Math Question - Latching Relay with Momentary Switch

Discussion in 'General Electronics Chat' started by kdrubio, May 2, 2019.

  1. kdrubio

    Thread Starter New Member

    May 2, 2019
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    Working with some Kemet EC2-5TNU double coil latching relays and trying to get it to work with a SPST momentary switch.
    I found an older article (here) with an example of how to implement this, but I am working with 5v not 12v.
    What is the math involved here to determine the correct resistor, capacitor and diode values?
    I have never had to dive this deep into circuitry before.
    Not looking for direct answer, willing to learn, just don't know where to start for something like this.
    Thanks
     
  2. KeepItSimpleStupid

    AAC Fanatic!

    Mar 4, 2014
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    Notice that there is a SET and a RESET coil with polarity markiings and the relay is shown in the RESET position.
    To move the coil to the SET position, you can do one of two things:

    1. Apply a 5V pulse with the marked polarity to the SET coil
    2. Apply a reverse polarity pulse to the RESET coil.

    and vice versa.

    Applying SET and RESET voltages simultaneously results in an unknown condition,
    Generally, you can apply the voltages continuously (a long time) if you actually need to.
    A permanent magnet holds it in the last position.
     
  3. SamR

    Active Member

    Mar 19, 2019
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    Shouldn't matter. Try it like it is. The diode is rated for current not voltage. The capacitors are storing the reversing charge for the single button and resistor throttling the rate of capacitor charge. You could play with the resistors a bit to tweak the rate of charge.
     
  4. kdrubio

    Thread Starter New Member

    May 2, 2019
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    Ok, so in a perfect world I would size my capacitor based on my coil requirements and size my resistors for the response time needed. Cool thats just 101 stuff that I need to learn up on. Ill give the example values a try and if they work then all done. With no Radio Shack left in my area I wanted to make sure the assortment part kits I was going to order would cover me.
    Thanks a bunch for dumbing it down for me.
     
  5. KeepItSimpleStupid

    AAC Fanatic!

    Mar 4, 2014
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    What i would try to do is make RC, the product of the resistance and capacitance about the same when you include the differences of the resistances of the relay coils.

    One rule of thumb is it takes about 5 time constants to charge a capacitor to about 98% or so. This isn't exactly what;s going on here, but the minimum activation voltage plays a role too.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    6,594
    My simulations show you likely need a capacitance of at least 100μF with those 5V relays.
     
  7. KeepItSimpleStupid

    AAC Fanatic!

    Mar 4, 2014
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    Makes sense. The coil resistance is about 4x lower.
     
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