Capacitor in parallel with resistor

Discussion in 'General Electronics Chat' started by Dritech, Jan 12, 2014.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi,

    I was looking for a shunt sense resistor to use with my H-bridge. While doing my research, I noticed that some use a capacitor in parallel with the resistor. What is the reason for this please?
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    I can imagine that the capacitor will make the current sensing less sensitive for spike currents.

    Bertus
     
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks for the reply. As an H-bridge I am using the BTS7960. It has the a status flag diagnosis feature. This is used to output the full current when when a fault occurs. Does anyone know how to get 5V from the IS pin? There is an equation in the datasheet, but I cannot full understand it.

    Any help will be appreciated.
     
  4. Gibson486

    Member

    Jul 20, 2012
    199
    12
    I have used that chip before. Just to let you know, that chip is being phased out. They are already 2 iterations away from that chip.

    Your output for that pin is dependent on your source voltage and your load current. I remember I spent a good two hours figuring out how to derive the resistor values like you are doing. I have it in my notebook at work on how to derive the equation. I'll update tomorrow.
     
    Dritech likes this.
  5. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    I will appreciate that a lot :) I was planning to get 5V when the flag is active.

    Another quick question. The datasheet states that the ON resistance is approx. 16m ohms. Assuming that the load consumes 20A, does that mean that the voltage drop is of 0.32V?

    Is the BTN7970 a better alternative?
     
  6. Gibson486

    Member

    Jul 20, 2012
    199
    12
    The ratio is 8500:1

    So, the equation becomes V(current) = current(out) / 85000 * R
     
  7. Gibson486

    Member

    Jul 20, 2012
    199
    12
    The BTN8962TA is the replacememnt. I am not sure they released it to distributors yet, but it is what the manufacturer told me. In the interim, I have used BTN8960TA.
     
  8. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks for the reply. Is V(current) is output voltage when the flag is active? and I'm assuming current(out) is the current output from the pin not the load current, right?
     
  9. Gibson486

    Member

    Jul 20, 2012
    199
    12
    Yeah, the whole point is that you adjust it so your max current leads to you flag voltage. At least that is how I understood it. It worked when I tested it, but we have not really implemented it since...
     
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