Relay driving circuit

Discussion in 'Analog & Mixed-Signal Design' started by Ramon Jimenez, Jun 30, 2016.

  1. Ramon Jimenez

    Thread Starter New Member

    Jun 30, 2016
    5
    0
    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!

    Problems:
    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.

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  2. Marley

    Member

    Apr 4, 2016
    142
    40
    There is a problem with this circuit. The supply is 110V AC. You are deriving power using the bridge rectifier D4-D7. The voltage across C4 is going to be in the region of 150V DC. The amplifier U1 can have max 15V on the supply rails!

    There are other problems, but I will stop here!
     
  3. Ramon Jimenez

    Thread Starter New Member

    Jun 30, 2016
    5
    0
    Thank you for your input! I am in the process of changing a few things and will reply with my changes.
     
  4. Marley

    Member

    Apr 4, 2016
    142
    40
    You are hoping to toggle the relay when an overload is detected. Therefore you need to measure the voltage dropped across the series resistor RS1. So your first amplifier U1 needs to be linear and have some kind of threshold point. The comparator connected as in your circuit will simply give a square wave output at pretty much any current.

    Also, in a practical circuit, a sensitive comparator connected directly to the main supply lines will have a very short life due to spikes and surges. It would be better to isolate your current measuring circuit completely from the supply by using a current transformer. And much safer too!

    So two things:-
    • An isolated power supply to generate a safe low voltage (say 12V) from the power line
    • A current transformer for measuring the current.
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,972
    3,219
    This is a line-power circuit and that circuit has many problems.
    Discussions about line-powered circuits are generally not allowed on this site.
    You need to do as Marley suggested.
     
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