truncation errors.

    That's the simple explanation. I've been told it is due to pivot errors or singularities in the matrix manipulations. I'll let the math gurus here try to explain that one!
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