Relays - Switch and Contact Protection

Discussion in 'The Projects Forum' started by jhausch, Jun 8, 2009.

  1. jhausch

    Thread Starter Active Member

    Jun 7, 2009
    This is probably a pretty basic question, but I'd like some reassurance that I am doing this correctly.

    On the attached diagram the 3 position switch runs the motor CW or CCW. When a limit is reached, the reed switch is opened by a magnet on the mechanical assembly preventing continued motion in that direction.

    I am pretty sure the diodes across the relay coils protects my reed switches (and the contacts in my 3-pos switch if it is opened prior to the travel limit). Am I correct?

    I am pretty sure the R-C between the motor connection on 0V protects my relay contacts. Am I correct?

    For the R-C I think I need to size the resistor to match the cold motor resistance and size the the cap at 1uf and 3x motor voltage - however, the wattage rating of those items is minimal Am I correct? (the duty cycle is low, perhaps one cycle under load for 10 seconds and then a 1 minute dwell).

    EDIT - I am 99% sure I have the R-C in the wrong spot. Maybe I can't protect the relay contacts using an R-C when using two relays to switch the motor? Maybe the resistance needs to be greater than the motor? I am confused now. . .

    Last edited: Jun 8, 2009
  2. jhausch

    Thread Starter Active Member

    Jun 7, 2009
    Respectfully, "Bump"
  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Nice clever circuit. Well thought out.

    As for the RC snubber, it is across the NC connection of the relay SPDT contacts, but the contact you are really switching is the NO contact. Thats is the contact you activate with your control switch and limit switches.

    You could just move the RC snubber across each relay NO contacts.

    If you are after a high reliability design you might want to add a series resistor between +12v power and the relays, this will take the edge off start surges and might save your bacon if a reed switch fails and the motor is at full power with a locked rotor. You could use a fusible resistor here, it would cook and save your motor in the event the operator doesn't notice the fault.

    I would try the largest resistor value you can get that still gives enough motor power for all expected input voltages.
  4. radiohead

    Active Member

    May 28, 2009
    A diode across the relay coil is intended to protect semiconductors from "kickback." When the relay coil is de-energized, the collapsing magnetic field will kick a current back through the circuit and could damage/destroy sensitive semiconductors. A 1N4148 will do in most cases. I tend to use the 1N4004.