Back EMF protection

Discussion in 'General Electronics Chat' started by xxxyyyba, Jan 25, 2016.

  1. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
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    Here is schematic that provides polarity inversion and switch off motor if it reaches one of its two end positions:

    [​IMG]

    SW1 is On-Off-On DPDT switch with the center position Off. SW 2 and SW 3 are limit switches. Motor voltage and amperage are 12V and 2A (max.).

    Should I add back EMF protection and, if yes, how can I implement it in my case?
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
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    Since you're using mechanical, you might be OK without back EMF protection. However, your switches might have degraded life-times, and it wouldn't hurt. You need two diodes across your DPDT switch on the motor end between "FWD" and the center contacts, and "REV" and center contacts, each reversed biased for normal current flow (ie, cathode on "FWD" anode to center contacts, anode to "REV" , etc.)

    However, you'll need to reverse the order of your FWD/REV switch and limit switches for this to work.
     
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  3. MaxHeadRoom

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    Jul 18, 2013
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  4. crutschow

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    Mar 14, 2008
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    You can also place the 4 diodes at the motor connection.
    Connect one at each motor connection to V+ (anode to motor) and one at each motor connection to V- (cathode to motor).
     
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  5. MaxHeadRoom

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    Jul 18, 2013
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    A motor that is revolving at turn off exhibits a BEMF in the same polarity as the supply, the level of which is dependent on rpm it is rarely needed in especially in a relay operated circuit.
    Max.
     
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  6. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
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    Like this?
    [​IMG]
    I thought it's better to open new thread, than to write in old one :)

    Do you have schematic because I'm not sure I understood? :)


     
  7. Brownout

    Well-Known Member

    Jan 10, 2012
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    That'll work.
     
  8. Dodgydave

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    Jun 22, 2012
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  9. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
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    There will be problem, when DPDT is in this configuration:

    [​IMG]
     
  10. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
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    Thanks for reply.
    On that way, there is no need for three wires, right?
    Similar to this schematic (thanks MaxHeadRoom):
    [​IMG]
     
  11. Dodgydave

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    Jun 22, 2012
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    Correct.
     
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  12. crutschow

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    Mar 14, 2008
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    Those diodes won't protect the limit-switch contacts when they open.
    For that you need an added diode at D1's anode (anode of new diode to B ground), and one at D2's cathode (cathode of new diode to V+).
    If you follow the direction of the inductive motor current when a contact opens, you will see why you need the added diodes.
     
  13. MaxHeadRoom

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    What about the generated voltage in the same polarity as the supply?
    You only have to back feed a PM motor to see this.
    Max.
     
  14. Brownout

    Well-Known Member

    Jan 10, 2012
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    Just add a pair of 15v zeners across the motor. Anodes connected to motor wires, cathodes connected together.
     
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  15. crutschow

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    Don't understand your question. :confused:
    The added diodes I suggested will protect the limit switches from inductive spikes for both motor directions.
     
  16. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
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  17. crutschow

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